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Teenagers are a puzzle, and not just to their parents. When kids pass from childhood to adolescence their mortality rate doubles, despite the fact that teenagers are stronger and faster than children as well as more resistant to disease. Parents and scientists alike abound with explanations. It is tempting to put it down to plain stupidity: Teenagers have not yet learned how to make good choices. But that is simply not true. Psychologists have found that teenagers are about as adept as adults at recognizing the risks of dangerous behavior. Something else is at work.

Scientists are finally figuring out what that “something” is. Our brains have networks of neurons that weigh the costs and benefits of potential actions. Together these networks calculate how valuable things are and how far we’ll go to get them, making judgments in hundredths of a second, far from our conscious awareness. Recent research reveals that teen brains go awry because they weigh those consequences in peculiar ways.

Some of the most telling insight into the adolescent mind comes not from humans but from rats. Around seven weeks after birth, rats hit puberty and begin to act a lot like human teens. They start spending less time with their parents and more with other adolescent rats; they become more curious about new experiences and increasingly explore their world. Teenage rats also develop new desires. It’s not just that they get interested in sex but also that their landscape of pleasure goes through an upheaval.


Miriam Schneider, a behavioral pharmacologist who studies adolescence at the University of Heidelberg, and her colleagues recently documented this shift. The scientists ran an experiment on a group of rats of varying ages, allowing the animals to drink as much sweetened condensed milk as they wanted. The amount of milk they drank, relative to their body weight, stayed fairly constant through their prepubescent youth. But when they hit puberty, they started to drink much more. Once they became adult rats, their rate of milk drinking dropped and then stayed steady as they got older.

To any parent who has observed a teenager guzzle a bottle of soda, this spike would look awfully familiar. But the behavior of adolescent rats is not simply the result of their being bigger than juveniles. Schneider and her colleagues trained their rats to press a lever in order to get a squirt of milk. The rats had to press the lever dozens of times before they were rewarded with a single sip, and each successive sip required two more presses than the previous one. This requirement allowed Schneider and her colleagues to measure just how much work the rats were willing to put in for a reward. They found that pubescent rats would press the lever much more often than rats of any other age, putting in far more work for the calories they were getting, given their size. In other words, they valued the milk more.

A number of other experiments support Schneider’s results. Whether rodent or human, adolescence makes us add more value not only to sweet drinks but to all sorts of rewards. A team led by Elizabeth Cauffman, a research psychologist at the University of California, Irvine, who studies antisocial behavior in adolescents, documented this shift with a game of cards. She and her team had volunteers play a simple gambling game with pictures of four decks of cards on a computer screen (pdf). At each turn of the game, an arrow pointed to one of the decks. The volunteers could either turn over a card or pass. Each card had a different amount of money on it—“+$100,” for example, or “-$25.” The goal of the game was to win as much of the imaginary money as possible.

The scientists had stacked the decks. Two of the decks had more losing cards than winning ones, and the reverse was true for the other two decks. When people play these games, they unconsciously shift their strategies as they see more cards. They pass more on some decks and take more cards from others. Cauffman and her colleagues tracked the strategies of 901 volunteers ranging in age from 10 to 30 years old and compared the teenagers with the other age groups. Across all ages, the older the volunteers were, the more they shied away from using the losing decks. But the scientists found a different pattern when it came to the winning decks. Adolescents tended to play the winning decks more often than adults or preteens. In other words, they were unusually sensitive to the reward of winning money but the same as others when it came to the risk of losing it.