Addicted to love, beauty or sex?

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On Valentine's Day it is appropriate to consider whether humans, and perhaps at least some other species, are so addicted to love that they cannot exist without it. Is love really like an addictive drug? What are the features that define perception of beauty and attraction to co-exist exclusively with others? Is sex without love simply a distraction from failure to get involved in the main event, or an admission that the main event is not primarily about sex?

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Gresham Professor of Physic


On Valentine’s day it is clearly highly appropriate to consider what makes us and other species so strongly attracted to members of a particular sex, why we all seem to have highly specific preferences for particular individuals and why some species form long-term attachments which are obviously not entirely about sex while for others they are short and appear highly sex orientated? Are we in the words of the song by the late Robert Palmer “Addicted to love”? A cursory look at the internet will show you that there are organisations catering for individuals who consider that they are either addicted to love or sex!

Even Einstein recognised how the power of love, just like pain, can appear to subvert the laws of time:

“When you are courting a nice girl an hour seems like a second. When you sit on a red-hot cinder a second seems like an hour. That’s relativity. (Albert Einstein)

Leaving aside for the moment whether love really is a form of addiction, there is no getting away from the fact that love, and all that goes with it, is more important to us humans than almost anything else. Failure to make lasting relationships, and relationship break-ups are a major contributor to stress, anxiety and depression. We write poetry, plays, songs and books, make films and gossip about love, relationships and sex far more than anything else and yet from a scientific point of view hardly anyone dares to study it. Perhaps it is, and should remain, one of the great human mysteries since having love finally “fixed and sprawling on a pin”, in T.S. Elliot’s words, could somehow devalue it. Perhaps it is something we simply take for granted and which needs no scientific interpretation. Perhaps when it is explained it will turn out to be something that is not uniquely human but a simple expression of the biological drive to reproduce that we see in all other species.


Why do we love?

This is a question that is addressed in considerable detail by Helen Fisher in her most recent book entitled “Why we love” (2004). In humans, at least, the symptoms of being in love with someone are relatively consistent across cultures and many of these are mentioned in Robert Palmers song “Addicted to Love”. Your love object becomes all important, taking on “special meaning”, and becomes virtually the total exclusive object of your attention (typically in your thoughts > 80% of the time) and intrudes constantly on your other thoughts, you exaggerate all of their positive features and blank out any negative ones, you feel energised, lighter than air (particularly women) and experience a torrent of intense emotions that often result in physiological reactions (turning pale, or flushed, heart palpitations and/or feeling it burning, trembling, sweaty palms, feeling dizzy, butterflies in the stomach, weak at the knees, unable to eat or sleep etc), you experience intense mood swings and the need for physical proximity and union with your beloved is intense. Love, in truth, is both a magnificent and debilitating obsession!

In short when in the throws of love you are pretty much useless for anything in life outside of trying to consummate your need for the person you are in love with. One might even go as far as predicting that more working time is probably lost as a result of workers being in love than from colds or back ache, especially where the work force is young and largely single!

However, thankfully this obsessive type of love that marks an initial relationship with another individual is not normally a permanent state. Estimates for the maximum time it lasts in humans are quite precise – around 1.5 – 3 years. This broadly corresponds to the time it takes for a child to be conceived, born and raised to the point where producing another child can be safely contemplated. The argument has therefore been made that love has evolved to ensure that couples stay together for a sufficient length of time to reproduce their genes and provides the basis for biparental care. Of course love does not just stop after a few years but normally evolves progressively into a form of attachment which maintains close bonds between individuals but is less inclusive and obsessive. However, with a sharp increase in the break up of partnerships between 4 and 5 years in humans it is clear that this attachment phase does not always work out and many individuals are drawn to finding another obsessive love “fix”.

This is not to say that love is distinct from attachment or lust for that matter. As Helen Fisher rightly points out in her book these three key features of romantic relationships can pretty much occur in any order or at the same time. The main question is whether romantic love as we experience it has only evolved in species which also form pair attachment bonds or whether it is truly a unique human attribute that has evolved from our increased capacity for self-awareness and empathic awareness of others?

For the remainder of this lecture I will focus on detailing what attracts us and other species to one another, what makes some species bond while others do not and whether the human brain in love looks qualitatively different from that of other species that are socially and sexually attracted to specific individuals either briefly or for long periods.


Animal attraction:

The first lines of a catchy song by the Bloodhound Gang make the simple statement: “You and me baby we ain’t nothing but mammals lets do it like they do on the Discovery channel”

In terms of the biology of the sex drive which makes us search for another individual to have sex with, this does appear to be strongly conserved across species and is highly dependent on the actions of the sex hormones testosterone and oestrogen on relatively primitive parts of the brain. The motivation to have sex is very much equivalent to that for eating when you are hungry, drinking when you are thirsty and, at least according to some scientists, showing physical aggression from time to time. Arguably for humans, and about 3% of other mammals, there is also a motivation to form close attachment bonds with individuals. Until these various “drives” are satisfied all animals experience an increasing level of need to find a release from them through the performance of the relevant set of actions – sex and attachment in the context of the current lecture

But what is sexual attraction really about? What attraction does is to focus your sex drive onto both a specific gender and then a specific individual of that gender. For the vast majority of individuals it also helps make sure that the object of your desire is the most appropriate one to reproduce your genes with although this is clearly not a conscious decision process.

In terms of promoting your own genes males and females can have very different agendas. For males, sperm are cheap to produce from an energetic and numbers point of view, and therefore normally the best strategy would be to have sex with many different females and adopt a simple “love them and leave them” policy and not bother about being too choosy. However, while males of many species do just this, the object is not simply to get females pregnant but to make sure that the offspring produced survive to reproduce themselves. With species where the young are vulnerable and females by themselves cannot easily rear them successfully then the male has to at least guard the females and their babies, and where mum cannot raise them alone without help then the males have to take on paternal duties as well. With either of these two scenarios males must immediately become much more choosey about which females they should reproduce with since the field has become more limited and their time investment higher. In addition, they need to make sure that the females they make this investment in do not nip off and reproduce with another male leaving them to do all the protection and dad stuff simply to promote another male’s genes! In general therefore males should be looking for females who will both produce the fittest (biggest) babies and who are both good mothers and easy to keep under control.

