Lots of animals choose their mates based on exaggerated features—think of the enormous antlers of moose or the elaborate plumage of many bird species. The explanation for this is what's sometimes termed "honest signaling"—if an animal has the health and metabolic resources to devote to growing these sorts of sex-specific features, then they've probably got the genetic wherewithal to produce healthy offspring. As long as nobody cheats—makes something that just looks like it took a lot of effort—the system works well from an evolutionary perspective.

Do humans engage in honest signaling? Clearly, there are features we associate with one or the other sex, and researchers have looked in to whether they might act as signals, feeding in to evolutionary selection. For example, some research has suggested that feminine faces on females act as a signal for fertility, as they're associated with estrogen levels. A masculine appearance, which is linked to testosterone levels, has been suggested to reflect health and disease resistance. And various studies have shown that the opposite sex appreciates faces that are strongly masculine or feminine.

So, in a neat and tidy package, we have an evolutionary explanation for both our appearances and our preferences for them. Or so a lot of people have argued. But a new study in PNAS argues that this is all an artifact of who we're asking. Do some studies in pre-industrial societies, and you get a very different answer.

The researchers behind the study (it was done by a large, international collaboration) point out that most of the studies of this sort are done with populations that are easy for researchers to access—typically, undergraduates at the same university. And those undergraduates are, in their terms, WEIRD: Western, Educated, Industrialized, Rich, and Democratic. If you want a picture of our evolutionary past, separate from modern social influences, then you have to look a bit more broadly than that.

And so they did, getting student populations from Canada, China, and the UK. But they also looked at Cree, Tuvans, Fijians, and other groups from Malaysia, South America, and Africa. These latter societies (excepting the Cree, who are integrated into the Canadian economy) farm, raise livestock, or in some cases forage. The status of these social groups was measured using things like the UN's Human Development Index, its measure of urbanization, or the World Health Organization's measure of years of life lost to disease.

The researchers took a set of facial photos from individuals of a number of ethnic origins, then used a computer to generate two alternate versions: one with a higher degree of masculinity and one with a higher degree of femininity. The subjects were asked to choose a photo that represented someone that might be interested in a long- or short-term relationship and the one they felt was most aggressive looking.

In general, the preference for a face that more strongly reflects the features of the opposite sex got stronger when the subjects came from a country with a high Human Development Index. This was true for both men and women, although among women the preference for short-term relationships was essentially random. There were some regional differences. For example, UK women liked masculine faces for short-term flings, but neutral faces for longer-term relationships—exactly the sort of thing that got evolutionary psychologists excited about the honest signaling model in the first place. But as the society became less industrialized, things shifted.

What about economic development was associated most strongly with this change? After running the statistics on multiple factors, urbanization stood out as the most significant predictor of the results.

There were two other notable results. Populations with higher disease burdens were less likely to prefer masculine faces—exactly the opposite result you'd expect if masculinity was signaling high testosterone and immune health. And the perception that strongly masculine faces were aggressive was also associated with economic development and urbanization.

Clearly, since urbanization is a relatively recent development, these results give us little reason to think that a preference for stronger signaling by the opposite sex is part of our deep evolutionary past. In fact, it's doubtful a strong preference could have evolved at all over the short time that industrial, urbanized societies have existed. Instead, it's likely that some social factor involved in urbanization is at play.

And the authors suggest one. The results, they write, "lead us to speculate (parsimoniously) that the "visual diet" of the observers may be an important factor in determining trait attributions and preferences." People in an urban society interact with many, many people over the course of one day and often have to make snap judgements about them. By contrast, people in smaller social groups have the time and capacity to get to know the rest of the group and don't have the need to rely on secondary cues to make judgements about them.

It's a compelling hypothesis, but for many people, so were the ones proposed by evolutionary psychologists. The research could be used to look at a larger group of societies, including some that only urbanized recently, before we get confident in the trends identified by the authors.

PNAS, 2014. DOI: 10.1073/pnas.1409643111 (About DOIs).

Listing image by Ines Seidel via Flickr