Blondes is a sign of high oestrogen levels and account for men's strong attraction to blondes. It is an indicator of fertility and its the likely explanation of the phrase "dumb blonde". Dumb blondes, so the jokes go, the have high fertility and low mathematical reasoning.
Research has also shown that teenage girls whose mothers took male hormones during pregnancy, performs better in academic discipline than other girls, and are more likely to get into university. The other side of the coin is that these girls are described as less feminine in nature and they tend to have more body hair.
After a blonde's first baby is born, her hair darkens because her oestrogen level drops. It darkens even more after her second child. The reduction in oestrogen levels is why there are few natural blondes after age 30.
In the Roman era, prostitutes wore more blonde wigs as an advertisement for their trade and this gave rise to the phrase, "Blondes have more fun".