I just read an article "Why Blondes Have High Fertility" by Allan & Barbara Pease. I also did a little research about it.
Do you know that blondes have higher oestrogen levels that give strong attraction to man? It is likely the explanation of the phrase "Dumb blonde" and was more exploited by commercials and media.
They say that blondes have higher fertility but very low mathematical reasoning, but after a blonde's 1st baby is born, her hair darkens because her oestrogen level drops. It darkens even more after her 2nd child. Therefore, we hardly seen pure natural blondes at the age of 30 and above.
My own explanation to this is, blondes become smarter after giving birth because they have learned their hard lessons. I think that is also the reason why many popular blondes end up as single mothers.
It is funny to realize, why Asian women are spending so much to color their hair blonde. More funny to see men doing this. Are they trying to look like dumb or they already are? :)