For parents of emotionally combative teens, new research offers a powerful biological reason for all the family feuding -- adolescent brain size.

A team of Australian scientists has found that when key regions of the brain known for controlling emotions are bigger, boys and girls tend to be more aggressive and more persistent during their fights with Mom and Dad.

"This is a bit of a unique study," said study author Nicholas Allen, an associate professor with the Orygen Research Centre at the University of Melbourne. "Because we've shown for the first time that in terms of aggression -- not physical, but being argumentative and unfriendly -- some of the differences in the way teen kids interact with parents are biologically based. The adolescent is developing, their brain is developing, and there's a link between the two."

The finding was published in this week's online issue of the Proceedings of the National Academy of Sciences.

The authors first videotaped 20-minute "problem-solving" discussions with 137 Australian teens between the ages of 11 and 14 and one of their parents. The interactions were spurred on by the introduction of provocative family issues, and analyzed for language and emotional content.

Following MRI scans of the teens' brains, Allen and his colleagues observed that children with large amygdala regions were more likely to engage in longer and more aggressive arguments with their parents.

They also reported that male -- but not female -- teens possessing an atypical unevenness in the size of two left brain regions -- the anterior cingulate cortex and the orbitofrontal cortex -- were also more likely to maintain aggressive behavior, as well as whiny and anxiety-laced behavior (collectively referred to as dysphoria).

In an attempt to explain gender variations in left-right brain asymmetry, Allen noted that left- and right-side brain regions are normally different in size, but that such differences are typically bigger in boys than girls.

"We think that when the difference is actually less than it usually is among boys -- but not girls -- this contributes to some mental health problems and, perhaps, aggressiveness," he said.

"But there's relatively little research -- almost none -- that shows a relationship between brain structure and actual behavior," Allen added. "So we can't say what the direction of causation is. It could be that environment has influenced teen brain structure. Or that brain structure is influencing their relationship behavior with the parents. Or it could be both. More and more, I think we are realizing that it's not a matter of a simple equation. It's a question of an interaction of both environment and biology. But we don't yet know how these two areas interact."

Elliott Albers, director of the Center for Behavioral Neuroscience at Georgia State University in Atlanta, agreed that many key questions remain.

"This study is very interesting and consistent with prior animal studies which support the concept that there are changes in limbic structures that can potentially relate to aggression," he said.

"However, we still don't know if conflict causes physical changes -- whether the social experiences of boys and girls cause different changes in their limbic system that can affect aggression -- or whether it's the other way around. Frankly, we still don't know much about what causes the neuroscience of aggression," Albers said.