Laws designed to decrease the number of teenagers who die in car crashes appear to have cut fatal crashes among 16-year-olds, but may inadvertently have increased deaths among 18-year-olds, according to new research.

In an attempt to protect young drivers, every state and the District passed a law establishing “graduated driver licensing systems” or GDLs. The systems require young drivers to get a lot of experience driving in low-risk situations, such as only during the day or with an adult, before they “graduate” to a full license. But those laws only apply to those younger than age 18.

Scott Masten of the California Department of Motor Vehicles and his colleagues analyzed data collected between 1986 and 2007 on fatal crashes involving drivers between the ages of 16 and 19 in all 50 states and the District.

In a paper published in Wednesday’s issue of the Journal of the American Medical Association, Masten and his colleagues found that adoption of such programs appear to be associated with a drop in the number of fatal crashes among 16-year-olds. But deaths among 18-year-olds appeared to increase, the researchers reported.

The study did not examine why the programs would increase deaths among 18-year-olds. But the researchers speculated that it could be that the programs do not allow teens to get enough experience driving by themselves before they get their full licenses.

“Some important lessons of experience, such as the need for self-regulation and what it means to be fully responsible for a vehicle, cannot be learned until teens begin driving alone,” they wrote. “Under GDL, this now occurs at least six months later, reducing the time that young drivers have to learn from driving on their own before they turn 18.”

Another reason could be that some may skip the restricted license and simply wait to start driving when they are 18 and have no restrictions, they wrote.

In an editorial accompanying the research, Anne McCartt and Eric Teoh of the Insurance Institute for Highway Safety in Arlington said the study indicates more research is needed to explore the findings.

“To the extent that some of the positive effects at earlier ages may be blunted, this is a serious issue deserving attention by researchers and policy makers. It is likely that further reductions in crashes involving young drivers can be achieved by strengthening individual components of licensing laws,” they wrote.