To some extent this may well be the case. In many human cultures, for example, men are more attracted to women with large breasts and hour-glass curvaceous figures (ideal waist hip ratio of 0.7) and all of these features are associated with greater fertility and nurturing potential. Also in mammals, sexual conflict can occur at the level of the genome through a class of genes called “imprinted” genes. These are genes that are monoallelically expressed with the mother’s or father’s copy actively suppressing their particular version of the gene to reduce its overall influence on their offspring. It is somewhat akin to having the right of veto! It is interesting that males try to use these genes to promote both the production of big babies and good maternal care by their female offspring. A case of rather long-term planning one might say, but evidence at least that male mammals do have a mechanism for trying to ensure that females exhibit the most attractive qualities from a reproduction point of view (see Isles and Wilkinson, 2000).

For female mammals, egg production, pregnancy and offspring care costs are very high and so it is absolutely imperative to have a highly efficient process for choosing Mr Right as far as the mating game is concerned. Exactly which characteristics Mr Right should have may be dependent to some extent on whether he needs to perform paternal as well as protection, nest-building and food supply duties. For monogamous species this can pose a few problems. Good attentive dads tend to be more in touch with their feminine side, have lower testosterone, be less aggressive and have genes that might not be optimum for survival when the going gets tough. On the other hand males with the best survival genes and highest levels of testosterone and aggression might be good protectors when they are around but are much more likely to be off seeking other females, fail to perform their paternal duties and forget to bring dinner home.

The solution of course is simple; set up home with a good dad and get macho man to make you pregnant. Broadly speaking observations from both monogamous birds and mammals show that this female infidelity game plan is being played quite widely. Females in monogamous relationships must be playing away (the scientific term for this is extra pair copulations or EPCs!) regularly since 10-25% of all offspring produced under these circumstances were not fathered by the male partner. Furthermore playing away is not a random process but tends to occur particularly around the time of maximum fertility. The basis for this is illustrated particularly well by the work of David Perrett’s group in the University of St Andrews. They have shown that females find male faces with a higher degree of feminisation (generally associated with lower testosterone and good parental qualities) most attractive for the majority of their menstrual cycle but prefer more masculine faces at mid-cycle when they have the highest chance of conception (Penton-Voak et al. 1999)

However, as one might predict, women appear to be able to adopt different strategies dependent upon how physically attractive they are. Recent work from David Perrett’s group (Penton-Voak et al, 2003) has shown that where females consider themselves to be attractive (and are considered to be attractive by others and have a low waist to hip ratio) then they prefer more masculine faces in the context of long-term relationships. However, less attractive individuals go for more feminine faces. One pragmatic interpretation of this is that attractive women feel they have both a good chance of attracting the good genes/macho males in the first place and can keep them interested thereafter to help with some parenting. Unattractive women on the other hand go for the safer option in making sure they hook up with a more faithful partner and good dad. It is perhaps worth speculating that whereas in American and European white cultures only 20% of women report being happy with their bodies, 60% of AfroHispanic women do. Perhaps therefore, cultural pressures on perceived beauty in white cultures are selecting progressively for more feminine men?

Other, more anecdotal work has suggested that women are more prone to wear their most revealing and sexually alluring outfits during the mid-cycle period. This might of course simply be to stimulate their partner into action but arguably since such outfits are usually worn in public then this could also be viewed as a wider advertisement of their attraction.

The health cost of promiscuity

Arguably for both males and females, embarking on the numbers game for achieving optimal reproductive potential, the health risks associated with engaging sexually with multiple partners are significant. As the words of a popular song go: “What do you get when you kiss a girl? You get enough germs to catch pneumonia.” Sexually transmitted diseases are a problem for all species, not just humans, and so choosing well to avoid having to play the field too widely is important for your health.

Strategies for choosing the right partner

I am sure that most of you out there would like to feel that who you fall in love with is largely a matter of free choice and in no way pre-determined. However evolutionary pressures have put a number of constraints on free choice that can have a significant influence on limiting the field. The problem really starts with investment banking. Just as an investor seeks to obtain the highest possible return when risking their hard earned money, so both males and females are looking to promote their genes optimally in their offspring. The problem is that you can only contribute 50% of your genes because you can only reproduce with a partner of the opposite sex. It’s a bit like giving 50% of your investment as payment to the organisation making the investment on your behalf!

However, all is not completely lost since you can try to find a partner who has very similar genes to yourself – sort of the next best thing to hermaphrodotism. How can this be done? Simply put, this means that you should be most attracted to individuals who are just like you. Indeed, research into many hundreds of animal species, including humans, confirms that for the majority of both physical, mental and emotional characteristics “birds of a feather really do stick together” and “opposites do not attract”.

As modern humans we could, for example, nip out and pay to have a DNA test done on a sample of hair or saliva and work out the degree of similarity. This seems unlikely to be popular and in any event, as we will see in a minute, you don’t want everything to be similar. The easy pragmatic way is simply to go for an individual that looks just like you since the degree of physical similarity is likely to be just as accurate as any DNA test. However, while this Narcissistic approach may be a possibility for humans and possibly for some of the great apes, it is not something that can be used by most animal species since the majority lack any clear ability to recognise representations of themselves (in mirrors for example). Having said this, some research in humans, again by David Perrett’s group, has come up with the startling conclusion that we do find opposite-sex versions of our own faces highly attractive even though we don’t actually realise that they look like us!

The answer for the majority of species who do not know what they look like is to use close f ami ly members who are genetically very similar as templates. The most appropriate individuals who can normally be guaranteed to be around during your early formative years are your parents. The idea that parents have any influence on what you find sexually attractive in others generally provokes incredulity and even disgust in many of us but there is growing support for it. This concept is called “sexual imprinting” and was called this by the famous ethologist Konrad Lorenz after he discovered that in birds such as ducks and geese, patterns of courtship behaviour as well as which species of bird was considered most sexually attractive, was more a matter of who raised you than who had similar genes.

