U.S. fertility rates continued their long-term decline in 2019, hitting another record low, according to a May report from the Center for Disease Control and Prevention. Though the causes behind this trend are complex, varied, and rest on large-scale cultural and economic developments, a recent study illustrates how more mundane factors can also play a role in declining fertility.

In their new paper, “Car Seats as Contraception,” Jordan Nickerson and David Solomon estimate that laws raising the age requirements for children to ride in a car safety seat have resulted in 145,000 fewer births in the U.S. since 1980. Since 1977, state laws have required children to be restrained by car seats, with the age limits gradually rising over time. The problem for potential parents is that many cars can accommodate two, but not three, car seats in the back row. Car seat laws thus require three-child households to purchase larger cars, indirectly raising the costs of having a third child.

Using census data, Nickerson and Solomon find that when a woman already has two children below the car seat age, the likelihood of her giving birth that year falls by 0.73 percentage points. Given that a woman age 18-35 with two children has 9.36% chance of giving birth in a given year, this is a considerable decline. In contrast, car seat laws don’t appear to have any effect on families with only one child of car seat age, or on families without access to a car, suggesting that car seat laws are the culprit.

By Nickerson and Solomon’s numbers, this unintended consequence actually affects more lives than the intended one. They estimate the laws lowered births by roughly 8,000 children in 2017 but prevented only 57 child car crash fatalities. In promoting car seat laws, surely no one had the goals of making children more expensive and lowering birthrates. But policies should be judged on their overall outcomes, intended or otherwise.

Policy analysts on both the left and right are increasingly giving attention to the implications of falling birth rates and considering policies to reverse them. This is difficult because people choose to have children for all kinds of reasons, and many of them—matters of culture, religion, or personal choice—are beyond the scope of most policy. However, economic considerations also play an important role. The Joint Economic Committee Social Capital Project aims to make it more affordable to raise a family. To that end, a commonsense first step would be considering removing regulations that, intentionally or not, create new obstacles for would-be parents.