Beauty may, as they say, be in the eye of the beholder, but what exactly is it that the eye sees that qualifies the beholdee as beautiful? Scientists - not to mention the lovelorn - would love to find out exactly what it is that makes one person more attractive than another, but while each individual's picture of the perfect mate differs, there are certain preferences that appear universal. One of these is symmetry. People whose faces and features are more symmetrical - think Denzel Washington or Cate Blanchett - generally rate as more appealing than those with imbalanced, disproportionate features. Among young and old from North America, Europe, Asia and Africa, studies have shown there is something about symmetry that makes a person more attractive.
The reason, according to Ian Stephen, a researcher at the School of Psychology in the University of St Andrews, Scotland, is that symmetry is linked to healthy genes and people who show a preference for symmetrical partners enhance their own chances of reproductive success. But why should symmetry be linked to such success? Mr Stephen says that as an organism grows it must deal with parasites or illnesses that interfere with its development. If an individual has fewer bouts of sickness, or has the genetic ability to develop normally despite falling ill, he or she ends up more symmetrical.
"How symmetrical you are as a young adult is found to correlate with how many illnesses you had as a child," he says. "If you get a severe illness, your body has to divert resources to fighting off the illness and doesn't have the resources to develop properly. It's the cumulative effect of all the illnesses that causes the asymmetry." According to Dr Ben Jones, a senior lecturer in the School of Psychology at Aberdeen University in Scotland, women's preference for symmetrical men is greatest during ovulation, suggesting the quality is strongly linked to reproductive fitness. It is also stronger in women who perceive themselves to be attractive.
"It might be that women who are very attractive feel able to compete for the higher-quality partners," he says. Just as symmetrical features are attractive, so too is having features that appear "average" - neither too big, or too small - as this is also suggestive of good genes. Unusual features, says Mr Stephen, can also result from inbreeding, which is linked to poorer life expectancy and fertility.
"Aristocratic European families were very inbred and sometimes had very distinctive features. You could look at the person and instantly say they were a member of that family," he says. "One example was a really, really small chin. It was a mutation in this family. Less inbred people will have more average features." But there is inbreeding - and inbreeding: when people who are distantly related have children, they tend to be healthier because, for any given gene, it is less likely they will inherit the same unhealthy gene from both parents.
The children might inherit a harmful gene from one parent, but its damaging effect will be masked by the normal form from the other parent. These individuals with different forms of a given gene are called heterozygotes. On the other hand, if the parents are closely related, their children will more often inherit the same form of any one gene from each parent - a state of affairs known as being homozygous - and among these will be harmful pairs that reduce life expectancy and reproductive success.
Heterozygote advantage explains why people of mixed race are often considered attractive. Research published in 2006 found that both white and Japanese people rated Eurasian faces as more attractive than pure examples of either race. Mixed-race individuals are likely to be heterozygous for many of their genes and, therefore, potentially healthier. This makes them good prospects as potential mates, so people who find them attractive will enjoy better reproductive success.
While symmetry, averageness of features and having mixed antecedents are traits favoured worldwide, other rules of attraction are rooted in the customs or conditions of particular societies. The degree to which women are attracted to men with masculine or feminine features is an example. European women, says Dr Jones, are keener on slightly more feminine men than are their counterparts in some other parts of the world. The reason, scientists believe, is that in European societies the male plays a significant role in bringing up children.
Feminine-looking men are seen as being more likely to play a full role in parenting than their macho counterparts. "When we test European women, they show a marked preference for feminine men, which is really quite surprising, given that in nearly every other species, the females show preference for masculine males," says Dr Jones. "When we go to some other cultures where there's less biparental care, for example in rural Jamaica, the women show preferences for masculine men. When a woman is less concerned with a man's willingness to invest in the relationship, you see a strong preference for masculinity."
Another culture-specific preference concerns steatopygia, the accumulation of fat around the buttocks, seen particularly among the Khoisan people in southern Africa. Larger fat deposits are linked to higher levels of the female hormone oestrogen, which is associated with higher fertility levels, so a woman with slightly larger buttocks will probably be a better bet for reproductive purposes. In the case of the Khoisan, the preference among men for women with larger buttocks has gone way beyond this, due to a phenomenon called "runaway selection".
"Once a preference exists, that preference will get passed on and the trait will get passed on in the population," says Mr Stephen. Runaway, or Fisherian selection, named after the biologist RA Fisher, who defined it in 1915, also explains the extraordinary tail plumage of male peacocks - this is nature's way of having too much of a good thing. As with buttock size, the degree to which fat people are found attractive varies from culture to culture. In general, in societies where food resources are scarce, being fat signals success, so is likely to be desirable in a mate.
"It shows you're particularly adept at hunting, gathering and storing food," says Dr Jones. Or, in the modern world, that you are in the kind of well-paid job which offers plentiful access to expense-account lunches. When preferences are this closely tailored to the environment, he says, it is often concluded that they must have been learnt; when preferences are universal, on the other hand, they are ascribed to genetics. "This view is crazy," says Dr Jones. "It's much more complex than that."
To demonstrate his point, Dr Jones and his colleagues asked women to imagine they were in hostile environments and judge the attractiveness of certain individuals. The women were then asked to do the same while imagining they were in a benign environment. Surprisingly, perhaps, women preferred feminine-looking men in more hostile environments, where resources were scarce. In such situations, a stable relationship is likely to be most conducive to the survival of the woman and her children.
"What seems to have happened is that people have evolved systems that allow them to fit what they like and what they don't like to their environment," says Dr Jones. firstname.lastname@example.org