I have a confession. When I first moved to New England from Michigan, I said hi to everyone. I waved at police officers. I asked gas station attendants about the weather and talked to or greeted whomever I bumped into. Eventually though, I started to notice that such cordialities were not always returned. Sometimes I got a stare. It wasn’t quite a dirty look, but a kind of squirrel-faced wondering about whether I wasn't from “around here” or was just slow.

Cultures differ in all sorts of ways—their greetings, clothing, expectations about how children should behave, coming-of-age rituals, expressions of sexuality, numbers of husbands or wives, beliefs in god, gods, or lack thereof. People celebrate but also wage wars about these differences. Usually such variety is attributed to the vagaries of history and chance. Some things, like religious expression and differences or, say, the fleeting popularity of tube socks, seem simply beyond the realm of explanation. And yet haven’t you ever wondered whether there is some reason we are all so different, some underlying cause to our great cultural diversity?

Recently a group of biologists has offered a theory that they say explains, if not tube socks, then nearly everything else. In a series of high-profile papers, Corey Fincher and Randy Thornhill, both at the University of New Mexico, and Mark Schaller and Damian Murray of the University of British Columbia argue that one factor, disease, ultimately determines much of who we are and how we behave.

Their theory is simple. Where diseases are common, individuals are mean to strangers. Strangers may carry new diseases and so one would do best to avoid them. When people avoid strangers—those outside the tribe—communication among tribes breaks down. That breakdown allows peoples, through time, to become more different.

Differences accumulate until in places with more diseases, for example Nigeria or Brazil, there are more cultures and languages. Sweden, for example, has few diseases and only 15 languages; Ghana, which is a similar size, has many diseases and 89 languages. Cultural diversity is, in this view, a consequence of disease.

Then Fincher and colleagues go even further. Where people are more xenophobic and cultures more differentiated from one another, wars are more likely. Democratic governments are less likely because the tribe or group comes first; the nation and individuals in other tribes within the nation come second. And finally, poverty becomes nearly inevitable as a consequence of poor governance, hostility between groups, and the factor that triggered this cascade in the first place—disease.

Other researchers have seen links between disease and culture; for example, prohibitions against consuming pork would have protected early societies from swine-borne diseases like trichinosis. But Fincher’s scope is broader. In the story of disease, Fincher sees no less than the sweeping story of humanity.

As a rule, it is good to be skeptical of biologists who, like Fincher and Thornhill, propose to explain a whole bunch of things with one simple theory. More so when those biologists are dabbling in questions long reserved for cultural anthropologists, who devote their careers to documenting and understanding differences among cultures and their great richness of particulars. Biologists, and I am no exception, seem to have a willingness–or even need—to see generalities in particulars. Fincher’s new theory would offer an example of these desires (and a little hubris) run amok, of biologists seeing the entire history of human culture through one narrow lens. It would offer such an example, if it didn’t also seem, quite possibly, right.

Fincher and colleagues tested their theories by simply looking at whether there were consistent patterns in how cultural practices vary among regions of the world, and whether the prevalence of disease varies in a similar way. Are the places with the most diseases also the most xenophobic? Yes, they found.

Consistently, in regions where deadly diseases are more common, people are more xenophobic, more strongly focused on the welfare of their group, and less likely to be nice to strangers. Where diseases are more prevalent, individuals are less open to meeting strangers and to new experiences. Where diseases are more prevalent, cultures and languages differ more from one another. Sure enough, all of the scientists' predictions seem to hold, or at least to not be easily refuted. If you meet someone who is wary or even openly hostile to you, who bows or shake hands rather than kisses and in general keeps their distance, chances are they come from someplace with a terrible prevalence of disease.

Could the prevalence of disease really impact people’s behaviors so completely and consistently? With a new hammer, sometimes every shiny thing looks like a nail. Then swine flu H1N1 emerged. When swine flu came, some people started shaking hands less, wearing masks and, pity the thought, even kissing less—all because of the potential of catching one disease. Handshakes were skipped at college commencements. Mexicans were urged to not kiss on the cheek. Churches stopped having parishioners drink out of a common, holy cup. Suddenly the idea that diseases affect our behaviors seemed less remote.

So far, Fincher and Thornhill’s tests are really just correlations, the coincidence of certain aspects of culture and disease. Cultures are, for example, more diverse where there are more diseases—but many other factors are also different.

For example, all the places with lots of disease are also places favorable for growing many kinds of food. Perhaps being able to grow more kinds of food historically allowed more cultures to coexist, without competing, as has been argued by the anthropologist Daniel Nettle.

Some of the places with few diseases are also isolated. Perhaps, as Mike Gavin, at Victoria University in Wellington, New Zealand, has suggested, cultures that are more isolated are more likely to become different and a little wary of strangers.

But whether or not disease has caused the pattern of cultural variation, the scientists' analyses show that the patterns in human behaviors and culture are not random. Whether disease or some other factor is responsible, we have less control of who we are and how we behave than we might like to think. Our language, our sexuality—even how we tend to greet each other—are influenced by forces far from our daily control.

More research may tell whether Fincher and Thornhill’s models are general enough to be both useful and true. To date, Fincher told me, most of the e-mails he has received have been supportive. Some biologists have suggested the work is revolutionary. A few of the biologists who wrote to him were interested but cautious, and one or two disagreed with him completely. Somewhere, probably, a cultural anthropologist is writing and rewriting a thorough and vehement response.

In the meantime, we go on living our lives, imagining that we decide for ourselves who we are and how to act. But when the flu comes back this fall, watch your neighbors. Watch to see if their actions change. If Fincher and Thornhill are right, wherever the flu strikes, people will become more wary of strangers. Hands once extended freely will search for pockets. Where the disease is worst, the changes will be most rapid and extreme. Whole countries may even shutter their borders. Because while it is very hard to predict the evolution of H1N1 and the deaths it will cause, at least to Fincher the changes in our own actions may be more foreseeable. We are like small boats, pushed and pulled in the tides of disease.

Rob Dunn is a biologist at North Carolina State University and the author of "Every Living Thing: Man's Obsessive Quest to Catalog Life, from Nanobacteria to New Monkeys."