Turns out Charles Darwin took this question very seriously! Who knew?

He was interested in understanding if hair color had effects on mate selection. Here’s an article from UK’s Telegraph describing what was uncovered as a result of the Darwin Correspondence Project which is attempting to compile 15,000 of the letters that Darwin wrote and received during his lifetime.

Oh, and Darwin initially thought perhaps dark hair in the general population was increasing because brunettes were more likely to get married while blondes tended to stay single and childless. Eventually he gave up on the notion and came to the conclusion that the experimental basis was not good enough.