A recent study by the National Institute of Child Health and Human Development found that when a girl is in a car with a young male driver, he slows down and drives more carefully. The opposite is true if there is another young male in the car.
However, the same cannot be said for girls. It seems that when a teenage girl is alone in the car, she drives more safely. Put another person in the car, boy or girl, and a teenage girl will drive with less regard for safely
The Insurance Institute for Highway Safety, a research group funded by the insurance industry, reports that when there is a teen passenger in the vehicle with a teen driver, the risk of crash is twice as high. When you have two or more passengers the risk can climb to five times as high, according to the institute.
All but 17 states restrict the number and/or ages of passengers that new drivers can carry. Parents are the main enforcers for these restrictions as law enforcement generally ignore this restriction unless the teen is involved in some other infraction.