A little-known region of the brain has been pinpointed as a key factor in the transformation of mother’s little darling into a rude and moody adolescent.

Scans of adolescent brains have shown that the length and intensity of their tantrums correlates directly with the size of their amygdalas. The bigger the amygdala, a region in the brain linked to anger, the bigger and more aggressive the fights with the parents are likely to be, according to research.

Teenagers with smaller amygdalas were likely to be passive, but those with an expanded version were identified as "nightmares."

Researchers identified the link between belligerence and the size of the amygdala when they carried out brain scans on 137 children who had been arguing with their parents.

Volunteer families were put in rooms and asked to talk through sensitive issues. Among the topics that quickly prompted fights, even in the laboratory, were homework, lying, bedtime, talking back, and the use of the Internet and cell phones.

Children with the largest amygdalas displayed the “longest sustained aggressive behavior," said Nicholas Allen of the University of Melbourne in Australia. “Some of the behavior of young adolescents isn’t driven simply by the environment. There’s also some biology involved.”

The findings, published in the journal Proceedings of the National Academy of Sciences, were part of a study to identify early warning signs of mental problems, including eating disorders and depression.

As children enter adolescence, their brains undergo radical rewiring to help them to cope with all the changes their bodies are undergoing, including puberty and hormones. Human brains continue to develop into a person’s early 20s, and many of the built-in neural safeguards against losing their tempers have yet to be put in place.

Allen said that children ages 11 to 13 had been chosen for the study to monitor behavior immediately after the onset of adolescence. “What we were able to do is look at the relationship between the kids’ biology, the brain structure and their behavior during their interactions with their parents,” he said.

“We were interested in the relationship between the brain structure and the kids’ behavior. The amygdala was one of the most important parts.”

Allen said that the research also cast light on why teenagers who one day approached tasks with a maturity beyond their years could act with immaturity the next. “Your 6-foot-2 son can manage some very complicated work yet still do these dumb things," he said.

“If you talk to parents of early adolescents, a lot of them will tell you they have quite a job adjusting to the changes in their child," he said. "These children can be moody, they can be argumentative, they can be angry. The brain is undergoing a lot of change. “These kids are still struggling to adjust — they are a work in progress.”