Here is a classic problem of cause and effect. Teenagers who give birth are more likely to be from households with lower income levels. Also, teenagers who give tend to end up later in life in households with lower income levels. But does the lower income level cause teens to be more likely to give birth? Or does giving birth cause as a teen cause that woman to be more likely to end up in a lower-income household? How can one untangle cause and effect? Melissa S. Kearney and Phillip B. Levine tackle these questions in "Why is the Teen Birth Rate in the United States So High and Why Does It Matter?" which appears in the Spring 2012 issue of my own Journal of Economic Perspectives. They have lots of interesting comments to make about variation in teen birthrates across states and countries. Here, I'll focus on their analysis of the cause and effect question, which surprised me and offers a nice example of how economist try to disentangle these sorts of issues.
"Our reading of the totality of evidence leads us to conclude that being on a low economic trajectory in life leads many teenage girls to have children while they are young and unmarried and that poor outcomes seen later in life (relative to teens who do not have children) are simply the continuation of the original low economic trajectory. That is, teen childbearing is explained by the low economic trajectory but is not an additional cause of later difficulties in life. Surprisingly, teen birth itself
does not appear to have much direct economic consequence."
Conceptually, how would one tell whether giving birth as a teenager is a cause of lower future economic prospects? Just comparing life outcomes for teenage girls who give birth and those who don't will give you a correlation, but not causation. "A comparison of the outcomes of women who did and who did not give birth as teens is inherently biased by selection effects: teenage girls who “select” into becoming pregnant and subsequently giving birth (as opposed to choosing abortion) are different in terms of their background characteristics and potential future outcomes than teenage girls who delay childbearing." The problem is made more difficult because some of the background characteristics may be measurable in the data (like family income level, or ethnicity, or if it's a single-parent family) but many other characteristics are not available in the data (like the personality traits of the teenage girl or the values lived by the family).
In an ideal experiment, one might want a research design in which a
random sample of teenagers becomes pregnant and gives birth, and then
you could track the outcomes. Of course, randomized pregnancy is an
impractical research design! But here are four approaches used by clever economists to disentangle this question of cause and effect.
A within-family approach. Look at life outcomes for sisters who give birth at different ages. The result of this kind of study is "once background characteristics are controlled for, the differences are quite modest. Furthermore, even these modest differences likely overstate the costs of teen childbearing, since the sister who gives birth as a teen is likely to be “negatively” selected compared
to her sister who does not."
Miscarriages. Of those teens who become pregnant, some will suffer miscarriages. Compare women who are similar in measured characteristics of family background, but some of whom gave birth as teenagers while others had a miscarriage. It turns out that their life outcomes look quite similar: that is, giving birth as a teenager doesn't appear to cause any additional decline in later life outcomes.
Age at first menstruation. Girls who menstruate earlier are at greater risk of becoming pregnant as teenagers. One can use a statistical approach to look at two groups of women who are similar in measured characteristics
of family background, but where one group has a higher pregnancy rate because they began their menstrual cycle
earlier. However, the life outcomes for these groups look quite similar; is not correlated with lower life outcomes: that is, a random chance of being more likely to give birth as a teenager (because of an earlier age of first menstruation) doesn't appear to cause any additional decline in later life
Propensity scores. Look at girls within a certain school, so that they live in more-or-less the same neighborhood. Using the available data, develop a "propensity score" that measures how likely a girl is to give birth as a teenager. Then compare the life outcomes for girls with similar propensity scores, some of whom gave birth and some of whom did not. There doesn't seem to be a difference in life outcomes, again suggesting that giving birth as a teenager doesn't much alter other life outcomes.
Kearney and Levine sum up the evidence on cause and effect this way: "Taken as a whole, previous research has had considerable difficulty finding much evidence in support of the claim that teen childbearing has a causal impact on mothers and their children. Instead, at least a substantial majority of the observed correlation between teen childbearing and inferior outcomes is the result of underlying differences between those who give birth as a teen and those who do not."
Kearney and Levine also offer an unexpected (to me) perspective on policies to reduce teen pregnancy:
"Moreover, no silver bullet such as expanding access to contraception or abstinence education will solve this particular social problem. Our view is that teen childbearing is so high in the United States because of underlying social and economic problems. It reflects a decision among a set of girls to “drop-out” of the economic mainstream; they choose nonmarital motherhood at a young age instead of investing in their own economic progress because they feel they have little chance of advancement. This thesis suggests that to address teen childbearing in America will require addressing some difficult social problems: in particular, the perceived and actual lack of economic opportunity among those at the bottom of the economic ladder."
The statement about teenage girls "choosing" nonmarital motherhood should be understood not as a claim that all pregnant 15 year-olds carefully considered their life options and decided on pregnancy! Instead, the economists' view of choice is that we all make groups of choices every day--say, choices about exercise and calories consumed--that make certain outcomes more likely. Decisions that are not well-considered, or that raise the risk of undesired side effects, still have a large ingredient of choice. For example, we typically view those who drive drunk as having made a "choice."
The cause-and-effect evidence here suggests that for many women who give birth as teenagers, their life outcomes like level of education achieved, income, employment, and chance of marriage are already so constrained that they are not made worse off by having a child as a teenager. Encouragement about contraception or abstinence can help reduce teen pregnancy on the margin. But what many teen girls from low socioeconomic status backgrounds need is a reduced prospect of marginalization, and a greater chance for personal and economic advancement.