For a long while it was thought that perhaps this was a bird thing because these species also imprint on the first salient object they see after hatching in a highly stereotyped manner (i.e. they will follow them and treat them in every way like a parent – even if the first individual they see is a human!). This is not something many mammalian species tend to do when they are born.

However, in 1992 I, and a colleague from the University of Pretoria, Professor John Skinner decided to test whether sexual imprinting did occur in mammals by carrying out a large-scale study using sheep and goats. We chose these two species both because we had the ability to control their maternal behaviour and since, like humans, they form an extremely strong attachment bond with their offspring.

The hypothesis was simply that if parents really did influence the social and sexual preferences of their offspring, then if goat kids were raised by sheep mothers they should grow up to act more like sheep and find sheep more socially and sexually attractive. On the other hand, lambs raised by goats should do the reverse. From the outset we decided that the most effective test was not simply to raise the cross-fostered young in isolation, with only their foster mothers for company, but to give them access to adults and juveniles of their own species at all times. Similarly, to test for competing or additive influences of sibling bonds we also raised some animals with a sibling of their own species, and others with a sibling of the same species as their foster mother. To act as controls we had normally raised sheep and goats kept in the same social environment as the fostered ones (i.e. exposed to both species). We thus hopefully had all the angles covered and could be confident of showing how much influence the bond between mother and offspring, as opposed to other bonds, could have.

The first observable change in the animals was in terms of their juvenile behaviours. Goats play and groom themselves much more than sheep and there was a significant shift in the fostered offspring towards the phenotype of their foster species – i.e. lambs raised by nanny goats played and groomed more than usual, and kids raised by ewes played and groomed much less than usual. However, a large number of other species-specific behaviours – patterns of vocalisations, browsing shrubs, climbing etc were totally unaffected – implying strong genetic inheritance of these traits.

When the animals reached sexual maturity they were given daily choice tests between tethered male and female sheep and goats. The results were almost unbelievable in that males almost exclusively chose to socialise and mate with members of their foster-mother’s species. Importantly, however, it was not that they would never mate with their own species since they could and did so on occasion – especially if given no option. The bottom line was that when they had a choice their preference was not for their own species but they still considered members of their own species to be socially and sexually attractive. Females also showed a significant shift in preference towards the foster species, but ended up choosing equally between members of the two species. The presence of a sibling of the same species did not weaken this dramatic effect and having a sibling of the same species as the foster mother did not strengthen it. Normally raised animals in the same social environment only chose to socialise and mate with their own species in spite of having been exposed to both species from birth.

Because these species, like us, use visual cues from the face both to recognise and become attracted to each other, we also tested their attraction to photographs of faces from the two different species. This showed exactly the same level of behavioural preference, with goats raised by sheep preferring sheep faces and sheep raised by goats preferring goat ones.

We could find no evidence for either goat or sheep mothers treating their male offspring differently from females, which might have explained the sex difference in maternal impact. We also found that the differential effects on social preference were not even dependent upon the presence of testosterone during the tests. Indeed, it seems most likely that the prenatal organising influences of testosterone on the male brain are responsible for enhancing the mother’s effect on the social and sexual preferences of their sons.

Of course, were this maternal influence to be transient in these animals then its significance would be of a lesser consequence. For the next four years we therefore kept all the animals in flocks containing only members of their own species. We then re-tested their social and sexual preferences annually. For males their preference for their foster species was almost completely maintained, whereas for females they changed to preferring their own species sexually within 1-2 years although they still maintained a social interest in their foster species (Kendrick et al., 1998,2001).

While no other large scale study of this kind has been carried out on other mammals, similar observations have been made in cross-fostered macaque monkeys by Fujita in Japan where visual preference for images of members of the foster mothers’ species were seen.

In humans there have, of course, been stories for generations suggesting that men are attracted to women who look like their mothers. Indeed, there is the classic joke stating that: “men who marry women who like their mothers don’t do so the second time around”. The first study I have seen which tried to ex ami ne this in at least a quasi-scientific manner was published by H ami lton and McGowan in 1929 and came up with two main conclusions:

There was no consistent trend for men to have partners they considered to be similar to their mothers.
A much greater proportion of men who did consider their partners to be similar to their mothers expressed themselves to be happier with their marriage arrangement.


More recently there have been a number of studies by David Perrett’s group at the University of St Andrews that have shown significant correlations between parental age, eye and hair colour and that of a chosen partner (Perrett et al, 2001). For both sexes these studies have shown that if your opposite sexed parent is older then you are more likely to be attracted to older partners. The same also appears true in terms of hair and eye colour. Other studies by McClintock in the USA have shown that women prefer men that smell similar to their father.

The acid test of this idea is to see what happens with adopted children and a recent study has now found that girls do find male faces that look like their adopted fathers more attractive than those that do not. An important factor for the strength of this effect was, as you might predict, the strength of the relationships between adopted father and daughter.

So all in all when Humphrey Bogart looks into Lauren Bacall’s eyes in the film Casablanca and says “Here’s looking at you, kid”, if he had been raised by a sheep he might have had to say “Here’s looking at ewe, kid”.

The importance of being the same but also slightly different

Whilst being attracted to individuals that are the same as you may make good sense in terms of genetic investment, it invites the possibility of incest and resultant inbreeding depression. For this reason there are a number of factors that counteract this possibility. In birds at least, detailed studies on attraction in Japanese quail by Patrick Bateson in Cambridge have shown that the individuals considered most attractive are about 12.5% different genetically (i.e. first cousins).

Another important experiential variable is the impact of being reared together with your brothers and sisters. Studies involving Kibbutz children and problems with Kimpua marriages in China (where young children intended for betrothal are brought together to live) have shown quite clearly that in terms of sexual attraction, f ami liarity during early life does indeed breed subsequent contempt. This hypothesis was first proposed by Westermarck in his book on human marriage published first in 1891.

