There’s a reason birth rates are an increasingly prominent feature in discourse and policymaking today. Population ageing and decline is one of the most powerful forces in the world, shaping everything from economics to politics and the environment.

But a weakness to the debate — perhaps even the term “birth rates” itself — is that it implies the goal is the same today as it was in the past: finding ways to encourage couples to have more children. A closer look at the data suggests a whole new challenge.

Take the US as an example. Between 1960 and 1980, the average number of children born to a woman halved from almost four to two, even as the share of women in married couples edged only modestly lower. There were still plenty of couples in happy, stable relationships. They were just electing to have smaller families.