What's more: once it kicks in, it doesn't kick in equally for everybody. The more racially diverse your peer group, the less strong the amygdala effect. At really high levels of diversity, the effect disappeared entirely. The authors of the study write that ''these findings suggest that neural biases to race are not innate and that race is a social construction, learned over time.''

There's a reason the previous sentence says "suggest" and not "prove." As the authors note, it's conceivable that "the increasing amygdala response to race [with age] may be driven by intrinsic factors of the child, such as puberty, rather than exposure to cultural messages." For that matter, the correlation between peer group diversity and dampened amygdala response doesn't mean the former causes the latter; it could work the other way around: maybe people with a mild response to racial difference wind up with more diverse peers.

But all of this is almost beside the point anyway, because there have always been plenty of reasons to believe that the amygdala response doesn't reflect an instinctive aversion to the racial "other." For example: The amygdala's response to African-American faces had been observed not just in European-American adults but in African-American adults--who aren't, in this case, the "other." Apparently whatever cultural information was inculcating a particular response to blacks in whites was having a similar effect in blacks.

I'm not a blank slater; I don't believe that we're born innocent, and only develop a dark side after bad tendencies are engrained by evil capitalists, or evil patriarchs, or evil warmongers, or evil whatevers. I think that, though we're not naturally racist, we're naturally "groupist." Evolution seems to have inclined us to readily define whole groups of people as the enemy, after which we can find their suffering, even death, very easy to countenance and even facilitate.

But when it comes to defining this enemy--defining the "out group"--people are very flexible. The out group can be defined by its language, its religion, its skin color, its jersey color. (And jersey color can trump skin color--just watch a brawl between one racially integrated sports team and another.) It all depends on which group we consider (rightly or wrongly) in some sense threatening to our interests.

It's in this sense that race is a "social construct." It's not a category that's inherently correlated with our patterns of fear or mistrust or hatred, though, obviously, it can become one. So it's within our power to construct a society in which race isn't a meaningful construct.