Two separate studies were recently published in the American Sociological Review, the journal published by the American Sociological Association, that find some interesting negative effects of divorce and single motherhood on children and mothers.
The first study shows that children of divorced parents often fall behind their peers in math and social skills, but only during the divorce period and not the period before or after. The second study show that women who had their firstborn child out of wedlock had much poorer health than their peers who married before having kids.
For more on these studies, see this article with more detailed information.

Comments