In February 2006 Britney Spears earned worldwide opprobrium after photographs surfaced of her driving a car while holding her infant son in her lap. Spears’s failure to strap the tot into a child car seat provoked consternation reminiscent of that which had greeted Michael Jackson three years earlier when he dangled his baby over a balcony railing. The Los Angeles County sheriff’s office consulted with local child-welfare officials before deciding not to press charges.

When your car stops suddenly, you keep moving. Seat belts are meant to keep you from flying into the dashboard or windshield, or the seat in front of you. The modern seat belt, which includes both lap and shoulder restraints, is designed to distribute the force of impact across the pelvis and rib cage to reduce the risk of internal injury. But adult seat belts are too large to properly restrain and protect small children. Moreover, during collisions, infants are particularly vulnerable to organ damage and injuries to the head and neck, so they need specially designed protection.

Today’s child car seats have been refined through crash tests, computer simulations, and lawsuits, as well as by the lessons learned from thousands of tragedies. But the child car seat started out as a convenience rather than a safety device. The first ones, which reached the market in the 1920s, were essentially sacks with drawstrings attached to the back seat. They were meant to keep young children in view of the driver and prevent them from getting loose and causing trouble, rather than to protect them in case of a crash.

Decades later, the parents of the first Baby Boomers, embarking on family vacations on America’s budding interstate highway system, could choose from more sophisticated and stylish child car seats, but their occupants weren’t much more likely to survive a crash. Well into the 1960s the typical child car seat remained little more than a legless, thinly padded high chair, sometimes with a toy steering wheel. An article in the March 1965 issue of Consumer Bulletin took a dim view of most child car seats on the market, though it praised the Tot Guard, from a Massachusetts company called Grow-Rite. Tot Guard seems to have been among the first child car seats available in the U.S. designed with more than a cursory nod to safety. Compared with the competition, it featured a more secure harness, extra padding, and flexible head-and-neck support.

As the nation’s highway death toll climbed toward a peak of more than 55,000 in 1972, however (the total for 2004 was 42,636, with more than twice as many passenger miles), states began passing mandatory seat-belt laws. These highlighted the inadequacy of adult seat belts for children, as well as the dubious safety benefits of most child car seats then on the market. In 1971 the National Highway Traffic Safety Administration imposed modest design and performance requirements on child car seats.

In the mid-1970s the American Safety Equipment Corporation was one of the first manufacturers to conduct extensive crash tests on its child car seats. Its $39.95 model included a harness equipped with a shield that distributed the force of an impact across the occupant’s pelvis and abdominal area. Yet the scope for truly detailed testing was limited, because in the 1970s most testing labs did not have child-size crash-test dummies. Instead they used dolls, which lacked the joint structure and sophisticated internal monitoring equipment of the adult dummies. This meant engineers did not have reliable data on what might happen to a small child’s body during high-speed impact. The situation changed in the 1980s, when facilities such as the Cornell Aeronautical Research Laboratory expanded the crashtest-dummy family to include models that reflected the size and physiology of infants and small children.

Yet these improvements meant little when less than 10 percent of families used child car seats. In 1978 Tennessee became the first state to require their use, though the law specified a number of exceptions, such as permitting adults to hold small children in their laps. Within a decade, all 50 states and the District of Columbia passed laws mandating some form of child restraint in automobiles. Most manufacturers began offering rear-facing seats, which provide better protection for the baby’s head, neck, and spine in head-on collisions. (In fact, rear-facing seats would be safer for older children and adults too, but lawmakers understandably balk at requiring them.) Parents could either upgrade to a larger, forward-facing seat as their kids grew or opt for convertible models that could be installed facing in either direction. Other models for infants featured a detachable base and an upper portion that could double as a baby carrier or be fitted into a stroller.

Today’s child car seats surround their occupants with even more padding and use harnesses proportioned for small children. While adult restraints usually consist of one strap across the lap and another that runs diagonally across the rib cage, harnesses on child car seats typically feature

two parallel shoulder straps that are buckled to a third strap that runs between the legs. Since the 1970s most child car seats have been made from injection-molded polypropylene, which is strong and lightweight and can be molded into structurally complex forms. And innovation is continuing: Earlier this year, the winning entry in the “American Inventor” television series was a new type of child car seat based on nested spheres.

No matter how well designed a child car seat is, its effectiveness is limited if it is installed improperly, which is often the case. For most of the last 30 years, virtually all child car seats were secured in place with the car’s seat belts. Installation was tricky because seat belts varied from car to car, and cramped passenger compartments made it hard to get enough leverage to pull everything tight. To simplify and standardize installation, NHTSA required that all vehicles and child car seats manufactured after August 31, 2002, adopt the LATCH (Lower Anchors and Tethers for Children) system. In this, the top of a child car seat is attached to a built-in anchor behind the car’s rear seat and the bottom is attached to built-in anchor points between the rear-seat cushion and the seat back.

Once kids outgrow their car seats, NHTSA recommends putting them in booster seats so they can use the car’s regular lap and shoulder belts. A diminutive child with extremely safety-conscious parents could conceivably hit puberty before graduating to grown-up seating. The back seat is the safest place for all passengers, regardless of age, and NHTSA specifically recommends keeping children in the back seat until age 12. After that, parents have four worry-free years until their son or daughter is eligible for a driver’s license.