An easy read, with anecdotal and statistical support. Correlation is not causation, so read this with at least a grain of salt. Bishop is not the first to claim assortative matching in areas other than human mating. While the net he casts covers many activities, his focus is largely on politics. Despite my doubts, I think his suggestion is closer to the truth than a previous comment blaming Sesame Street for the disappearance of the middle from American political life.