At the risk of making people angry...as a parent and a degreed philosopher, author and lover of quiet thinking time I have noticed a correlation between not having children and a part of the would-be adult (educated/philosophical) mind that itself doesn't want to give up childhood and it's unencumberances. In other words, there's a selfish streak in philosophers. They may blame the choice to not have children on overpopulation and whatnot, but if you dig a little deeper, the real answer is that they don't want to have to give up their "game time", ie their puzzle solving time.
Thinking is addictive and for deep thinkers, there are endorphins involved in puzzle-solving, not unlike what other gamers feel when they say, successfully raid on WoW.
There does seem to be an inverse relationship between intelligence and reproduction, witness the American South.

(I can say that, I'm from Texas

)