More Evidence on Car Seats vs. Seat Belts

Things move quickly in the modern world. Within two hours of posting my academic paper on car seats vs. seat belts on the Freakonomics web page (the first time this paper had seen light of day), another economist found the paper and tested its hypotheses on a very different data set and reported back the results.

The economist is Paul Heaton, a graduate student at the University of Chicago and a former co-author of mine. (Our affiliation is largely irrelevant here — he just happened to have data handy to test my results — but I mention it in the interest of full disclosure). The data he has are from New Jersey motor vehicle crashes. The big difference between his data and mine are that he has all crashes (even if no one dies. I only had access to crashes in which someone died. This difference is important, because a concern in the fatal crash data is what economists call “sample selection.” The choices people make about what safety device to use will affect whether they die, which in turn affects which crashes I see in my data.

Heaton replicated the most basic specifications in my paper. His results are remarkably similar. He found no difference in the death rates or incapacitating injuries for children in car seats versus children using adult seat belts. Like me, he found a slight advantage for car seats in preventing non-incapacitating injuries relative to adult seat belts (car seats offered a 10% improvement for these less serious injuries in both of our samples). The only difference is that in his data the gap in injuries between car seats and seat belts is statistically significant, whereas in my data set it was not.

When I compare my findings to the existing NHTSA estimates on fatalities, I can see how our approaches differ and why we get different answers. What is more puzzling to me is why my results and Heaton’s both suggest very little injury benefit of car seats, but the medical literature often finds 70% (!!) reductions of injuries with car seats relative to seat belts. We find reductions that are an order of magnitude smaller. They use very different methods — surveying people in the weeks after crashes for instance — but still it is really a puzzle. Which is why, when you read my paper, I am extremely cautious in interpreting the injury findings.

I hope that the medical researchers, Heaton, and I can all work together to try to make some sense of the conflicting results being generated by these different methodologies to resolve this important question.


Whenever I hear someone complain that cars aren't big enough for safety seats and that this forces parents into bigger vehicles I stop and think about a young couple who live in the same suburban community I lived in until a few months ago. This young, obvious poor couple drives an ancient car that seems to be held together by prayer and bungie cords.

The backseat of this little car has two things in it. A dark blue Hot Wheels booster seat and a bright pink Barbie booster seat.

No. No other adults can sit back there. No. Even if they could afford it, they can't put anything from Whole Foods in the back.

But their two school-aged kids are belted up in safety seats.

I regard arguments as to space as excuses. If this young, poor couple, can do it, so can a working class or middle class family.


Hi - original "Anonymous" poster re. car size back to respond to the idea that even young, poor people can have a large car if they get an old one. The deal is, if you have to get an old car to afford a big one, then perhaps your family would actually be safer in a smaller, but newer and more tech advanced car w/o car seats for the 7 year old kids. I don't know the answer to that, but you have to be willing to consider that tradeoff.


The argument that a newer, smaller car is better for protecting children doesn't really work: you're tossing decoys into the water and hoping the ducks won't notice the difference.

You're making the basic assumption that new and shiny=better and safer and that the safety of the car is somehow related to the reason we put grade-schoolers in booster seats.

Does a 7 year old minivan that's not sexy and doesn't look so hot provide more and better safety than a brand new Civic that does look kewl?

Does my poor, young family's car (If memory serves it was a little Toyote, about 1993 vintage or so.) actually provide less safety to a child in a back seat wearing only a safety belt than someone in a brand new Toyota?

The reality is that that question is a decoy. The safety seat advocates don't even address that issue because the point is a decoy. Their argument is that the safety belt itself causes injuries in kids: because their bodies are smaller, they tend to wear seat belts across their abdomens instead of their hips, leaving them to sustain internal injuries in an accident from the same seatbelt that kept them from flying about the car.

In other words, the safety of the car itself is NOT the primary reason for a booster seat. The design of the seatbelt is.

And seatbelt design hasn't changed in many, many years.

I stand by my original argument: rationalization is rationalization no matter how realistic your decoys are.




The reason we don't "legalize drugs and let the junkies do us all a favor" is because many drugs when used reasonably are safe and highly enjoyable, and the government is only interested in citizens pursuing happiness rather than actually achieving it. Mellow nation is hardly fit for world domination. Now, why we need world domination, and what government doesn't want world domination?


It seems clear that both car seats and seatbelts protect children better than no restraints, so did you consider that young kids who are in car seats are usually unable to undo the more complex harnesses themselves whereas kids in seatbelts are able to get out of them more easily. In that case, having a child in a carseat _does_ protect children better than seatbelts.

Stephen J. Dubner

Anonymous said...
It seems clear that both car seats and seatbelts protect children better than no restraints, so did you consider that young kids who are in car seats are usually unable to undo the more complex harnesses themselves whereas kids in seatbelts are able to get out of them more easily. In that case, having a child in a carseat _does_ protect children better than seatbelts.

