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I'm looking after my eight-year-old nephew while my brother and his wife are on vacation, and they insist that I put him in a booster seat in my car. They're helicopter parents who don't let the child play in the front yard by himself and they can't actually tell me why he needs to be in the thing, other than, "It's required." Seat belts, I get – they keep us from being thrown out of the car. What's the purpose of the booster seat, other than treating children like babies for as long as possible? – Tara, Edmonton

Sure, a seat belt keeps you from hitting the windshield – or flying through it – but that's not its only job.

"Contrary to popular belief, the main purpose of a seat belt is not to keep people from being ejected out of a vehicle," said Takuro Ishikawa, a doctoral candidate at the University of British Columbia. "It redirects crash forces to the strongest parts of the body, the hips and chest."

And, like most things in your car, apart from maybe the back-seat entertainment system, seat belts are designed for grown-ups.

"Kids between 4 and 8 are too small and the seat belt usually ends up on their belly and across the neck," said Ishikawa, whose research is probing the attitudes that keep parents from using booster seats. "That's redirecting crash forces to the internal organs or to the neck."

In a crash, that force could be 544 kilograms (1,200 pounds). If the shoulder belt has slid off, or behind the child, the child jackknives over the belt and that crash force gets directed entirely to the abdomen – resulting in severe internal injuries to the spleen, liver and bowel, which have been collectively called seat-belt syndrome.

For younger children, car seats – if they're properly installed and used – are designed to protect children where the car can't. Rear-facing car seats support babies' weak necks in a crash. When they grow out of them, they move to forward-facing car seats.

"There are forward-facing car seats that are made for children up to 30 kg," Transport Canada says on its website. "Even if your child weighs more than 18 kg (40 lbs) and your provincial/territorial law says you can use a booster seat, your child is safer in the forward-facing car seat as long as he or she is still below the car seat's weight and height limits and fits in the car seat correctly."

After that, enter booster seats. They raise children so seat belts fit – and protect – where they're supposed to. They're required everywhere in Canada except Alberta, the Northwest Territories and Nunavut.

Generally, booster-seat requirements are based on height (shorter than 145 centimetres, or 4 feet 9 inches), weight (less than 36-45 kg, or 80-100 pounds, depending on the province) or age (between 7 and 10).

In reality, some kids may not meet the height or weight requirements until they're 11 or 12.

The Canadian Paediatric Society (CPS) says to check the upper weight and height limits posted on your booster seat.

"In a collision, children using seat belts instead of booster seats are 3-1/2 times more likely to suffer a serious injury and four times more likely to suffer a head injury," it says on its website.

Helicopter parenting in the minivan?

Are we holding children back by keeping them in booster seats longer? A car is one of the places where we should be treating kids with kid gloves, said developmental child psychologist Mariana Brussoni, who has done research on the benefits of childhood risk for the BC Children's Hospital Research Institute.

"Letting kids climb the monkey bars to test their limits has developmental benefits," Brussoni said. "But in the case of cars and booster seats, the child is not playing and they don't get any developmental benefit from not being properly protected in a crash."

Car crashes are the No. 1 cause of death by preventable injury for Canadian children between the ages of 1 and 9.

Provinces without booster-seat laws have the highest rate of childhood motor-vehicle related fatalities compared with the Canadian average, a paper presented last month at a CPS conference said.

In Alberta, that rate was 4.18 per 100,000 people in 2012, compared with 2.27 in Ontario.

In the United States, a 2012 paper by the Insurance Institute for Highway Safety looked at crash data from five states – Missouri, North Carolina, Pennsylvania, Wisconsin and Wyoming – that had passed laws for booster seats for older children.

After the laws were in place for two years, booster-seat use tripled and there was a 17-per-cent reduction in deaths and serious injuries.

So if booster seats save lives, why are 50 per cent of children who should be in booster seats, riding in cars with just seat belts? Ishikawa thinks it's because parents don't understand the purpose of seat belts. His research is testing that. "Before I started studying this, I was absolutely sure that the point of seat belts was to prevent ejection," he said. "I suspect more people may be like me, and if they are, then we can target that with education."

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