What makes teenagers moody and impulsive? The answer used to be raging hormones plus a dearth of(短缺) life experiences. But three years ago this simple equation was blown apart by evidence from brain scans of strange goings-on behind the teenage forehead. Till then, scientists had thought the brain's internal structure was fixed by the end of childhood. The new scans showed the brain's frontal cortex(皮层) thickening just before puberty(青春期), then slowly shrinking back to normal during the teenage years. Suddenly, the erratic huffiness(发怒) seemed to make sense: the teenage brain was a work in progress, a house in the process of being rewired. Now comes more evidence of neural turmoil. According to psychologists in California, the speed with which youngsters can read the emotional expressions on people's faces dips suddenly at around the age of 11 or 12 and takes years to get back on track. The latest study, like the brain scan research before it, is a welcome and necessary part of building up a picture of a typical teenage brain so that scientists can get a better handle on what might be happening in the mental illnesses that appear to be afflicting children and adolescents in ever greater numbers. But there are dangers. Scientists still have no idea how to interpret the subtle changes seen in adolescent brain scans. Yet in the wrong hands, these findings could be used to justify hothousing, impulse control training and other dubious attempts to get the most out of malleable teenage brain cells. The science could also spark a new wave of moralising based on a perceived need to protect teenagers' evolving brain connections from evil or toxic influences. Incredibly, some scientists have already suggested in the press that the brain scan evidence somehow proves that it is biologically bad for teenagers to play video games or lie on the couch watching MTV. A hundred years, ago one well-known 'expert' urged teenage boys to drink six to eight glasses of hot water a day to flush impure thoughts from their bodies. Have we really learned so little? In the past it was thought that teenagers were moody and impulsive because of______.