A fun one which came up recently on IRC: everyone thinks that how your parents raise you is incredibly important, this is so obvious it doesn't need any proof and is universal common sense, and you can find extended discussions of the best way to raise kids from Plato's Republic to Rousseau's Emile to Spock.

Except twin studies consistently estimate that the influence of 'shared environment' (the home) is small or near-zero for many traits compared to genetics and randomness/nonshared-environment.

This is quite possibly the most comforting scientific result ever for me as a parent, by the way.