For every 1,000 unmarried U.S. women ages 15 to 44 in 2013, there were 44.3 births, down 2% from 2012 and 7% from 2010, CDC data show.
In contrast to unmarried women, birth rates for married women increased 1% in 2013 from 2012 to 86.9 births. In fact, they’re up 3% since 2010, after declining 5% between 2007 and 2010. (The absolute number of births among married women in 2013, 2.34 million, remained slightly below 2010’s 2.37 million.)
That piece, and this one, view this as a change in behavior, as if a constant group of married women decided to have more children, and a constant group of unmarried women decided to have less.
However, there is another possible explanation. Suppose that the two constant groups are “planners,” meaning women who only have children once they are married, and “non-planners,” meaning women who are willing to have children while unmarried. Also, suppose that among planners the rate of child-bearing is highest between years 3 through 10 of marriage. What happens if the marriage rate declines among planners because of a weak economy? Because they are unmarried and will not have children, you will see an increase in unmarried women not having children. Because the proportion of new marriages (where couples are not ready to have children) drops, you will see a bit of an uptick in married women having children. No fundamental change in behavior, just a decline in marriage rates among planners due to the recession.
I am not claiming that this is the explanation. But I need to see better quantitative analysis to rule it out.
Good lesson, whenever you compare groups over time, you need to think about whether the composition of the groups might have changed. Another example is comparing groups based on educational attainment over decades, as a higher share of people go to college.