Will Millennials Have Fewer Children?

Yes.

Leslie Root, a PhD student in demography and a Russianist, disagreed with my take on fertility. Her point is simple: she says that all I’m observing is a “tempo effect.” That is, TFR projects current fertility rates for each age group into the future, with age-specific birth rates held constant. TFR, then, is a kind of “naive forecast” of completed fertility.

But of course, age-specific birth rates are not constant over time. So what happens if they change over time? What will happen to completed fertility?

I’ve discussed this extensively before. Here’s one where I show much of diminished completed fertility is due to changes in marital status: changes that are accelerating and becoming more extensive with time. Here’s one where I impute historic completed fertility from well before the current period, showing the very long-run decline. And here’s one where I simulate the effect of different future TFR trajectories on population and completed fertility.

Let me do another round of simulations. Leslie responds to my dismal scenario by suggesting that the impacted women were just delaying having kids. It’s true: we are seeing rising fertility among older moms! We can calculate age-specific birth rates from two major sources: the American Community Survey and the CDC’s births data. Here’s a comparison of their data from 2001–2017. Notably, ACS does not track births among females under age 15 or over age 50, while CDC does. These births are an exceedingly small fraction of total births.

As you can see, there are marked differences, with ACS tending to have lower estimates of younger fertility, higher estimates of older fertility. Part of this is due to sampling frame: CDC reports age of the mother at birth, while ACS asks women if they’ve had a birth in the past year, so women could be up to a year older when ACS measures them than they were when the child was born. There are a number of other survey biases at work too, like the ACS not updating for more recent underlying population estimates.

But on the whole, while levels vary, trends look very similar! Falling fertility for younger moms, rising for older!

So then, what if these trends continue? From here on out, I’ll be using just the CDC age-specific fertility rates (ASFRs). I will run three ASFR scenarios, shown below. Here are their total fertility rate outputs:

Scenario 1 makes a very simple assumption: current age-specific trends continue. Basically, the average annualized percentage decline/increase in age-specific fertility over the last 5 years continues. So young births keep falling, older ones keep rising. I taper off growth near the middle of the century to avoid preposterously high or low ASFRs. Scenario 2 assumes declines continue for younger groups, but assumes we get even more aggressive increases in older fertility than we’ve had in recent years. Scenario 3 builds on Scenario 2, but assumes that 20-something fertility stops its decline earlier than Scenario 1.

Put another way, Scenario 1 is “current trends,” Scenario 2 is “improved reproductive tech,” Scenario 3 is “Tech + Slightly Younger Marriages.”

Here are the ASFRS:

Now then. Let’s simulate completed fertility for each birth cohort from 1988 to 2020. Basically, we will have a woman be “born” in 1988, begin having a chance of having kids at age 11 (I’ll backcast under age-15 fertility from for birth cohorts 1988–1992), and continue having a chance of having kids until age 55. In each year, her odds of having a kid are the ASFR for her age.

Using these ASFRs, we can develop simulated CFRs by birth year: