Here is an interesting article about how religious people are out-reproducing the non-religious. In the modern world this effect is becoming particularly potent, as in secular society women wait longer to have children (if they have them) and have fewer of them on average.

Puberty Blues

This article asks why children are hitting puberty earlier and earlier, with some girls menstrating at 6 or younger! Scientists say that some consequences of early puberty include obesity, reproductive cancer, depression, and anxiety. The average age of puberty for American girls has gone from 17 in the 19th century to about 8.

To contrast, the average age of sexual maturation in hunter-gatherers is from 15-18 years.