America’s birth rate is falling far below what is needed to maintain economic growth, and below what women themselves say they want. But while this demographic decline is easy enough to describe, solutions are harder to come by. Quick policy fixes, like more parental leave or financial incentives, may have some effect but are likely to be quite expensive relative to their modest impact on birth rates.

A large part of the decline in birth rates can be directly accounted for by a factor unlikely to be influenced by those birth-targeted incentives: marriage. That is, most long-run change in fertility can be accounted for by changes in the marital composition of society.

While women are married, they tend to have very high birth rates. Note that the chart above does not show that married women will havefour or 5 kids; it means that the average birth rate for married women ages 15 to 50 sums up to 4 or 5 kids. But that’s a 35-year span, when the average woman will only actually spend between 12 and 20 of those years married.

As you can see above, there’s been a decline in married fertility since the peak in 2009. But married fertility rates today aren’t actually much lower than married fertility rates in the mid-2000s. For divorcees and widowers, age-adjusted birth rates are actually higher now than they were before the recession. Never married women, again, have about pre-recession levels of fertility too.

By fixing age-specific marital status to 2001 or 2008 levels, we can model a counterfactual scenario of what fertility might have been had people gotten married and formed families at the same rate as formerly.

Essentially all of the decline in fertility since 2001 can be explained by changes in the marital composition of the population. Married, single, and divorced women are all about as likely, controlling for age and marital status, to have kids now as they were in 2001. But today, a smaller proportion of women are married during those peak-fertility years…

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