Do car seats prevent the birth of children? In the case of car seat laws, the statistics suggest that they do. This is a case where the law of unintended consequences applies.
The law of unintended consequences, often cited but rarely defined, is that actions of people—and especially of government—always have effects that are unanticipated or unintended. Economists and other social scientists have heeded its power for centuries; for just as long, politicians and popular opinion have largely ignored it.”The Library of Economics and Liberty
Car seat laws are relatively recent; It was only in 1986 that all 50 US states required car seats. Over time, the age and size requirements for car seats has gone up. Jordan Nickerson of the University of Washington and David Solomon argue that car seat laws have prevented 145,000 births since 1980.
How? One possible explanation is that it is very difficult to find a car that will fit three car seats in the backseat, and purchasing a new car can be cost prohibitive (having a child isn’t exactly inexpensive in the first place). Certainly Nickerson and Solomon make a good case for it- they found that the likelihood of having a third child in any given year was .73% lower (small- but significant considering that the baseline likelihood is only 9.36%) for women that already had 2 children in car seats.
Small percentage- but cast it over all of the women considering having a third child in the US during a given year, and the number looks larger. Their study estimates that in 2017 car seat laws saved 57 lives and prevented the birth of 8,000.
In Montana, car seats (child restraint systems- so booster seats included) are required if a child is under the age of 6 and less than 60 lbs. Montana does not require children ride in the backseat. So even in Montana, the argument that women are likely delay (or opt against entirely) a third child until having one doesn’t require three children in car seats has face validity.
Whether or not Nickerson and Solomon are right about the reason (and their research looks fairly solid to me), they are certainly correct in observing the decline in birth rate and fertility in the United States, though car seat laws are probably only one among many contributing factors.