behavior and the biome: love and early childhood diarrhea

scientificamerican |  It’s not everyday that love and diarrhea come together in theoretical matrimony. Recently, however, a study by an interdisciplinary team of scientists managed to form this near-perfect union. Oh relax. I’m not about to share the sordid details of a revolting new sexual fetish. (That’s for another post.) The research I’m about to tell you about is actually bigger than that. In fact, it’s a pity that the report appeared without much fanfare last year in Evolution and Human Behavior, because when looked at in the right light, there’s a certain quiet beauty to these data in the way that they subtly illuminate gene-environment interactions.

Before we can hope to understand the curious connection between the hellish expunging of our intestinal contents and the type of person that we’re in turn most likely to marry—or simply to screw—it’s necessary to first step back to look at the wider framework in which the study’s main hypothesis was based. Unless you reject evolutionary psychology out of ignorance or spite (or some combination thereof), it’s noncontroversial to say that, all else equal, it’s biologically more adaptive to sexually reproduce with a healthy than with an unhealthy partner. Needless to say, “biologically adaptive” is rarely isomorphic with “nice and kind”, but kind or not, most of us don’t instinctively gravitate to strangers with a death rattle for a cough or get turned on by the sight of someone with random body parts sloughing off. Beyond such truisms, having children with a chronically ill person makes it not only difficult or impossible for that poor individual to “invest” in your mutual offspring, leaving you to shoulder the “costs” of rearing them on your own, the health of your kids (who, let’s not mince words, are the cooing keepers of your eternal genetic promise) may also be compromised if your partner’s disease is heritable.

While it’s all well and good to aim for a healthy baby-mama or baby-daddy, one hitch is that, in the real world, we can’t always count on a conspicuous cue like emaciation or puddles of pus to tip us off about a prospect’s dubious hardiness. Even if that guy giving you eyes looks fit enough now, who’s to say he won’t be the first to drop when the next scourge rolls in? Our ancestors would have faced similar challenges discerning the relative health of viable members of the opposite sex.

The solution to this adaptive problem was mindlessly ingenious. Among other fixes, our brains became aesthetically predisposed to faces that were the best genetic gambles in a world brimming with deleterious pathogens. When looking at aggregate data, these appealing faces tend to belong to folks who are more disease-resistant across the lifespan. Lucky bastards. Now, how to spot these anti-pathogenic lovers. That’s to say, what’s a “pretty face” exactly, and why do we often disagree on others’ attractiveness? Yes, there’s that old facial symmetry giveaway, but debate rages on about the relationship between this variable trait and perceptions of beauty. If you really want to stack the fitness odds in your offspring’s favour, there seems to be a more reliable marker of a person’s health than mere symmetry: having an extremely sex-typical mug. An already impressive, yet still-mounting, body of evidence reveals that the degree of “masculinity” in a man’s face correlates positively with his lifelong health, while the degree of “femininity” in a woman’s face does the same for hers.

“That’s nice,” you’re probably saying to yourself. “But where does diarrhea come into it?” I thought you’d never ask. The authors of this new study, led by the psychologist Mícheál de Barra of Stockholm University, suspected that our behavioral immune system, which is a hypothetical collection of evolved cognitive biases that spur us on to make adaptive decisions in the domain of disease avoidance, may get a sort of laxative fine-tuning during our early child development.