Car Seats as Contraception

Fertility rates in Western countries are near historic lows. Last year, not a single Western country was above the replacement level of 2.1 children per women.

Various factors have been proposed as potential causes of low fertility in the West, including female emancipation, the decline of religiosity and post-materialist values. One that has received little attention until recently is car seat laws.

In a fascinating preprint, Jordan Nickerson and David Solomon argue that car seat laws have prevented more than 130,000 births in the US since 2000. How have they done this? The proposed mechanism is simple. Since most cars cannot fit more than two child seats, if a couple with two children wants to have a third, they have to buy a much larger car. Yet doing so is impractical or unaffordable for many families, so they stop at two.

The authors begin by noting that car seat laws have become much more stringent over time. Between 1980 and 2020, the average age (depending on the state) when a child can ride without a car seat has increased from zero to eight, with most of most of the increase having occurred since the mid 1990s.

Note that in Europe, car seat laws are even more stringent: children must ride in a car seat until they are 1.5m – which corresponds to an age of about 12!

Nickerson and Solomon then report their main findings. Using data from the US census, they ran a model of the probability that a woman gives birth in a given year. The predictor of interest was an indicator for whether the woman had two children below the age mandate in the relevant year and state.

They found that compared to women who had at least one child above the age mandate, those who had two below were significantly less likely to give birth in a given year. Moreover, they effect was limited to households with a car – as you’d expect based on the proposed mechanism.

How large was the effect? Women with two children below the age mandate were 0.73 percentage points less likely to give birth in a given year, which represents a 7.8% drop relative to all women with two children. While this might not sound like much, it’s similar to other effects in the literature. For example, one study found that compulsory schooling reduces teen births by 8.8%.

Car seats clearly aren’t a major contributor to low fertility in the West, but it’s likely that fertility would be in their absence. Interestingly, the authors also examined their impact on car crash fatalities and found that it was small, amounting to only 57 in the year 2017. This means that for each life saved, 140 third births were prevented.

This comparison would suggest that stringent car seat laws may well fail a cost-benefit test. Like many policies, they sound good – but you always have to think about unintended consequences.