Wow, that article was crazy. I'm incredibly pro-open borders, pro-almost unlimited immigration, pro-cultural mixing, pro-genetic mixing, but I can see how those findings would come about. Got any other relevant links on the topic? Incredibly thought provoking.
For about 100,000 years, your ancestors have gotten optimal results by trusting people who looked like them, and distrusting people who didn't. That would be a very tough instinct to change.
The implicit association test usually shows that you like the folks you look like. And Putnam's study reveals the same thing, on a large scale. Once you consider the fact that for most of human history, we were part of small tribes of close genetic relatives (so kin altruism was good for our tribe's genes, which we are likely to share), you have an explanation that fits the data and makes sense.
If that weren't true, you'd have some data that don't make sense, and an evolutionary hole to fill (how would the non-prejudiced tribes outcompete the prejudiced ones before the invention of trade?).
