If you’re a teenager, don’t read this. Federal scientists may have discovered a biological excuse for laziness.

Studies conducted on adolescents and young adults show significant differences between the two age groups in the brain region that governs "drive," the internal momentum to work for a reward.

This region, barely active in adolescence, apparently doesn’t come into its own until the early 20s.

Scientists at the National Institute on Alcohol Abuse and Alcoholism used brain scans to test whether the developing teenage brain is any different from the mature brain of an adult when faced with an opportunity to make money.

James Bjork and colleagues found that as adults worked to make money in a research task, their brains experienced an increase in blood flow and volume in the nucleus accumbens, a region deep in the middle of the brain.

"We actually see the anticipation for winning," Bjork said.

In contrast, adolescents between ages 12 and 17 who performed the same research task had half the blood flow and volume in this region, Bjork said.

"We have got to take seriously how big these developmental differences are," said Dr. Hans Breiter, director of motivation, emotion and neuroscience at Massachusetts General Hospital. "This is a beautiful piece of work."

When the volunteers received their rewards, the scans showed activation of another brain region, and there was no difference between the age groups in this part of the study.

"It tells us that teenagers love stuff, but aren’t as willing to get off the couch to get it as adults are," Bjork said. "The good news is that the brain does mature" and these motivational circuits become more active.

The findings appear in this month’s Journal of Neuroscience. Scientists recruited 12 adolescents from 12 to 17 years old and 12 young adults, 22 to 27 years old. Both males and females were tested. The volunteers were taught to play a computer game where they worked to win money, the amount varying between 20 cents and $5. Volunteers in both groups succeeded about two-thirds of the time. All the while, they were hooked to a functional-MRI that measures blood flow throughout the brain.

A second test challenged them to avoid losing $20 they were given at the start of the game — the same computer task, but with a different motivation. In this study, the nucleus accumbens was not activated in either age group. Dr. Daniel Hommer, chief of the section on brain imaging at the institute, suspects that the idea of winning money is more of an energizer or motivator than trying to avoid losing money.

The federal scientists found that blood flow and volume increased with age, a suggestion that motivation increases gradually with age in a normal developmental process.

So, parents, just wait. They’ll get there.