IT IS not just a sense of fairness that seems to be calibrated to social circumstances (see article). Mating preferences, too, vary with a society's level of economic development. That, at least, is the conclusion of a study by Ben Jones and Lisa DeBruine of Aberdeen University, in Scotland, published this week in the Proceedings of the Royal Society.

Dr Jones and Dr DeBruine, themselves a married couple, examined what might be called the Deianira paradox. Hercules, demigod and paragon of masculinity in the ancient world, was indirectly done for by his own sexual prowess—his jealous wife, Deianira, accidentally poisoned him with a potion she thought would render him eternally faithful. Deianira's predicament is a woman's ultimate dilemma. In a man, the craggy physical characteristics associated with masculinity often indicate a strong immune system and thus a likelihood of his producing healthier offspring than his softer-featured confrères will. But such men are also more promiscuous and do not care as much about long-term relationships, leaving women to raise their kids alone.

Nowadays, sound parenting is often more important to the viability of a man's offspring than Herculean strength. That, some researchers suspect, may be changing the physical traits that women look for in a mate, at least in some societies. A study carried out in 2004, for example, discovered that women in rural Jamaica found manly types more desirable than did women in Britain, which led to questions about whether those preferences were arbitrary or whether women in different parts of the world might be adapting to circumstances that place different emphasis on manliness in the competitive calculus.

Dr Jones and Dr DeBruine therefore looked to see if there is an inverse relationship between women's preference for masculine features and national health. Sure enough, they found one. In environments where disease is rampant and the child-mortality rate is high, women prefer masculine men. In places like America and Britain, where knowing how to analyse health-care plans is more important than fighting off infection, effeminate men are just as competitive.

To test their thesis, Dr Jones and Dr DeBruine showed around 4,800 women from 30 countries 20 pairs of faces, in their online laboratory, faceresearch.org. The faces were masculinised and feminised versions of the same features. The women, who were all Caucasian in order to control for racial differences, were asked to rate each face for its attractiveness. The results allowed the researchers to calculate an average “masculinity preference” for women from the countries in question.

Having done so, they compared their results with national-health indicators in each of those countries, including child mortality, life expectancy and the prevalence of communicable disease. Since there is a strong correlation between these variables and a country's wealth, they controlled for GNP per head by measuring whether it alone accounted for the difference in mating strategy. They also tried to control for cultural factors using a questionnaire called the sociosexual orientation inventory index, which measures differences in mating patterns across cultures—specifically whether women prefer long- or short-term relationships.

Neither wealth nor mating pattern had much impact on women's preferences for manly men. Disease rates, by contrast, seemed to be directly related to how they went about choosing a mate—the healthier the society, the less women valued masculinity. Hygiene and wimps, it seems, go hand in hand.

These results echo earlier research by Dr DeBruine's thesis adviser, evolutionary biologist Bobbi Low. She found that polygamy is more common in societies that have more disease. In those societies, a modern Hercules can have his way because women prefer to share him rather than have a wimp to themselves. Healthy living, though, seems to have tipped the balance towards Deianira.