However, there are also genetic determinants of “negative imprinting” that do the very important job of making sure you are attracted to individuals who have a slightly different immune system to your own. This is the best way to provide your offspring with the strongest possible immune system. Research on rodents, such as mice, has consistently shown that animals exhibit the strongest sexual preferences for individuals who have a different immune complex (primarily the major histocompatibility complex –MHC) to their own. Indeed, such is the acuity of the sense of smell in mice that they are able to distinguish between individuals with only a single gene difference in their MHC.

What about humans? Well our sense of smell may be a little more limited but evidence suggests that we too prefer the odours of individuals with different immune complexes from our own as far as sexual attraction is concerned. These are the so called “sweaty tee-shirt” experiments where men and women are asked to rate the attractiveness of shirts worn by members of the opposite sex for 24-48h. However, one interesting twist to these findings is that women on the contraceptive pill do not show this same preference. The hormones contained in the pill mimic pregnancy, and it might normally make more sense to be attracted to close relations during this pregnancy since they are most likely to give you and your offspring support. It has been argued therefore that women normally switch their preferences away from individuals with different immune systems when they become pregnant and that is why they also seem to do the same thing while on the contraceptive pill. Whichever way you look at it this may not be particularly good news for the woman who is attracted to, and perhaps marries, an individual while on the contraceptive pill and then finds she is less attracted to him after all when she comes off the pill in order to start a f ami ly!


Gender orientation

This is a big and controversial topic and one that I have covered in more detail in a previous lecture (“Sex, hormones and animal passion: making pleasure out of necessity” – September 2002). It is now clear that both physiological and experiential factors can contribute towards the development of homosexual preferences. For many years it was considered to be some kind of human specific unnatural deviation whereas it is now the case that it has been shown to occur in a number of other species (see Bruce Bagemihl (1999) “Biological Exhuberance), the best documented of which is sheep. Bonobos are of course a completely special species where anyone can do it with any member of any sex and at any age, but this is not quite the same thing. There are even structural differences in the brains of male human and sheep homosexuals where the general observation is that the neural substrates are more like those of females rather than males.

In any event it is not my purpose here to consider in detail what may contribute to a homosexual phenotype but merely to point out that the majority of what I am now going to say about what attracts us sexually to other individuals, and even to bond with them, applies pretty much equally whether one is considering heterosexual or homosexual relationships. The brains of all species have developed an exceptional ability to discriminate the gender of other individuals and have clearly been organised in such a way that a sexual response will normally only be elicited by members of one sex rather than another. However, both male and female sexual brain circuitry exist in both sexes although organisational effects of sex hormones during early development and puberty usually result in one being very active and the other less so. Similarly, which of the social cues from other individuals eventually predominate in gaining access to drive these sexual control centres will be influenced both by experience and by the relative balance present between the dominance of either the male or female sexual response centres in the brain.


What specific physical features are the biggest turn on?

We all know that each of us have rather different views as to what attracts us towards others and what does not. To a large extent this is not a rationale process but what can most easily be described as a kind of biological reaction which our thought processes can control or accentuate but cannot by themselves easily generate.

Just as we normally form opinions about the suitability of an individual for employment within the first 30 seconds or so of an interview, we generally do much the same thing in deciding whether we find a person we have just met attractive or not. This is pretty much the philosophy of speed dating. If you don’t feel attracted to someone within a couple of minutes then you probably never will. The only immediate qualification to this statement is that if you spend the whole of your first few minutes of meeting someone new thinking about someone else, then this immediately invalidates the principle. Selective attention is a very powerful attribute and means we can effectively shut out taking on board some or all of the key features of another individual which we might find attractive. This is why psychologists looking for preferences across a wide set of different stimuli in a particular individual generally make them perform distraction tasks for a period between each test (usually something really taxing mentally such as counting backwards in 7s). I am not aware that speed dating agencies use this principle!

A quick look at other species will show quickly that in general size, age and physical health are very important. Big, older males in their physical prime, and who know how to be attentive at the right moments do it for females, whereas younger females with key offspring-bearing qualities, good health and a willing eye do it for males. However what attracts individuals to one another in different animal species can be quite varied:

The scent of a woman

There is little doubt that smell is a highly evocative sense and many other mammals rely strongly on this to detect what is happening around them. There are two smell systems in the brain, the vomeronasal, or accessory olfactory system which has evolved to detect liquid borne pheromones and the main olfactory system which detects airborne odours. In the context of sexual attraction considerable attention has focussed on the more primitive accessory system since this bypasses all of the brain’s thinking processes and directs its information exclusively towards the regions that control sex and aggression and hormonal regulation. Male hamsters, for example, have an absolute requirement for this system to be functioning in order to be attracted to and mate with a female hamster. Male mice require this system to be able to distinguish between males and females and female pigs require it to be turned on by boars.

In pigs, at least, the actual pheromone has been isolated and is called “androstenedione”. For pigs this is bottled up and sold under the name of “Boar Mate” and works pretty well in the sex attraction department. It is also claimed that this works well for humans even though this pheromone detection system is somewhat vestigial in humans and no longer seems to have receptor genes that are actually capable of functioning. So perhaps for the time being it may be better to assume chaps that if you spray on androstenedione you might not necessarily be a hit with the ladies but things could be somewhat different if you decide to visit a local pig farm!

Indeed, there is plentiful evidence that heady sexual aromas are highly species specific. The smells from male elephants in musth or male ungulates such as goats in the rut, seem anything but attractive as far as we are concerned, but for the females of these species the smellier and secretion covered a male is, the more they appear to go for him! So in short what does it for an elephant or a goat may well do quite the opposite for us!

Although some sexual arousal effects may also be transmitted through the main olfactory system this is unlikely to represent a “love at first smell” phenomenon since this is only one source of information that our senses receive from a prospective mate and smell is not usually our most dominant sense. Even the intriguing sweaty T-shirt studies only reveal a very small increased preference for smells from individuals with a different MHC complex!

