A new study from the U.S. goes a long way to confirm what many parents already suspected, roads would be a lot safer if there were fewer teenage boys driving, especially with their mates as passengers.
The study suggests that when a teenage boy is the front-seat passenger, a teenage driver, whether boy or girl, becomes more careless.
The study, by researchers from the National Institute of Child Health and Human Development, also found that when a teenage boy was driving and a teenage girl was in the passenger seat, he drove more safely.
However if a girl was driving with another girl as a passenger, she tended to be a bit less careful than a girl driving alone.
The researchers reached these conclusions after monitors watched the driving performances of students as they left 10 high schools in the Washington area.
While one team of monitors noted who was in the cars as the teenagers left the schools.
Another team, a short distance away, used equipment to calculate speed and whether the students were tailgating.
These two measures were used in the study to assess driving safety.
The researchers found that most of the students tended to drive a faster than the general traffic and to leave less room between their car and the car in front.
But these inclinations were more so when a boy was driving and another boy was in the front passenger seat.
Then, say the researchers, the drivers tended to go 15 miles per hour faster than the speed limit.
Of the teenage boys observed driving dangerously, one in every five had a male passenger, but only one in 20 had a female.
The risk-taking was greatest when teenage drivers of either sex were accompanied by a teenage boy, said researchers.
Overall, 14.9 percent of teenage boys and 13.1 percent of teenage girls were seen driving dangerously, which included speeding and tailgating.
The lead author of the study, Bruce Simons-Morton, chief of the agency's Prevention Research Branch, said it was unclear how passengers were affecting drivers, but says it was a surprise to find that females had a moderating effect on male teen driving.
In all, the researchers tracked 471 teenage drivers and compared their driving with a large sample of adult drivers.
Simons-Morton says the study shows that parents need to be extra vigilant when their teenage children are driving with other teenagers in the car.
The study appears online in the journal Accident Analysis & Prevention.