In about '78, Joan Claybrook, Jimmy Carter's Sec. of Transportation announced that there would be airbags. There were several proposed specifications. One of these specs said that an airbag should be able to save the life of a 200 pound man in a 30mph head on collision even if he wasn't wearing a seatbelt. Detroit pointed out that this spec resulted in airbags which came out fast and hard (200mph) and would kill smaller people. They wanted a spec that was for people who were belted in. Joan insisted that they would find a way get past the laws of physics and make it work, and passed the law.
There have been numerous studies of airbag effectiveness, most done by universities as far as I know. Transport Canada found that airbags reduced the death rate for men by 11%, increased the death rate for women by 9%, and increased the death rate for children by 21%. They decided this means airbags reduce the death rate by 2% (11% - 9%, ignore the 21% number entirely, don't you just love new math?) In this study, Transport Canada, which had originally estimated that airbags would save 400 lives per year, found that in fact over a seven year period they had saved no more than 300 lives total. Since 300 is apparently 11%, we can conclude that in the same period they killed 245 women and an unknown number of children.
Most studies conclude that airbags have no effect on the death rate for adults which are not belted in. All studies conclude that airbags increase the death rate in children under the age of 12 or 13, belted in or not. Typically it's found that the death rate for kids increases by 20% or so.
In about 1990, Leonard Evans, a retired General Motors researcher said: "We already know that 158 people have been killed in the United States by air bags in crashes in which they would otherwise not have been seriously injured or, in many cases, not injured at all."
According to statistics released by the NHTSA in 1999 air bags were determined to be the cause of death for 146 people -- 84 children and 62 adults. You'll note that this number is not in agreement with the number the GM researcher released almost 10 years earlier.
In 1998 the law was finally changed, we now have "second generation airbags," which really means the spec changed and now airbags come out slower. Also, the car companies were allowed to put in the passenger-side override switches. Finally, the NHTSA recommended that children under 13 should ride in the rear seat. As of Jan 1 2005 it's illegal in California for kids under the age of 6 to be in the front seat. Laws are killing our kids? No problem, pass more laws.
Draw your own conclusions. My conclusions: 1) yet another example of gross incompetence on the part of the government, 2) if your wife weighs less than about 140 pounds and you like her, you want the airbags in her car disabled, and 3) you want your passenger side airbag disabled pretty much at all times. The exception would be if your passengers nearly always weigh more than 150 pounds and are nearly always taller than about 5'6".
None of this analysis applies in Europe, btw. Their original airbag spec called for deployment to save the lives of people belted in who weighed about 175 pounds. Now I understand they have their own second-generation airbags which are two-stage, faster deployment in higher speed crashes, slower in low speed crashes. I haven't followed the studies in Europe, but I understand their airbags have killed far fewer people, it's never been an issue over there.