By Anthony Little and David Perrett

What makes a face beautiful? What makes people seek out and desire to mate with the owners of beautiful faces? In recent years, scientists have turned to the theory of evolution to help us understand why some faces are judged to be more attractive than others.

According to the evolutionary view, the attractiveness of individuals is directly linked to their value as mates. A "high-value" mate is someone who best enhances your reproductive success. Going back into the evolutionary past of the human race, someone who noticed the cues to the value of a potential partner, and intentionally selected a high-value mate, would leave behind more children. These children would tend to inherit genes for attentiveness.

Attention to attractiveness is thus part of our evolutionary design.

This scientific analysis is reflected in the fact that our magazines and television screens are filled with attractive people. It's obvious that both women and men are highly concerned with good looks in a partner.

The same is true across the animal kingdom. A diverse range of species relies on external factors to attract mates, such as the size, shape, and colour of their feathers, fur and antlers. Why has evolution accentuated these particular characteristics? A variety of mechanisms may be responsible. The most obvious is that attractiveness is associated with the quality of an individual's genes.

The testosterone link

One link between "good genes" and attraction could be the masculine face-shape.

We can all spot the difference between the faces of grown men and women, and researchers have identified the reasons why. Boys and girls have similar face shapes. At puberty, hormones act on their faces to masculinise or feminise them and produce distinctive features of mature men and women. Testosterone provokes the growth of certain facial features - such as the jaw and cheekbones - so boys' faces grow more than girls'. Female faces remain relatively childlike. High levels of oestrogen in growing girls prevents the growth of facial bone, and leads to increased thickness of lips and fat deposition in the cheek area.

Theoretically, men with more 'masculine' faces should be more attractive, because there are evolutionary costs involved in developing such characteristics. The reason comes back to the testosterone levels associated with the very male face-shape. Testosterone decreases the effectiveness of the body's immune system, and so only healthy individuals with high quality immune systems can afford to produce the hormones required to produce masculine characteristics.

The quality of our immune systems is linked to our genes. So an attractive masculine face should reflect the attractiveness of underlying good immunity genes.

Macho-face and baby-face

Faces with masculine features - such as a large jaw and prominent cheekbones - appear dominant, and dominance is associated with male reproductive success in many species, including humans.

For example, surveys show that male teenagers with dominant facial features report sexual intercourse at an earlier age than less dominant looking adolescents. In some situations facial dominance can predict career success. Researchers have found, for instance, that the facial dominance of graduates from a military academy predicts their final rank at the end of their careers. Increasing testosterone also has negative effects. Married men with high testosterone are more likely to suffer troubled relationships and to have extramarital affairs.

At the other extreme are feminine or 'baby-like' faces, characterised by smaller chins, high eyebrows and larger eyes. Both men and women with baby-like faces are seen as being warmer, more honest, and more sincere - but also more naïve and less physically strong. In simulated court trials, baby-faced individuals are less likely to be found guilty of charges involving intentional criminal behaviour. They are also given lighter sentences. Both results reflect the effects of attributed naïvety and honesty.

These stereotypes appear to reflect reality. Researchers have found that the more baby-faced a man looks, the more he perceives himself as approachable and warm, and the lower he ranks in terms of aggression. In other tests, people whose faces were rated as being 'less honest' were more likely to volunteer for experiments that involved them in deception than people who were judged to look more honest.

Reading a face

Our research here at the University of St Andrews has demonstrated that people both in the UK and Japan prefer a feminised male face-shape to a masculinised one. This finding probably reflects the fact that people read different personalities into subjects' face shapes.

They tend to associate feminised male faces with positive traits such as honesty, warmth, co-operation and skill as a parent. Conversely, traits such as dominance are associated with masculinised face shapes.

The personality of a potential partner is an important factor in reproductive success, so it's not surprising that the personality people 'read' from a face influences their perception of how attractive that face is. Femininity in male faces may be attractive because it is associated with positive personality traits.

But the matter is not as simple as that. Other studies have shown that people find masculinity and dominance in male faces to be attractive. The contradictory findings may reflect the costs and benefits of masculine and feminine faced males. A high status/testosterone partner may offer good immunity genes but such a partner may possess negative personality traits and be more likely to desert the female who chooses him.

A resolution to this conflict could be that very attractive male faces possess a combination of both masculine and feminine features. The most attractive faces could indicate both a dominant and a co-operative partner.

Manly men and short-term relationships

We all know - often to our cost - that relationships are very variable in duration: some last for a long period time, while others are over in a single night. How long a relationship lasts is dependent on the choices of both the individuals involved in it - and that may be reflected in face shapes.

Masculine-faced males are thought to make poor parents but have high quality genes, so they may make bad long-term partners but be attractive in the short-term. Conversely, feminine-faced males are seen as better parents and more co-operative, and so we might expect that they would make attractive long-term partners.

We recently tested this idea with an experiment conducted on British television, which involved over 18,000 participants. We presented images of a masculine-faced male and a feminine-faced male, along with two dating adverts. The viewers had to link each male with one of the adverts. The only difference in the adverts was the potential length of the relationship sought: short-term or long-term.

We found that people associated the masculine-faced male with this advert:
Attractive, young (single) professional, back in town for short period, likes pubbing, clubbing, being funny and plenty of sports, would like to meet someone for fun and laughter.

And they associated the feminine-faced male with this advert:
Attractive, sporty, young, single, male with good sense of humour, professional job, looking to settle, into pubs, clubs etc, seeks someone fun for love and to cherish forever.

The results show that people think men with masculine faces are more likely to pursue short-term relationships, while more feminine-faced men are thought to be interested in long-term relationships. Why? Possibly people think that masculine-faced males are likely to invest less in relationships than feminine-faced men. Or women may select more masculine-faced males for short-term relationships.

Time of the month

Other research at St Andrews suggests a reason why females may show preferences for masculinity in some studies and femininity in others: they employ a mixed mating strategy, which varies with their menstrual cycle.

According to the St Andrews results, women prefer more masculine faces at peak fertility in their cycle - when they are most likely to become pregnant. This implies that women are more attentive to good immunity genes when they are most likely to conceive, and at this time they are less interested in the long-term potential investment from feminine-faced males.

Work on the timing of affairs has shown that flings or affairs tend to coincide with a woman's peak fertility. Putting together the two findings - masculine male faces are more attractive at peak fertility when affairs are most likely and they are also associated with short-term relationships - suggests that women may be making the best of both worlds. They take a more feminine male partner for long-term investment, while occasionally having affairs or short-term relationships with masculine males to provide good immunity genes for some of their children.

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