Love at first taste

While we don’t generally go around tasting each other prior to deciding whether there is sexual attraction or not, there is a lot of taste as well as tactile stimulation going on when we kiss one another, particularly on the lips. With other animal species there is often much more oral contact between individuals either through social investigation or grooming behaviours. Like smell it is unlikely for humans that this is the single cue that drives attraction but such oral investigation behaviours must have evolved for a reason. Kissing is a particularly efficient way for two individuals to be able to sample the chemistry of each other’s body. Kissing can also give information on health and hormonal status and even more prosaic stuff such as the quality of food an individual is managing to acquire! Not a particularly romantic interpretation I will admit, but coming up with another satisfactory explanation for how kissing evolved is difficult from the perspective of an evolutionary biologist.

Love at first touch

Touch is certainly a very good sense for communicating romantic intentions to others and we, together with many other species, use it quite extensively and often with great subtlety. The feel of another individual’s skin can also tell you a large amount about their health and even hormonal status and state of arousal. Memories of caresses received can be extremely poignant. However, once again for most of us this information is unlikely to be the key trigger for attraction although for an animal like the star-nosed mole that has phenomenal touch sensitivity compared with its other senses then it could well be a different matter.

Love at first sound

Certainly we and many other species find voices attractive and in humans deep, husky voices are considered sexually attractive by both sexes and these correlate with high testosterone levels and therefore potential high sex drive and good genes. With many bird species a large component of courtship revolves around the complexity of an individuals song and how good they are at singing it although with most modern human cultures you are more likely to be the recipient of a professional singer’s CD recording rather than being exposed to any direct attempt at courtship by personal serenade! True romance established purely by long-distance phone class is also something of a rarity and, rather like internet romance, can fall apart when the most important sense finally comes into play – sight.

Love at first sight

For species with good eyesight, visual displays seem to play a very important role in triggering sexual attraction and hence all forms of visual adornment are used to help attract members of the opposite sex. It appears that human males are particularly influenced by visual cues from females and of the two sexes, tend to fall in love the quickest

Esoteric and surprising visual cues can sometimes be important for some species. Whereas the story goes for humans that gentlemen prefer blondes, lionesses, for example, prefer their males to be deep brunettes. Why you might think in either case? Leaving aside for a moment recent findings that men find intellectual women less attractive with contemporary “blonde joke” humour suggesting that fewer blondes fall into this category, why are lionesses so observant of hair colour? Well, here we are on much safer ground since male lions who have dark brown manes have higher testosterone levels (i.e. more macho and with good genes) than ones with light brown manes. There is a problem however with selection for such secondary characteristics in this case. On the African plains dark brown manes absorb more heat, making their owners body-heat rise which then depresses their sperm count.

Many species also advertise their reproductive status through engorgement and colour of sex skin and these are highly visual, eye-catching displays. However, for humans and many other primates, faces are the single most important feature for attraction and when we meet individuals, a large amount of our visual attention is focussed on their face particularly the eyes and mouth. Humans and apes also have a wide variety of facial expressions that they use to communicate their emotional state and smiles, in particular, are highly attractive. This is not to deny that there is also significant attention paid to other more reproductively oriented body regions such as the breasts and genitals.

Even sheep who, like us, have excellent face recognition skills, will exhibit consistent preferences for particular sexual partners based purely on choices between face pictures. There is also the touching example of a female gorilla, called Koko in the USA who has been taught to use American sign language by Dr Penny Petterson and who has been filmed making a choice between potential mates through observing a video of them. With her language skills she leaves you in no doubt whatsoever as to who she wants and who she does not - “bring, you bring – Lip (girl) heart throb”.

As far as humans are concerned we are all aware of changes in cultural norms for visual aspects of beauty over the centuries but gross features such as the waist-hip ratio and proportional body mass and symmetry all appear to be general norms. Symmetry of body features in particular has been shown to be important in many species and is proposed to be a general index of good genes since the greater degree of asymmetry in the body, the more likely you are to be carrying mutant genes that will impact negatively on your level of fitness (in the Darwinian sense) and will be passed on to your offspring. In males, possession of good symmetrical features is also more associated with high testosterone levels, further reinforcing this idea of their being representative of “good genes”. Another example of this importance of symmetry in humans is that we generally find averaged faces highly attractive and these, through the process of averaging, are the most symmetrical.

The concept of divine proportion

Although Einstein did not trouble himself to determine whether there were universal principles associated with the perception of beauty, this has not deterred others from trying to do so. It has been argued, for example, that whether we are talking about buildings, musical instruments, body parts or faces, everything is based on the simple fact that adjacent features are in the proportion of 1 to 1.61803398874989… (otherwise known as Phi Ф ). If true it certainly might be considered another example of relativity! It does seem, for example, that if the length of the hand is 1 then the length of the forearm is 1.618 times longer. The same is true even for the width of your teeth and the shape of your mouth when you smile. For your face, if the distance from the corner of your mouth to the bottom of your chin is one then the distance from the corner of your mouth to the corner of your eye is 1.618. A maxillo-facial surgeon Dr Stephen Marquardt has used this principle to develop a beauty mask overlay which fits like Cinderella’s slipper uncannily well onto the faces of both women and men from many different cultures and ages considered to be the most beautiful.

Love your personality

It is likely that if you go to a dating agency you will end up filling up a number of personality questionnaires. While it is true that, as with your appearance, you are more likely to be attracted to someone who has similar personality traits and interests as yourself, these are unlikely to stimulate your first feelings of attraction. However, this is something that females are likely to take more account of than males since, as you would expect from my previous reproduction investment arguments, females need to be more choosey in picking a mate than their male counterparts do.

What if you have not got all the key attraction qualities?

I have already discussed changes in female preferences towards lesser genetically advantaged males if they themselves feel lacking in attraction qualities and this seems a reasonable compromise. However, if you are determined to get the best then one important rule is that if you have not got the key attraction attributes then hang out with a member of your sex who does have and you may benefit from confusion, frustration or even his or her generosity. This is a strategy that can be seen in many species which live in social groups.

