Everything About Car Seats

I am not so old that I do not remember car seats as a child. However, I definitely do not recall them as a permanent fixture in the car. There were many, many car trips with three kids and my grandmother in the back seat, the smallest child held (buckled!) in her lap. I doubt my family ever used a booster seat in the car, and by 8 or 9 I was permitted in the front seat. To be clear, my family was on the risk-averse end — seatbelts at all times, defensive driving, and so on.

Car seat rules have evolved over time. In the present moment: children under age 2 are in infant seats, usually facing backward. Age 2 to 4 brings the giant five-point-harness car seat (when we finally disposed of the last of these, I can only describe my husband as gleeful). And in many places, kids up to age 8 are still required to be in booster seats. Recommendations often suggest continued booster use up until the age of 12.

These regulations have accelerated since the 1980s, and especially since the mid-1990s. In a somewhat surprising fact, a 2020 paper argued that the regulations have impacted fertility. The idea is that people do not want to have a third child because they cannot fit in another car seat. Seriously, that’s what the paper says. Almost a 1 percentage point drop in third births! Once you hit kid number three, you’ve got to go to a minivan. Maybe it’s not worth it?

For the most part, car seat behavior is dictated by these regulations. But many people wonder why these are in place — what is the evidence? Beyond this, we may wonder whether it makes sense to go beyond the recommendations — if the recommendation is that a child be rear-facing until they are 2, should you go further? 

I want to focus on these two questions. 

• What’s the evidence on front- versus rear-facing for kids, especially those above 1 year?
• What is the evidence on booster seats for older kids?

Car seat transition #1: Front- versus rear-facing

Infants sit in an infant seat, and they face backward. This is understood and accepted, and, indeed, they will not let you leave the hospital until you show that your child is in the car seat correctly. It has always been recommended that children continue to face backward until the age of 1. 

Recently, this age has been moved to 2, and many people feel it should go further. We may reasonably ask: why 2 years? If rear-facing is safer, why should there be an age cutoff at all?

The idea is driven by the biomechanics of what would happen if you were in a car accident. Tests with crash test dummies sized between 1 and 3 years (i.e. this paper) suggest less risk of injury (to the dummies) with rear-facing than forward-facing car seats. These tests are very convincing, since they are run in controlled environments, with dolls equipped with fancy accelerometers.

Translating this to data in the real world has been more of a challenge. In 2007, a very influential paper was published comparing injury rates in crashes between children using forward- versus rear-facing car seats. You can see the paper here. Using data from the National Highway Traffic Safety Administration, the authors argue that children ages 12 to 23 months are five times as likely to be injured in a side crash if in a forward-facing seat, and twice as likely overall. This paper played a large role in the push to staying rear-facing for longer.

However: this paper has since been retracted.

In 2017 the journal was contacted by an outside researcher who had been unable to replicate the analysis. The original paper writers then revisited their analysis and found that they were also unable to replicate it. The issue, according to the journal, was thought to be a mishandling of survey weights, which caused their sample size to seem bigger than it was.

The authors retracted the paper and redid the analysis, now with more years of data, publishing an update here. Their conclusion was that while the direction of the effect was still in favor of rear-facing seats, it was not significant and the effects were small (0.5% of children 12 to 23 months in forward-facing seats were injured, versus 0.2% of those in rear-facing. But the actual number of injuries was only 14 and 3, much too low to get any significance).

This is all consistent with a world in which the biology/mechanics/theory shows that rear-facing has advantages but the real-world data suggests these may be quite small. And there are some tradeoffs: as children get larger, facing backward becomes more cramped and uncomfortable. Kids who get carsick facing backward may feel better facing forward. There is a balance here. If your child is happy facing backward, it’s great to leave them. If they are puking and screaming on every car ride, it may make sense to turn them sooner. 

Car seat transition #2: Out of the booster seat

Second big question: Does your child need to be in a booster until they are 8 or 10 or 12? My older kid is quite tall, and by the time she moved into a real seat, she was as tall as some short adults. Was there any real point in maintaining the booster for so long?

(Note: I’m focusing here on “boosters” in general, although most kids spend some time in five-point-harness car seats, which combine a booster seat with a harness structure. Here, I’ll tackle the question of transition out of any booster option.)

The arguments behind a booster seat are also based in biomechanics. Children are short. Seatbelts are built for taller people. This means they are less suited to protecting little kids in an accident than protecting adults. Booster seats (or “belt-positioning boosters,” as they are sometimes called) are designed to get kids tall enough to optimize seatbelt functioning.

A number of years ago, Steve Levitt (of Freakonomics fame, but also a friend of mine) wrote several papers arguing that, in fact, booster seats didn’t protect children from serious injury or death. This conclusion was subject to a lot of criticism and scrutiny. Notable to me, though, from a parenting standpoint, is that the papers do find that boosters reduce less-serious injuries. I would leave my kid in a booster just for that.

But even if you aren’t as concerned about less-serious injuries, follow-up work suggests larger safety improvements including for older kids. One set of example papers are this one and this one, which use data from State Farm Insurance on younger and older children to examine the effect of booster seats on injuries. The authors argue that use of booster seats reduces injuries, even for kids 4 to 7. An obvious complaint with both papers, though, is that they use parent-reported injuries and parent-reported seat use. Variations in honesty or memory could bias results.

Better, I think, is this paper, which relates variation in state booster laws to estimate the relationship between boosters and accident fatality rates. The authors find that fatality rates among 6- and 7-year-olds dropped in places that introduced booster laws for those age groups, relative to those that did not. (They find similar results for 4- and 5-year-olds, but there is less variation in those groups, so the results are weaker.) I have some quibbles with the paper (I do not like their figure presentation), but this is a good approach to the problem and an important finding.

There isn’t any real data I can find on booster seats for kids beyond the age of 8, partly because they aren’t legally required (even if they are recommended), which makes it hard to have any true variation (more-careful drivers are likely to keep children in boosters for longer, for example). But important to note is that it’s not so much about age as size. A very tall kid may be ready to be out of a booster before a short one.

The upshot is that even though boosters are annoying, my read is that even in the absence of regulations, it is a good idea to keep children in them until they are fairly old.

The bottom line

• For any car seat transition, it’s not really about age but size. Especially on the issue of boosters, the concerns arise because children are shorter than adults. This means that some of the choices about boosters should reflect the size of your kid.
• The most dangerous thing is for kids to ride unrestrained. No seatbelt or restraint is much worse than an incorrectly used one. This might be obvious, but in case it is not: Click it or ticket!

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