Car seat laws are acting as a form of contraception, a study has found, as women with two children are eight per cent less likely to have a third as they can't find enough space in the vehicle.
The legal age at which a child must ride in a car seat has been increasing gradually in the USA since 1977.
This means mothers are having to wait longer for their first two children to grow out of them before having a third child as most cars don't accommodate a third seat.
While the legislation resulted in 57 children being saved from fatal crashes in 2017, it also led to 8,000 fewer births occurring that year, according to researchers from Massachusetts Institute of Technology and Boston College.
Third child fertility has been reduced by 7.8 per cent overall, their analysis estimated, with 145,000 fewer births in total since 1980.
Prof Lauren Jones, from Ohio State University's department of human sciences, said the decline in third child fertility was an "unintended consequence" of trying to keep children safe in the event of a car accident.
"Here we find a great example of a policy whose intended purpose – protecting children in car crashes –had a profound effect on a seemingly unrelated family decision – fertility," she told The Telegraph.
"I am sure no legislator who passed a law to expand car seat requirements for older children suspected that these laws could diminish a family’s capacity to have a third child. However, the authors have nicely demonstrated that the laws – which in most of the US now require children as old as eight to be restrained in child seats – have diminished third-child birth rates by up to 10 per cent.
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"As a new mother who has recently tried to wrestle just one car seat into the back of my mid-sized car, I can attest to the fact that it would be nearly impossible to have three children in car seats without moving up to a much larger car. This could certainly discourage many families from having a third child."
Professor Jones said the researchers' cost/benefit analysis shows that the number of third children which would be born if car seat laws weren't so strict would be far greater than the lives saved by car seats.
"This confirms my own work showing that increasingly strict child safety seat laws save relatively few children from car crash fatalities each year – likely fewer than 100 per year across the US. This number is very small compared to births women would have had if not for the car seat requirements," she said.
"Weighing the life-saving benefits of these laws against the costs of prevented births leaves us with what the authors call a “puzzle” – should policy weigh a life saved so much more heavily than a life prevented?"
However, Professor Jones said injuries potentially prevented by car seat laws, in addition to lives saved by them, should be taken into account.
"We do not yet have good estimates as to whether car seat laws have reduced child injury in crashes. If the laws do prevent injury, their benefits may be much larger than what this study considers," she said.
Weighing the life of an existing child against that of a potential one is a minefield of "philosophical and religious complications" which are "beyond the paygrade of most economists", she added.
To view the study, which is yet to be peer reviewed, click here.
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