Ways to improve chances of mutual attraction

It goes without saying that possession of a technique for making others romantically attracted to you, no matter what your level of compatibility on other measures might be, could make you both very popular and exceedingly rich. There are of course all kinds of things that can be done to make you look and even smell good but we all know that these do not provide any kind of reliable guarantee. Sharing good and pleasant experiences together might also seem to be a reasonable candidate for success, but in fact research shows that it is shared experiences of fear and terror that are more likely to result in mutual attraction. This was first shown in a classic study by Dutton and Aron in 1974. They had an attractive woman ask for interviews of young men both on a swaying rope bridge, 200 ft above a river, and also on terra firma. Mid-way through the interview, she gives them her phone number. Over 60 % from the rope bridge called her back, versus 30% from terra firma.

A more recent study has replicated this effect of increased attraction following a first meeting in a high anxiety state with couples riding a roller coaster (Meston and Frolich, 2003). The authors entitled their paper “Love at first fright”! Various arguments have been put forward to explain this finding. On the one hand it could simply be confusion that the high levels of arousal elicited were caused by attraction rather than fear. However, studies on other animals have also implicated stress hormones in the processes of both attraction and bonding in monogamous species and so perhaps the secret of success on first dates is to share some frightening experience. This might of course backfire if one partner blames the other for putting them in the frightening situation against their will. In this case a second date might not be forthcoming.

The other way to boost the likelihood of attraction is to stare deep into one another’s eyes for several minutes. This is perhaps more easy to arrange in the context of a controlled psychological experiment than in real life since this is not normally something we do with other individuals until after we have fallen in love with them. However, apparently when strangers are forced to do this in the cause of science, they report strong levels of attraction for one another after it. This might of course be another example of shared stress although it is well known that the eyes are a primary focus for attraction. The use of belladonna by women to widen their pupils and make their eyes seem bigger is a good example of how this can be used to great effect!

Other things that tend to increase attraction are well known factors such as novelty. F ami liarity it seems always breeds a measure of contempt. Most of us find limited amounts of novelty extremely stimulating no matter in what context but as far as sexual partners are concerned the phenomenon of novelty-induced enhancement of sexual arousal is called after an American President, no less – the “Coolidge” effect. The story goes that President Coolidge and the first lady were visiting a chicken farm and at one stage the first lady was shown a rooster being extremely attentive sexually towards some hens. The first lady politely enquired whether the rooster kept this up for a long period and received the reply that “yes, he did”. The first lady then directed the farm hand to inform the President of this when he came around. The farm hand duly did this when President Coolidge finally arrived. The President turned to him and simply asked whether the hens given to the rooster were changed from time to time and was told “yes, they were”. He turned to the farm hand and asked him to convey this fact to the first lady!


Attraction and the brain

We know a reasonable amount about the pathways in the brain that regulate male and female sexual responses and the effects that sex hormones have on them to promote the sex drive. Primarily it is the older subcortical structures in the brain that are involved - at its base in the hypothalamus and in other regions controlling emotional behaviour, called the limbic system. These structures also link up with regions of the brain which control feelings of pleasure, often termed reward centres and which utilise a particular brain transmitter called dop ami ne.

There is relatively little work establishing in detail just how specific sensory cues trigger a sexual response although some of the work we have done with sheep is illustrative of what it may be like for species that do not bond with one another and where females undergo dramatic changes in their sexual interest in males as a function of their ovarian cycles. Sheep, like us, are attracted to faces and when a sexually receptive female sheep sees the face of a particular male she fancies this causes increased activity in both the brain regions controlling attention and face recognition and also in those controlling emotional and sexual responses and reward. However, when she sees the same male a few days later but is no longer interested in sex, then his face has a fairly minimal impact and just activates the face recognition areas but none of the regions controlling emotional and sexual responses.

Similarly we know that when a receptive female sees the face of a particular male that she fancies then this releases the key transmitters in the brain that make you feel pleasure: dop ami ne; which direct your attention towards particular objects: noradrenaline; and make you feel aroused and compelled to perform some kind of action: serotonin. Humans in love also seem to have altered dop ami nergic, noradrenergic and serotonergic activity.

A number of recent brain imaging studies carried out by Andreas Bartels and Semir Zeki at University College London and by Helen Fisher, Lucy Brown and Art Aron in New York, have found that when individuals view pictures of someone they are intensely in love with, this particularly activates dop ami nergic centres in the brain that promote pleasureable feelings. This is a very similar activity profile to one you would see in people looking at something they are addicted to, leading to a possible conclusion that love can be considered to be a form of addiction. However, pretty much anything you desire at any time activates brain pleasure centres and not all of these are considered addictions although in some cases they can become so!

Over time it seems that some cortical areas of the brain also start to become recruited in individuals that have been in love for a year or so, suggesting a shift perhaps towards a less intensive and exclusive emotional experience. Some of the studies have also shown that viewing individuals you love whether in a romantic or parental context, actually suppresses activity in parts of the frontal cortex that are important for making moral judgements. This finding has been loosely interpreted as providing an explanation as to why someone in love with another individual is incapable of considering them capable of doing any wrong.

The suppression of activity in the frontal cortex is also indicative of potentially reduced serotonin activity in this region, an event which is significantly correlated with anxiety and depression disorders, in particular obsessive compulsive disorder. Indeed, an argument has been made that the brain in love has many similarities to the brains of individuals with obsessive compulsive disorder. Such individuals feel compelled to make large numbers of repetitive actions, such as hand washing because they feel dirty, and this has some parallels with individuals in love who become obsessed with their need for close proximity to their desired lover. Similarly the patterns of activity seen in the dop ami nergic reward centres are very similar to those seen in people suffering from drug addiction and so it is easy to see how human love can be interpreted as something akin to a clinical obsession/addiction syndrome. Indeed, there is a clear parallel in terms of feelings of craving, although an important distinction is that drug addiction is associated with increased tolerance to the effects of a drug leading to greater and greater amounts of it needing to be consumed in order to obtain the same effect. For the majority of individuals there is no clear indication that addiction to love is such that you end up needing more and more love experiences to get the same degree of fix. If this were the case then we would be in distinct trouble.

