Why Has the Proportion of Children Born to Unmarried Mothers Increased?

According to the NCHS, about 40 percent of births in 2007 were to unmarried mothers. A quick Google search reveals that the conservative family values crowd is jumping on this as the next sign of the apocalypse brought on by promiscuity.

I greatly enjoyed and recommend Robin Hanson's posts on the subject. He discusses a new equilibrium for male characteristics in a world where fatherly responsibilities disappear for child rearing.

My suspicion though is that we are seeing a lagged effect from an increase in cohabitation, or a progression in cohabitation as a social phenomenon. In other words, either people who started cohabiting before are now having children, or people view cohabitation as a permanent relationship rather than a "trial run" of marriage. In which case, men will continue to play a fatherly role, and we can spend less time in the gym.

By all accounts cohabitation has been increasing considerably, particularly from 1990 to 2000. The best data on the subject seems to be this page from the U.S. Census, where Table C-3 in 2008 and 2007 gives us the living arrangements for children and the marital status when both parents living with them. The percentage of children living with unmarried parents increased from 2.92 to 3.18 percent in those two years, so we don't learn much.

If we look at data from the NCHS report, while there is nothing even remotely definitive, I think it is more supportive of my suspicion. Notice that while unmarried teenagers are not getting pregnant at any higher rates, compared to 1995 there is a notable increase in unmarried births for the groups that would have been cohabiting in the 1990's to early 2000's: