There are plenty of explanations for teenage turmoil. The newest theory is that uneven brain development may be responsible for the changeable moods and unsettling behavior of adolescence, reports the July issue of the Harvard Mental Health Letter.

Although many teens have fairly advanced intellectual and reasoning ability, recent research has shown that human brain circuitry is not mature until the early 20s. Among the last connections to be fully established are the links between the prefrontal cortex — the seat of judgment and problem-solving — and the emotional centers of the brain. These links are crucial to emotional learning and high-level self-regulation, explains the Harvard Mental Health Letter.

Another circuit still under construction in adolescence links the prefrontal cortex to the midbrain reward system, where addictive drugs and romantic love exert their powers. Brain scans hint at why most addictions get their start in adolescence. Teenagers and adults process reward stimuli differently; adolescent brains react intensely to novel experiences, making those experiences more enticing.

Hormonal changes are at work, too. The adolescent brain pours out stress hormones, sex hormones, and growth hormone, which in turn influence brain development.

Teenagers’ problems have many causes — social and individual, genetic and environmental. Since the brain is still forming, things can go wrong in many ways, and some of them involve the onset of psychiatric disorders. Scientists are looking at typical adolescent brain development to provide clues to the ways in which things go wrong. “The more we know about how psychiatric disorders and adolescent problems develop, the easier it will be for us to develop better treatments,” says Dr. Michael Miller, editor in chief of the Harvard Mental Health Letter.