U.S. researchers say a teen's brain undergoes a previously unsuspected biological makeover not complete until they're 25, and that could explain a few things.

Researchers at Temple University in Philadelphia say the findings may help explain why teens are more prone to crash a car and why they are more likely to engage in risky sex, drug abuse or delinquency, the Chicago Tribune reported Friday.

"The reason that kids take chances when they drive is not because they're ignorant," said Temple University psychologist Laurence Steinberg. "It's because other things undermine their better judgment."

Researchers had earlier found that a baby's early exposure to the outside world enables the brain to have a synaptic growth spurt. Synapses are the body's connectors that allow, for example, the brain to tell a hand to move.

The Temple University researchers, who used imaging technology to look inside the brain, say teens have a similar brain synaptic growth spurt -- synapses that are not incorporated into neural networks for memory, decision-making and emotional control are eliminated to make way for a leaner, more efficient brain in adulthood.

Copyright 2006 by United Press International