Teenage mood swings are known to be down to hormones, but scientists claim they have identified the specific one that makes adolescents so volatile.
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A team from the State University of New York identified a hormone which normally acts to calm anxiety, but the effect is reversed in adolescence.
Writing in Nature Neuroscience, the researchers say it may be possible to reverse the puberty effect.
And they add the study should help parents and teachers understand teens.
A hormone called THP is normally released in response to stress.
It usually behaves like a tranquiliser, acting at sites in the brain that calm brain activity and, in adults and pre-pubescent children, helps someone cope with stress.
But a mouse study by the New York team shows THP actually increases anxiety at puberty.
'Things are harder for teenagers'
They found that the target for the hormone, a specific receptor, is more prevalent in the part of the brain which regulates emotion during puberty.
This appears to reverse the normal calming effect.
Dr Sheryl Smith, who led the study says it is so far unclear why this happens, but she suggests it is because of the action of all the other hormones which come into play at puberty.
Dr Smith and her team were able to genetically alter the receptor to reverse the puberty effect.
She said it might also be possible to block the effect of the hormone.
But Dr Smith said more research was needed before that was possible, and a more immediate benefit of the research might be to help parents and teachers understand what teenagers were going through.
"This research has revealed that there is a biological basis for a teenager's mood swings.
"They can be frustrating for parents and teachers - as well as the adolescents.
"As adults, we just deal with things, but it is harder for teenagers because of their biology. I think it's important for people to know that."