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 linked to anger, the bigger and more aggressive the rows with the parents are likely to be, according to research.

Teenagers with smaller amygdalas were likely to be delights to have about the house but those with an expanded version were identified as real nightmares.

Researchers identified the link between stroppiness 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 rows, even in the laboratory, were homework, lying, bedtime, talking back, and the use of the internet and mobile phones.

Children with the largest amygdalas displayed the “longest sustained aggressive behaviour”, Nicholas Allen, of the University of Melbourne in Australia, said. “Some of the behaviour 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 twenties, and many of the inbuilt neural safeguards against losing their tempers have yet to be put in place.

Professor Allen said that children aged 11 to 13 had been chosen for the study to monitor behaviour 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 behaviour during their interactions with their parents,” he said.

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

The area of the brain controlling all the other regions is at this stage switching from the greater limbic region to the prefrontal cortex, and researchers believe that this developmental stage is when many of the seeds of mental illness are sown. By better understanding the processes by which the brain grows, researchers hope to gain insights into the causes and potential treatments of psychiatric disorders. A further finding was that in boys the ratio of assymetry of the anterior cingulate cortex and orbitofrontal cortex was linked to aggressive behaviour.

Professor 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 6ft 2 son can manage some very complicated work yet still do these dumb things. ‘What were you thinking?’ has been asked by every parent of teenagers,” 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. 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.”

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