"Who knows the secrets of the human heart?" asked the bartender at the end of 1992's gender-bending caper The Crying Game, a meditation on love, attraction and human nature. Well, scientists are still wrestling with the rules of attraction, but the nose, rather than the heart, may be the place to start looking for an answer, suggests one new study.
Since 1995, researchers such as Claus Wedekind of Switzerland's University of Lausanne have pursued the notion that scent may play a role in human mating, following the observation that mice, rats, sand lizards and even fish prefer mates with immune system genes unlike their own. In animals, scents provide the signal of this difference, which evolutionary biologists find leads to offspring with healthier immune systems who are able to fend off a wider variety of ailments. Pheromone experiments, most famously done with scents from men's sweaty T-shirts evaluated for attractiveness by women, have found some evidence of dissimilar immune system preferences in people.
So, "Birds do it, bees do it. Even educated fleas do it," as Cole Porter once penned. "Let's do it, let's fall in love."
But is it so simple? Not every study has found the pheromone effect. A 2002 study looking at T-shirt odors, for example, found the preference instead was for scents from men with similar immune systems rather than dissimilar ones. Hoping to look at the question from another angle, a recent study in PLoS Genetics led by statistician Raphaëlle Chaix of the University of Oxford analyzed the genes of 30 couples from Nigeria and 30 couples from Utah to see whether they preferred a certain set of immune characteristics (technically speaking, the Major Histocompatibility Complex, or MHC) in their mates.
Well, yes and no -- for the Americans, the study finds a "small but significant" tendency for couples to have dissimilar immune system gene sets, Chaix says. But among the Africans, no effect is seen, which suggests social factors there overpower any immune system preference. Or perhaps among the genetically diverse but poor African population, Chaix suggests spotting people with a weak immune system is easier, alleviating any need to sniff it out when looking for a mate.
So, if you are lonely and living in Utah, all you need to do is mutate your immune systems and so long, empty datebook, right? No, says Chaix. Pathogens will likely get you if your immune system is too screwed up, and that may explain why preferences aren't for extremely different immune system smells in past studies. Too much variety in your offspring's immune system genes might also lead to autoimmune diseases and other problems.
"In this context, we may think that sexual selection is a weaker evolutionary force in comparison to selective pressures from pathogens," she says. "But we still have to test it."
"Our study indicates that the relative importance of biological and social factors varies from one population to another," the authors conclude. Future studies of more couples should include "just married" childless couples, they add, and look at more locations to build a better picture of the secrets of human attraction.
Or you could just ask your bartenders. No doubt they've seen it all by now.