I'm very glad you raised this point, Anon. It is in fact a common misconception that kids can undo seat belts but not car seats. The fact is that car seats are held in place *with* the seat belt, so undoing the seat-belt buckle -- which my daughter started doing regularly at age 2 -- results in a really bad situation: the child is in her harness, in her car seat, which is unattached to anything else. So there's the illusion of restraint but in fact there is no restraint at all (except the harness, which in the event of a crash would mean that she only has a car seat strapped to her back to give added weight as she flies through the windshield, e.g.).



I am sorry to have insulted you when I claimed that schlepping the seat between cars was a big deal. It isn't, however even small amounts of time are economic concerns. One of the questions is, what is the net value of the seat, and if that value could provide more safety in other manners. EG if it takes me 5 minutes to move 2 car seats, does that put me 5 minutes behind that I need to make up by driving x% faster. That may not ever happen with you, then again I would doubt that you are a target of the laws that are being questioned. Also from Drubners comment above, while it make be just as easy to unhook a car seat harness, I heard years ago by a car seat advocate that children in boosters were more likely to sit comfortably due to the fact that they were higher, could see the road, outside the windows and into the front seat. Thus they were less likely to unhook and stand in their seats. Which is an action has the potential to create chaos in the vehicle and distract the driver. I don't think that the data in your analysis is useful for this question.



Imagine three children:


A is in a car seat. B is in a seat belt. C is in neither. You are obsessing over A vs. B. Isn't the question of how to increase the percentage of kids who are A or B and not C infinitely more important than quibbling over whether A or B is better?

Wouldn't a real economist be interested in how to decrease the size of C, rather than nitpicking over A vs. B?

You have an unbelievable affinity for trivia, even when you stumble on real issue (see abortion research).

Stephen J. Dubner

Anon. said:
Wouldn't a real economist be interested in how to decrease the size of C, rather than nitpicking over A vs. B?

Dear Anon:
If you read the NYT article, you'll see that we explicitly encourage restraint of whatever variety vs. non-restraint.


"I don't think that our public resources should be spent on ticketing people who choose not to put their children in car seats, refuse to wear helmets, or insist on doing stupid things unless they are putting others in harms way."

You forgot that choosing to not put your child in a car seat is putting another in harm's way while refusing to wear a helmet is putting yourself in harm's way. Your child is another - he or she isn't yourself. It's like the way brushing your kid's teeth won't keep your teeth clean, putting a sweater on your torso won't keep your kid warm, etc.


Dubner: If you read the NYT article, you'll see that we explicitly encourage restraint of whatever variety vs. non-restraint.

What New York Times article?

Stephen J. Dubner

Anonymous (10:03) said...
Dubner: If you read the NYT article, you'll see that we explicitly encourage restraint of whatever variety vs. non-restraint.

What New York Times article?

Sorry if there's been confusion here. The blogging on these pages have been in support of this NYTimes article:

We also have a page elsewhere on this website in support of the article:


Steven -- As long as you are on a automotive safety jag, please take a look at the cost/benefit of front airbags versus seatbelts. I've known a number of engineers who worked on airbag development, and almost all doubted that airbags would be in cars if the FDA, say, rather than the NHTSA were performing the analysis. One actually removed airbags from his wife's car. (In contrast, I believe a good case can be made for side bags, as alternatives such as belts offer essentially no protection in side crashes)

One problem, of course, is that an airbag can add energy during a crash, particularly if the driver is small and sitting close to the steering wheel. A large number of fatalities have been reported of small women and children from airbag deployment, and I suspect that these are seriously underreported because of selection bias. The airbag fatalities reported are almost always in such low speed collisions that almost no other explanation is possible. In contrast, in higher speed crashes, the EMTs or police shake their head and think, "Even the airbag couldn't save them." The police reports that NHTSA statistics are based on then reflect this. As the injury mechanism is often a momentarily lifting of the skull from the spine with consequent cord damage, and as it takes a very good autopsy to reveal this, I suspect that the actual number of fatalities from airbag deployments is a multiple of 2 or 3 of the official NHTSA statistics. Huelke's work at the University of Michigan is probably a good place to get background on this.

Three-point belts offer almost as much protection as front airbags, and four-point belts would offer a further improvement. At an annual cost for airbags in new vehicles in this country of about $7 billion, it's almost impossible for me to believe that an economic case can be made for their mandatory fitment.


Tom West

One possibly relevant point: Does the imposition of child-seat laws increase the number of children who are buckled up at all? In the A,B,C explanation above, perhaps legislating A is the most effective way to decrease C.

Anecdotally, I know a doctor who now does hip operations on children who were wearing seatbelts in accidents full-time because they are so professionally "challenging".

It's the booster seat for my 6 year-old.

Mrs. Coulter

It would seem that many of the posters have little familiarity with the actual universe of car seats.

There are three basic types of car seats:

Infant seats (those are the ones you see people toting little babies around in)
Convertible seats
Booster seats

Infant seats are generally good for only about the first 6-8 months of a child's life, since they have a weight limit of around 20lbs, and a height limit of 26-28", depending on the manufacturer.

