I can find a lot of web sites that make this claim, I can find no scientific studies that make this claim and the web sites don't cite any. I have found this relevant study: Intended and Unintended Effects of Youth Bicycle Helmet Laws by Christopher S. Carpenter and Mark Stehr. It is a long study packed with statistician jargon. In brief it uses DOT fatality data from 1991 - 2005 along with CDC helmet usage data from the same period. Their conclusion is this:

"4. Discussion and Conclusion
The results above show that state laws adopted over past two decades that require youths to wear helmets when riding a bicycle reduced youth bicycling fatalities by about 19 percent, increased helmet use by 20-34 percent, and (unintentionally) reduced bicycling by 4-5 percent."

You can read the entire report and you will see that there are some uncertainties about the exact values yet the conclusion is pretty clear, helmet laws do produce the decreases that you say they should produce. Previous studies that failed to show this were very much smaller and less sophisticated. I ultimately don't care if you or anyone else here wears a helmet. Do what you want to do and Godspeed to you! And I do not support mandatory helmet laws or any similar laws. What I do with my life is none of the government's business. I do support government efforts to standardize and improve the safety equipment on the market for all activities. I do support possible government efforts to make motor vehicles safer for cyclists and pedestrians. And I do support government efforts to produce and disseminate valid scientific reports on the effects of helmets, etc, so that people can make informed decisions. I do support efforts by cyclists to promote helmet usage among themselves. It seems to me the information is out there, it is clear, and you ignore it at your risk.

Ken