Since 1991 the teenage pregnancy rate has fallen by about 22 percent, reversing a 40 year trend. In a lengthy story, the NYTimes suggests that learning from the hard experience of others is the explanation for the drop without explaining why it should take 40 years for this learning to take effect. They do note “teenage pregnancy had already begun its decline in 1991, well before welfare changes and the economic boom, and well after the first round of sex education programs.” The Times, however, does not examine the most controversial but well-supported explanation, the introduction of legalized abortion in the 1970s.
If this explanation rings familiar it should. In a very controversial paper, Steve Levitt and John Donohue provided evidence that legalized abortion in the 1970s reduced crime some 18 years later. The theory is simple. Abortion rates are higher among the poor, the unmarried, teenagers, and African Americans than among other groups and children born to mothers with several of the preceeding characteristics are at increased risk for becoming involved in crime. Legalized abortion gave these mothers an option and thus reduced the number of at-risk children who might otherwise have grown up to become criminals (note that abortion doesn’t mean fewer children per-se, it may simply delay childbearing to when the mother is not poor, a teenager or unmarried which works just as well.)
In brief, the evidence for the Levitt-Donohue theory is a) the timing is consistent, b) states that legalized earlier had earlier drops in crimes, c) there is a dose-response effect i.e. states that had more abortions had bigger drops in crime, d) the drop in crime in the 1990s occured among those cohorts who were potentially affected by abortion policy in the 1970s (and not among say 40 years olds.)
Joined by co-author Jeff Grogger, Levitt and Donohue apply the same idea to teenage pregnancy and find very similar results – thus reinforcing their earlier story. They write:
Parents who are least able or willing to begin caring for a newborn are most likely to make use of abortion. The abortion rates for teens, the unmarried, and the poor are substantially higher than for the general population. Children who are born unwanted are subjected to poorer care both during pregnancy and the early years of life. With the legalization of abortion, mothers with unwanted pregnancies suddenly had a new recourse. Consequently, the number of children raised in adverse environments dropped substantially. Donohue and Levitt  showed how this change reduced crime among the subsequent generation by 15-25 percent. As teen childbearing is a closely associate social pathogen, the magnitude of the drop should be similar.
Our empirical evidence suggests that birth rates as teens are strongly negatively associated with being born in a state and time period in which abortion rates were high. Our results suggest that teen birth rates today may be 20 percent lower as a consequence of legalized abortion in the 1970’s.