I have thought about this question and now I see a new paper (ungated here) on the topic:

Genes and culture are often thought of as opposite ends of the nature–nurture spectrum, but here we examine possible interactions. Genetic association studies suggest that variation within the genes of central neurotransmitter systems, particularly the serotonin (5-HTTLPR, MAOA-uVNTR) and opioid (OPRM1 A118G), are associated with individual differences in social sensitivity, which reflects the degree of emotional responsivity to social events and experiences. Here, we review recent work that has demonstrated a robust cross-national correlation between the relative frequency of variants in these genes and the relative degree of individualism–collectivism in each population, suggesting that collectivism may have developed and persisted in populations with a high proportion of putative social sensitivity alleles because it was more compatible with such groups. Consistent with this notion, there was a correlation between the relative proportion of these alleles and lifetime prevalence of major depression across nations. The relationship between allele frequency and depression was partially mediated by individualism–collectivism, suggesting that reduced levels of depression in populations with a high proportion of social sensitivity alleles is due to greater collectivism. These results indicate that genetic variation may interact with ecological and social factors to influence psychocultural differences.

Still, I can't see the evidence. I don't see the case for causation. Let's say something about a group's serotonin level made it more susceptible to social stress: couldn't that lead to either greater individualism or greater collectivism? Is collectivism so calming and are social institutions so functional so as to respond to how stressed we feel from social interactions? If I were a very stressed out person (I'm not), wouldn't I prefer to live in or construct the social institutions of Sweden, which in this context counts as individualistic?

You might respond that the evolving alleles should be linked to earlier Swedish society and not Sweden today. But then one needs to measure collectivism vs. individualism at that earlier point in time. I wouldn't be surprised if China in the tenth century were "more individualistic" than Sweden in the age of the Vikings, and so on.

(By the way, Is "individualism vs. collectivism" the right spectrum? We individualistic Americans seem especially apt at being trained to kill people and fire when ordered. We also seem especially patriotic.)

If you pull out the strangely-placed Colombia from Figure 1 in the paper, it's basically a Europeans vs. Asians effect driving both the genetic contrasts and the collectivism vs. individualism contrasts. We're left with two quite general contrasts and no theory connecting the two or much of a good reason to think they should be connected.

Don't we just have two data points here — "Asia" and "Europe" — and the split of the data into countries is a phony way to boost apparent statistical significance?

It's a broader question what effects higher serotonin levels have. I've tried to read a few papers on this topic and I've seen high serotonin levels correlated with both anxiety and calm. To be sure, this may reflect the inability of this non-specialist to see through to the best and best understood results, but still the relevance of serotonin to human behavior hardly seems like an open and shut question. I'd sooner suggest that right now we don't understand serotonin very well, at least not as it shapes broader social interactions.

I thank RR for the relevant pointer.