This seems a wee bit reductive to me, but it’s still an interesting hypothesis:

One of the more intriguing patterns in psychology is that different cultures are characterized by different personality types. A team of psychologists has proposed a new explanation: the legacy of disease. They matched the personality scores of people to historical data on the prevalence of major diseases in each country. They found that a history of disease in a country corresponded to a personality characterized by a less promiscuous orientation – especially for women – and by less extraversion and openness to experience. The idea is that more inhibited personalities evolved to prevent the spread of disease by minimizing risky social contact.

Note the verb “corresponded,” which has to do a lot of heavy lifting in that sentence. Cultures are incredibly complex entities, so I think one has to be careful coming up with simple explanatory relationships. (We should make every researcher attempting to “explain” history via data-mining read Isaiah Berlin’s classic essay on Tolstoy and history.) This could also be a case of looking for the keys under the streetlight: there is so little quantifiable historical data from a wide-variety of cultures that it can be tempting to put the numbers we do have through the statistical grinder, looking for subtle correlations between rates of leprosy and introversion, etc.

That caveat aside, speculation sure is fun. Personally, I’d be interested to look for correlations between measures of personality in various cultures and the preference for certain types of drugs. Some cultures prefer beer or wine (and have for millenia) while others favor opium or coca. How do these intoxicants influence, over vast stretches of time, the collective habits of people?