Opinion | Could Peer Influence Be a Cause of the Global Baby Bust?

I read several papers on peer effects on fertility with Angrist’s caveats in mind. One, by Jason Fletcher and Olga Yakusheva, looked at American teenagers and found that a 10 percentage point increase in pregnancies of classmates is associated with a 2 to 5 percentage point greater likelihood of a teenager herself becoming pregnant.

Disentangling causality is “a really hard problem,” Fletcher, an economist at the University of Wisconsin’s La Follette School of Public Affairs, told me. He and Yakusheva, who is at the University of Michigan School of Nursing, tried to identify causality by using a trick similar to the one Angrist used to study the impact of military service on veterans’ earnings. They needed a factor that was correlated with a girl’s likelihood of becoming pregnant, but with no risk that the factor could be caused by the girl (reverse causality). They found two such factors, called instrumental variables: whether her classmates began to menstruate early and whether the classmates were themselves children of teenage mothers. Those factors made the classmates more likely to become pregnant, which in turn influenced the fertility outcomes of the girls being studied.

Amalia Miller at the University of Virginia is one of four scholars who studied fertility in workplaces in Denmark. If two women in an office got pregnant around the same time, it was hard to tell if A influenced B or B influenced A. They came up with the idea of looking at the fertility of A’s sister, realizing that if A’s sister got pregnant, A herself was more likely to get pregnant. That was their instrumental variable. Miller and her research partners found that for more-educated women, a colleague of the same education level having a baby increased their own chance of having a baby. Less-educated women were less likely to have a baby when a colleague of the same education level had one. The negative peer effect for less-educated women “could come from a desire to distinguish oneself from one’s peers,” among other factors, they speculated.

The third person I interviewed is Nie Peng, a professor at Xi’an Jiaotong University in Xi’an, China. He and two co-authors studied Chinese women ages 18 to 49. They found that an increase in the fertility of peers reduced the probability that women would want only one child and increased the probability that they would want three, four or more children. The research “provides support for the role of social norms in the fertility choices of reproductive-aged Chinese women,” they wrote.

Nie told me he read Angrist’s paper years ago and kept its caveats in mind as he designed his team’s research. In any case, he said, peer effects aren’t the whole story. Even though Chinese authorities desperately want to raise the nation’s birthrate, it’s still hard for young families to raise children, he said. He said he and his wife have one child, a 5-year-old girl. When he picks her up from kindergarten, he said, there’s a long line — of grandparents, because all the other parents have to work.