How do teens alternate between shoplifting a case of beer, then "borrowing" a car and at other times scoring the winning goal or singing the National Anthem at perfect key?
The answer, of course, is that their brains are a work in progress, still churning out new bundles of cells and knitting them together with connections that may not be fully fashioned until they're well into their 20s.
Researchers are finding more and more evidence that those cells and connections play multiple roles in developing not only self- control and forethought, but personality, social skills and, on the downside, mental disorders.
A recent study of brain cells in mice done at Yale University showed that when the creation of new cells is interrupted during adolescence, adult mice later show impaired social behavior, basically ignoring other mice.
"These mice acted like they did not recognize other mice as mice," said Arie Kaffman, an assistant professsor of psychiatry and senior author of the study.
When the same cell generation process was blocked in adult mice, there were no similar behavioral changes noted, the researchers said.
Significantly, schizophrenics have difficulty generating new brain cells in the hippocampus, one of the key areas for neuron development. And many of the symptoms of schizophrenia first start to emerge in the teen years.
Most researchers think the hallmark of brain maturity is the growth and repair of white matter, a process known as myelination, which creates speedier, more efficient connections between frequently used regions of the brain.
Scientists know that some key areas of the brain, such as those around language centers near the back of the brain, tend to get connected early, before 13 or so. But connections are much slower to settle in at the front regions of the brain that govern inhibition, high-level functioning and attention, among other things, often not fully networked until the mid-20s or even older.
When myelination is impaired or becomes damaged, it can result in mental retardation or developmental disabilities. A recent study by scientists at the Children's National Medical Center found a gene that helps regulate this process and which could be a key for understanding the origin of many such disabilities.
Brain scans done of more than 100 people aged 5 to 32 at the University of Alberta confirmed that brain wiring in the frontal lobe continues well beyond the teen years. Researchers speculate in a recent issue of the Journal of Neuroscience that the continued development of the brain in young adulthood is needed because people in their 20s have so many novel experiences to process -- college, a first job, living independently and forming new social and family relationships.
But there's also evidence that the growth of brain connections in teens helps improve function. As more white matter makes teen minds work more quickly, it literally makes their brains seem smarter.
Scientists at the University of Texas at San Antonio analyzed results of 12 intelligence and mental speed tests administered to nearly 7,000 teens aged 13 to 17.
Writing in the journal Psychological Science, the team found that older teens consistently score better on the tests than younger adolescents, but they also worked faster at solving problems, all related to how fast the brain is able to work with steadily improving white matter.