Even more challenging: We need that message to be true. This is a much more difficult proposition, but all of the evidence suggests that this is what is required -- interventions that change the life trajectory of girls on the path to teen motherhood.

Why Contraception Isn't Enough

The evidence on targeted teen pregnancy prevention approaches is almost always disappointing. In previous work, we have shown that access to free family planning services for low income teens reduces the likelihood of giving birth, but not by very much. The most rigorous studies assessing the impact of access to Plan B emergency contraception points against there being an effect on pregnancy or abortion rates. Abstinence only programs also have no discernible impact on teen births. Sex education has been shown to have an impact on some outcomes like delayed initiation of sexual activity, but the evidence regarding its effectiveness in reducing teen childbearing is weak at best.

Why don't these policies have more of an impact? We believe it is because they do not address the fundamental forces that drive most teens to have children. They focus on the immediate precursors to pregnancy, and miss a lifetime of behaviors and decisions that build towards it.

Advocates for these types of short-term interventions will point to research on brain development demonstrating that teens are less capable of thinking beyond the moment. Therefore, they argue, we should craft policies aimed at making sure, for instance, that teens have a condom handy or hear consistent messages regarding abstinence. We have no doubt that some teen pregnancies happen because young people fail to consider the long-term implications of their actions. But we do not believe that explains the majority of them. One needs to look no further than the enormous geographic disparities in teen birth rates to suspect that something other than the adolescent brain is at fault. Why would teens in Mississippi or New Mexico have so much more trouble controlling their impulses than their peers in New York or New Hampshire?

An alternative view -- the one we favor -- is that teen childbearing is a symptom of living a life full of obstacles. Facing limited education and job prospects, as well as a slim chance of finding a suitable man to marry, some low-income girls simply ask, "Why not have a baby now?" Ethnographic work on young, single mothers supports this theory. So does economic research showing that the financial and social problems teen mothers and their children experience are mostly driven by the mother's socio-economic background, not her decision to have a baby early in life.

Why Income Inequality Plays a Role

This view also helps explain why income inequality seems to encourage teen pregnancy. We have found that girls from disadvantaged backgrounds who live in places with a larger gap between the poor and the middle class are considerably more likely to give birth as a teen than girls who have similar backgrounds, but face less inequality. Income inequality is strongly linked to lower economic mobility -- the ability to improve one's station in life. And so our findings seem to suggest that girls who don't see a chance to better their lives are more likely to have a child.