For the curious, total N in the Rosenberg et al study is 421, with 65.2% Caucasian, 6.67 Asian-American, 5.71% Hispanic-American, and 8.81% African-American (13.57% were of "Other/Unknown" ethnicity, but it looks like most of them didn't complete personality questionnaires anyway).

It is known that the MAO-A locus has been under positive selection*, and because its effect on phenotype is to generally make people less Agreeable,

I don't understand this statement. The locus is a location where different alleles can be present, right? So how can a *locus* have a general effect on phenotype? Were you just saying that the variants that were selected for are ones that make people less Agreeable?

Also, why assume the existence of alleles at modifying loci? Seems plausible enough, but wouldn't the default hypothesis be that the recent evolutionary environment has favored nastier people?

The locus has seen, experienced, selection in its alleles. The variants have undergone selection, etc. And yes, the selected variants are the "warrior" variants.

Re: modifying alleles, there's nasty and then there's nasty. I assume that recent environmental change induces a quick response that possibly overdoes it when unchecked, which then requires other alleles to restrain the "bad cop" allele.

Alternatively, the kid might inherit the "docile" variant but would also inherit the modifying alleles from the parent with the warrior variant. Thus, there would be a team of restraining cops with no bad cop to contain, perhaps resulting in an overly meek and credulous phenotype.

Malcom Gladwell?

(i) Thirty three DNA samples were provided by Coriel
Cell Repositories, Camden, NJ. These consisted of: seven
Pygmy samples, nine Aboriginal Taiwanese, three Chinese,
two Japanese, five Mexicans, and seven Russians.

(ii) Samples
from 23 unrelated individuals were provided by the National
Laboratory for the Genetics of Israeli Populations at Tel Aviv
University; these came from two ethnic groups: Ashkenazi
Jews (13 individuals) and Bedouins (10 individuals).

This does not seem fairly representative...

So, due to selection, the Meek will probably not inherit the Earth ;-)

This does not seem fairly representative...

The aim of the study was not to estimate the frequency of alleles in various populations, in which case of course studying just 2 Japanese wouldn't suffice. The aim was to detect signatures of natural selection on alleles at a particular locus. Since selection leaves certain "calling cards," detecting these tells you that the alleles at the locus have been subject to some kind of selection or other. Our blogger friend Evolgen wrote up a series on this:

http://scienceblogs.com/evolgen/ ...election_pa.php