It is probably easier to view love and attraction as a biological drive, like hunger and thirst, which is impossible to ignore, gets stronger and stronger the longer the need goes unresolved and which gives intense pleasure when satisfied, followed by a period when there is less interest in satisfying it again. This is slightly different to an addiction or even an obsession although there are many shared elements.


Differences between males and female brains in love

So far nothing major has been shown to distinguish the brains of human males and females in love, although I will discuss some potential differences in relation to bonding later. Helen Fisher’s group in New York has found that parts of the female brain that are associated with memory are more strongly activated and has suggested that this might reflect the females’ greater need for assimilating all the quality control information about a prospective mate more thoroughly than their male counterparts. However at this stage it is probably safest to conclude that there are not any major fundamental differences.

The road from romance to bonding

The links between attraction and bonding have mainly been revealed in a small number of monogamous rodents, most notably the Prairie vole. Prairie voles are a highly social species and when individuals are attracted to one another and mate, they form exclusive long-term bonds and these are strengthened progressively the more they mate with each other. The link between mating and bonding however is not absolutely rigid since both partners play away on occasion but do not appear to bond with the individual they have casual sex with. Possibly this is simply a matter of a differential intensity of bonding with the f ami liar partner as opposed to the casual one but it may also reflect the fact that attraction and bonding can be separated, especially when a bond is already in place.

For female voles we know that the most important stimulator of bonding is a small peptide hormone called oxytocin. When oxytocin is released during sex it does three main things. In the first place it helps the female learn to recognise the smell (and perhaps cues from other sensory modalities) of the individual male she had sex with. In the second place it stimulates parts of the brain dealing with reward so that recognition of the male is associated with a pleasurable experience. Lastly the peptide also promotes a form of social anxiety which stimulates the individual to obtain more social contact.

Whilst the cells and pathways that make up the brain oxytocin system are very similar across mammals, including man, and mainly comprise those parts of the limbic system and hypothalamus dealing with social behaviours and recognition, sex, emotion and stress, there are differences in the distribution patterns of its receptor. Only one oxytocin receptor has so far been identified and a distinguishing feature of its distribution in monogamous mammals is that it is more highly expressed in the nucleus accumbens; a region of the brain particularly associated with reward. It has thus been hypothesised that for females, pair-bonds with males may particularly be reinforced by this link between sex-induced oxytocin release and the stimulation of pleasure centres in the brain. Oxytocin acts to produce this pleasurable aspect of bonding through altering dop ami ne release in the nucleus accumbens which acts on D2 receptors. Thus giving drugs that stimulate D2 receptors will also cause pair bond formation. Oxytocin can also promote the release of endogenous opiate systems in the brain (endorphins) which cause feelings of euphoria.

What about human females? We know both that oxytocin is released in women following orgasm and that the areas of the brain containing oxytocin receptors in humans are active when we see pictures of an individual we are in love with. This would therefore suggest that similar mechanisms are at play in women as in the female prairie voles. While we don’t know if female voles experience something akin to sexual orgasm, it would seem that a high frequency of pleasurable sex should act to strengthen female bonds with their male partners and so impatient males and females who make a habit of faking orgasm should perhaps take note that this could be a recipe for break-up! This would also seem to be a strong advertisement for the likely utility of the “orgasmatron” for helping to form or maintain relationships!

With males the situation is slightly different. Although they also have oxytocin pathways and receptors these play less of a role in pair-bonding. Instead, a closely related peptide called vasopressin is more important. As with oxytocin in the female, the distribution of this peptide within the brain is very similar in different mammals. It is also very similar to that of oxytocin.

However, it is once again the pattern of one of vasopressin’s receptors – the V1a receptor – that is altered in monogamous mammals. Thus in monogamous marmosets, prairie and pine voles and the Californian mouse this receptor is more highly expressed in another region of the brain associated with reward – the ventral pallidum. As with oxytocin in the female, brain infusions of vasopressin can only increase the formation of a partner preference and general social behaviour, in species that have this pattern of receptor localisation. Indeed, by increasing the level of expression in this region in a monogamous vole they can also bond with a partner without sex.

An important discovery has been that this altered pattern of V1a receptors is directed by an additional sequence inserted into the gene for this receptor. When Larry Young and Tom Insel at Emory University created transgenic mice with this version of the vole gene the animals developed a monogamous pattern of V1a receptor distribution in the brain. While they did not become monogamous they did start to show increased social behaviours in response to brain infusions of vasopressin. More recent experiments have shown that introducing the monogamous version of the gene just into the ventral pallidum area can convert a non-monogamous species of vole into a monogamous one. So a mutation in a single gene can, it seems, convert a philanderer into a constant partner – at least as far as voles are concerned!

Vasopressin also seems to act by modulating dop ami ne release but another thing it appears to do is to reduce serotonin concentrations. In males, one consequence of this can be increased aggression but it may also induce obsessive attraction towards a member of the opposite sex in the same way it is claimed to do in humans. In all likelihood oxytocin may have a similar influence on brain serotonin levels.

All this should of course mean that it may be difficult to fall in love and bond with someone if you take drugs that keep serotonin levels in the brain high. Certainly one of the most common side effects of serotonin selective reuptake inhibitors such as Prozac, is a reduction in sex drive.

One thing that is clear for monogamous species is that there is a potential additive level of pleasure which can be derived from sexual partnerships since both sexual attraction and sex itself may be pleasurable for all species but for those that bond there is an additional boost to the system caused by the bonding chemistry. So while short-term attraction and sex can be fun, longer term attraction, sex and bonding can be even better.

An interesting further observation is that stress activates both oxytocin and vasopressin systems in the brain and this may provide a mechanism whereby shared experiences of fear and anxiety can promote attraction.