The next stage is a convertible seat (really, except for preemies, even a newborn can sit in a convertible seat, but the convertible seat doesn't pop out so you can use it as a convenient carrier). Convertible seats can rear face or forward face; infants under one year *and* 20lbs should rear face (the seat then absorbs much of the energy in a collision, rather than the head snapping forward).

Once your child has achieved the 20lb and one year milestones, you can turn that seat around so that the child faces forward. Most seats are rated to either 40 or 60lbs, depending on the model; a toddler is supposed to ride in this seat until s/he is 40lbs *and* 4 years old.

After crossing that milestone, the next stage is a booster seat. The laws/recommendations on booster seats vary widely by state. I believe I have seen 8 years *and* 80lbs as the recommendation, but I have also read that it is height that matters more than weight.

In terms of cost, the really expensive seats are the convertible toddler ones, though perfectly acceptable convertible seats are available in the $60-70 range. However, you are going to use that seat for nearly four years, so you might as well get the best one that you can afford.

In any case, I would never drive anywhere without my one-year-old tightly strapped in her car seat. Otherwise, she'd be climbing all over the back seat and probably into the front. I know plenty of people who bring car seats onto airplanes not for the safety benefit, but because then they know that the child will stay seated in one place and not protest and try to get down and run around.

I have trouble visualizing how a two-year-old strapped in with sufficient tightness (have you ever been to a car seat check? appropriate tightness is tighter than you think!) could undo the seat belt restraint on the car seat we own, which not only requires you to undo the seat belt but also to unclip two locking mechanisms. Nevertheless, children can be amazingly resourceful when it comes to self-endangerment. I personally fear that mine will figure out what the little red button in the middle of buckle does (why does it have to be such an obvious and appealing color?) and refuse to listen to all pleas to leave it alone.

I suppose that I can accept that the marginal safety of a six-year-old is not significantly improved by a booster seat vs. a seat belt, but I just can't see how a toddler isn't safer in car seat.

Lastly, to the parent vacillating over whether his teenaged daughter will be safer in an SUV or a passenger car: from what I've read, SUV drivers are more likely to do risky things because they feel overconfident in their large, tank-like vehicle. This effect is magnified when the driver is inexperienced, which means that SUVs are even more dangerous for teenagers.



Perhaps the data is too sparse (or just plain missing), but there are two important technological advances that may be improving the argument for the car seats (at the infant stage at least).

Newer cars are required to have a standard anchoring system for car seats. I've never used these because our cars are too old so I can't comment from experience, but in theory they should reduce misinstallation (that was the design goal) and child-initiated release.

Second, for the last 6ish years cars & car seats have had rear anchors to prevent the seat from rocking forward in a crash. These are pretty hard to bungle completely (and pretty easy to get completely right) & should restrain a kid in a crash & be immune from child's fingers. They _MIGHT_ even improve things if the seatbelt isn't attached at all; it would seem logical they help when it is loose.

So, the test would be if these innovations really help -- are deaths & serious injuries lower with their use than without. Ideally one would have some police organization collect really detailed stats on all this to be analyzed carefully.



Not being in the US, and not being a parent...

I cannot imagine taxis fitting and re-fitting kiddie seats all the time, nor can I imagine the law being so ridiculous as to ban kids from them, so what happens when kids travel in taxis?

Stephen J. Dubner

Anonymous (9:40 p.m.) said...

I cannot imagine taxis fitting and re-fitting kiddie seats all the time, nor can I imagine the law being so ridiculous as to ban kids from them, so what happens when kids travel in taxis?

There are a number of exemptions to car-seat laws including taxis, school buses, etc.


First of all, the poster who says that seat belts haven't changed much in the last 10 years or so hasn't ridden in the back of many small sedans, I think. 10 years ago, a lot of cars still only had lap belts in the middle position, and many had only lap belts in the side positions. Lap belts are known to actively *cause* fairly serious injuries in children. However, there are few boosters that work safely with lap belts. So in older cars with only lap belts, there really was very little difference between belts and boosters.

IN your review of the literature on this topic, did you read this?
It discusses the full history of what kinds of restraints are out there - its dated, but it addresses some of the questions being raised here about injury vs. fatality, lap vs. lap/shoulder, etc.
The other problem is that those belts in the back need to be designed to fit everyone from a 4 year old just out of a forward facing carseat to a 200 pound adult. What's safest for one can make the belt less safe for the other.

Finally - to say that instead of requiring booster seat use, we shoudl require car manufacturers to develop seat belts that adjust to accommodate everyone from 3 feet to 6 feet, from 40 pounds to 200 pounds, from age 3 to age 90 - isn't that just shifting those same millions of dollars from the parents to the car companies? And won't the car companies pass that cost on to *everyone*, not just parents, in the form of higher car costs? Past attempts by car companies to include built-in child restraints haven't been well recieved by the market; I think almost all of them have been dropped as too expenseive to continue designing into every car.



Just a small note... Once a child reaches 30 lbs, s/he can be strapped into the car's lap/shoulder belt with one of these: $13.56 regular price and they go on sale frequently and can be had for a few dollars less. It's just not true that the only choices for the over-2 set are a $200 carseat or strapping the child into the car's seatbelts with no other adaptation.

cathy :-)