Hormones have long been blamed for mood swings in teenagers, even though the specific scientific causes have never been identified, making it hard to understand and treat adolescent angst.

Now, scientists have discovered that a hormone normally released in response to stress, a steroid called THP, actually reverses its effect at puberty, when it increases anxiety.

Sheryl Smith of the State University of New York Downstate Medical Center in Brooklyn reported the potential cause of pubescent mood swings yesterday in the journal Nature Neuroscience by a team led.

In adults, the hormone THP, tetrahydropregnanalone, normally acts like a tranquilizer, acting at sites in the brain that “calm” brain activity.

But the team found that in adolescent mice, THP acted on an unusual type of inhibitory receptor — where a receptor can be thought of as a kind of protein switch — and actually increased anxiety.

In adult mice, THP inhibits stress by acting on GABA receptors and calming the nerve activity. But in the adolescents, these GABA receptors increased dramatically at puberty in a part of the brain where conditions caused them to be inhibited by THP, making the brain more excitable.

In many species, puberty is the time to leave the home environment and make one’s way among strangers. Anxiety during adolescence could contribute to caution, increasing the odds of survival.

Further research is needed to determine whether THP has similar effects on GABA receptors and anxiety in human teenagers, though this seems likely: The human form of the GABA receptor was also inhibited by THP.

This adds to evidence that the adolescent brain is different from an adult one. Scientists used to think the brain stopped developing within a few years of birth, but MRI scans of adolescent brains published in the past decade have shown that not only is there major reorganization in the teenage brain, but it continues to develop until the early 20s.

Myelination, the growth of fatty insulation around nerves, which increases the speed of signaling between brain cells, continues during puberty and in some brain regions, including the prefrontal cortex.

This accounts for the behavior of television parodies such as Kevin the teenager, or Vicky Pollard, since this part of the brain is responsible for empathy and self-awareness.

Research has already shown that one effect of this brain reshaping is a 20% dip in the ability to judge emotions from faces. This may make teenagers less able to read social situations or empathize with others.

So, next time a Kevin or a Vicky fixes you with defiant stare or moans about feeling “soooooo tired,” spare a thought for the complicated events still taking place between their ears.