With humans, voles and other monogamous species, loss of a partner can result in the surviving partner never being able to bond again. It seems likely that one thing the bonding hormones like oxytocin and vasopressin do is to re-organise the link between sensory systems, sexual arousal control centres and pleasure centres in the brain such that no other potential partner is able to fully activate the whole system optimally again. So while the development of bonding may enhance the pleasure and duration of specific sexual relationships it may also restrict the possibility of having pleasure through a relationship with anyone else.

But is formation and maintenance of a romantic bond really dependent upon having sex? The answer to this as far as females are concerned is almost certainly no since oxytocin also promotes long-term attachment bonds with their babies. While such bonding is also triggered by vaginal and cervical stimulation through giving birth, it is maintained simply by physical contact thereafter and even the sight of a lamb for a mother sheep can provoke oxytocin release in the mothers brain (and probably in the lambs brain as well). Similarly when we humans see pictures of our loved one this particularly activates parts of the brain that contain oxytocin and again in sheep when they see faces of an attractive male this can provoke brain oxytocin release independent of sex. Such a release of oxytocin should also promote pleasure through activating dop ami nergic reward centres. So while having sex and achieving orgasm may be important ways of strengthening romantic bonds they are clearly not essential for them and just seeing and being with another individual can both cause pleasure and strengthen romantic bonds by themselves.

In all likelihood the same is probably true of males but we don’t yet have the experimental evidence. What would seem to be highly likely however is that the long term link between attraction and bonding is much more widely developed in female mammals than in males because it occurs in the females of many species whereas it is relatively absent in their male counterparts. Indeed this may explain why males seem to have highjacked a different hormone for promoting bonding with both females and their offspring which actually normally plays a role in promoting aggression. So in some sense when females bond romantically with males they are treating them rather like their babies whereas when males bond with females they are treating them more like male sparring partners!


Bonding and unrequited love

We really understand relatively little about the impact of being spurned by a desired lover other than the simple fact that in all species it provokes behaviour associated with aggression, frustration or depression. Helen Fisher’s group in New York have carried out a brain imaging study non humans in the throws of unrequited love but the results have yet to be analysed. My own thoughts on this are that it may only really be species that are capable of bonding romantically which suffer the true negative consequences of Cupid’s misdirected arrow. While most regard love and attraction as distinct from bonding the reality is that they occur simultaneously. Oxytocin, and probably vasopressin, should be released when you think about or see someone you are attracted to and want to be with on a permanent basis even if the relationship never develops fully. Under these circumstances perhaps the impact on dop ami nergic reward centres diminishes as the relationship fails to develop and instead all you get is a steady increase in social anxiety, obsessive behaviour and cravings similar to a form of addiction. The experience of these can become highly painful and distressing. The net result of such pain should of course be to drive individuals even harder to seek fulfilment of their drive elsewhere although the intensity of the compulsion experienced towards a specific individual can be very hard to break. These are only speculations at this stage but it is perhaps the link between attraction and bonding in humans and some other species that results in the complex psychological, emotional and physiological experience that we call “love” Without the capacity for bonding this experience would probably only be by comparison, a pale imitation.


So are we really so different from other animals?

From all that I have said in terms of what science has managed to uncover to date, human love does not seem to be qualitatively different from the sexual attraction and bonding that can be seen in some other animal species although clearly our enhanced capacities for voluntary control of conscious thought, self awareness and empathy can considerably accentuate and prolong our feelings of romantic attraction towards others.

The integral link between attraction and bonding would also seem to be of major importance with the pleasures of reciprocated sexual attraction and bonding being at least additive and more potent and enduring than sexual attraction alone. By the same token all these advantages leave us extremely vulnerable to the pain of failure in obtaining such relationships. To this end the main purpose of love might simply be viewed as the establishment of long term attachment bonds which provide continuous sources of pleasure with or without sex and which facilitate the raising of offspring needing long periods of care. The natural conclusion from this is that attraction and sex without love are indeed simply a distraction from the main event, since the main event is not primarily about sex.


Some final conclusions:

Love is……..
Similar to sexual attraction and bonding in other monogamous mammals
Intensified by voluntary control of conscious thought, self awareness and empathy
An obsessive drive that has the potential to become an addiction
A potential economic disaster in the work place
Seeing only the best in someone else
Finding someone who looks like Mum or Dad!
Less about personality, kissing, smells, songs and touching hands … or even being single, sexy and promiscuous
More about sight, symmetry and Phi
Frightening him/her to death while looking into their eyes
Making sure he/she is wearing their bonding genes in the right place
Remembering his good points when he can’t remember yours.
Knowing that attraction and sex without it are a pale imitation


© Professor Keith Kendrick, Gresham College, 14 February 2005



Selected references:

Bagemihl B (1999) Biological Exhuberance: Animal homosexuality and natural diversity, St Martins Press, New York

Dixson, AF (1998) Primate sexuality. Oxford University Press, Oxford.

Helen Fisher (2004) Why we love. Henry Holt, New York (available from Amazon)

Insel TR and Young LJ (2001) The neurobiology of attachment. Nature Neuroscience Reviews 2: 1-8.

Isles AR and Wilkinson LS (2000) Imprinted genes, cognition and behaviour. Trends in Cognitive Sciences 4:309-318.

Kendrick KM et al., (1998) Mothers determine sexual preferences. Nature 395:229-230.

Kendrick, KM (2000) Oxytocin, motherhood and bonding Experimental Physiology 85S 111S-124S.

Penton-Voak IS et al. (1999) Female preferences for male faces change cyclically. Nature 399: 741-742.

Penton-Voak et al (2003) Female condition influences preferences for sexual dimorphism in faces of male humans (Homo sapiens). Journal of Comparative Psychology 117:264-271.

Perrett DI et al., (2001) Facial attractiveness judgements reflect learning of parental age characteristics. Proceedings of the Royal Society London B 269:873-880.



This event was on Mon, 14 Feb 2005

professor keith kendrick

Professor Keith Kendrick

Professor of Physic

Professor Keith Kendrick is Systems and Behavioural Neuroscientist and was Gresham Professor of Physic between 2002 and 2006.

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