More fathers married when their first child is born? Probably not

A startling data brief from the National Center for Health Statistics reports that the percentage of fathers who weren’t married at the time of their first births fell from the 1980s to the 2000s. Here is the first “key finding”: “The percentage of fathers aged 15–44 whose first births were nonmarital was lower in the 2000s (36%) than in the previous 2 decades.”

That is shocking. How could we have a falling percentage of fathers not married at the time of their first births? The author, Gladys Martinez, writes:

Results from this study indicate that in the 2000s, the percentage of fathers with nonmarital first births declined. However, the percentage of fathers whose nonmarital first births occurred within a cohabiting union increased. This pattern differs from that for the mother. Data for women showed that the share of all births that occurred to unmarried women has doubled between 1988 and 2009–2013, and that the increase was driven by an increase in the share of births to cohabiting women.

Here is the main figure, showing the decline in nonmarital first births for fathers:

But I think this is not correct (this concern was first raised to me by Pew researcher Gretchen Livingston). Here’s why. As the figure shows, the source for these three decades of data is the National Survey of Family Growth. The earliest this survey captured men’s births (awkward phrase, but you know what I mean) was in 2002. And the ages included in the survey were 15-44. But the figure has information about births in the years 1980-1989. By my math, the oldest a 15-44-year-old in 2002 could have been in 1989 is 31. So that 2002 survey is only returning data on the marital status of men ages 15-31 in the 1980s.

I always have to do one of these to make sure I’m not crazy when I’m trying to work something like this out. This is how old 15-44 year-olds in 2002 were in the 1980s, excluding those under 15 (click to enlarge):

They’re all 15-31 (or younger) in the 1980s. In contrast, if they combine the 2006-2010 survey (collected over 5 years) with the 2011-2013 survey (collected over 3 years), they have men ages 15-42 in the 1990s and 15-44 in the 2000s. So, as the age of the men in the sample rose, the proportion married when they had their first birth rose, too. This is what we would expect: younger first-time parents are much less likely to be married.

Consider, then, the followup finding from the brief: for men of every age the proportion unmarried at the time of their first birth has increased:

How can it be that the overall proportion unmarried is falling, while it’s rising for each age group? The answer in the data brief is that first-time unmarried fathers are getting older. But remember — the samples are getting older across these decades, because of the timing of the surveys: they age from 15-31 to 15-44. That explains the next figure perfectly. Look at that increase in the proportion of unmarried first-time fathers who are 25-44:

In the 1980s, just 8% of first-time unmarried fathers were age 25-44, compared with a whopping 33% in the 2000s. But doesn’t it seem likely that you’ll have fewer men ages 25-44 in a group that only goes up to age 31, versus a group that goes all the way up to age 44?

This stuff gets confusing, but I’m pretty sure this is right. That is, wrong. I do not believe that there is a falling percentage of fathers having first births when they’re not married. What looked like a weird, complicated demographic problem — falling unmarried first-fatherhood along with rising unmarried first-motherhood — is probably an artifact of a weird, complicated problem in the analysis.

There is nothing in the data brief to suggest there was an adjustment for the changing age composition of the data for these decades, but maybe they did something I don’t understand. If not, I think NCHS should correct or retract this report.