Similarly, statistics show that substances of abuse are much more readily available than they have been in the past, and unlike other generations, today's teens only have to send a text on a smartphone to gain instant access to a source of illicit drugs.
Scientists have a term for risk-taking, "suboptimal choice behaviour", and for the most part adults chalk up the suboptimal choices of teenagers to their impulsivity, irrationality, youthful egocentricity, or pervasive sense of invulnerability. Even Aristotle weighed in on "crazy" Greek adolescents more than 2000 years ago when he wrote that young people thought and behaved differently from adults because they were "passionate, irascible, and apt to be carried away by their impulses."
He concluded that teenagers are so self-focused, so unreasoning and so prone to feeling invincible that they never consider the possibility that they might hurt themselves doing something that adults would never do.
Yet "irrationality," "self-absorption," and "invincibility" are labels for an adult who would do these risky things. It is hard to use these terms in the same way with a teenager. No matter the evidence of their peculiar, sometimes infuriating behaviour, teenagers are not irrational.
Contrary to that popular misconception, a person's reasoning abilities are more or less fully developed by the age of 15. In fact, adolescents appear to be just as adept as adults in their ability to logically assess whether a certain activity is dangerous or not.
So why do teens do some of the crazy things they do? In general, teen brains have a more active sense of reward than adult brains and the release of, and response to, dopamine is enhanced in the teen brain. This is one of the reasons sensation-seeking is correlated with puberty, a time when the neural systems that control arousal and reward are particularly sensitive.
In adults, the frontal lobes can help inhibit risk-taking. However, the connectivity to the frontal lobes does not reach adult levels until the mid-20s.
So because the frontal lobes are still only loosely connected to other parts of the teen brain, adolescents have a harder time exerting cognitive control over potentially dangerous situations.
Adults also are better able to access a network of frontal brain areas than adolescents, whose brain regions engage in more "connectivity" to assess risks, rewards, and consequences.
In a study of 245 people, ages 8-30, University of Pittsburgh researchers monitored the ability of subjects to inhibit their eye movements. Instructed to look at a light on a screen in a dark room, the volunteers were told to look away from a second, flickering light when it appeared.
The natural tendency of the brain is to be curious and to follow the novel information - especially if it is forbidden. This response inhibition, as it is called by psychologists, is poor in children and much better in adolescents. In fact, by the age of 15, if teenagers are sufficiently motivated, they can score nearly as well as adults.
What fascinated the scientists was the difference in brain scans between adolescents and adults. Although adolescents scored similarly to adults, adults used far fewer brain regions but could engage their frontal lobes, and this made them better able to resist temptation. Hence, adolescents had to put much more effort into staying away from what was forbidden.
In another unusual brain-scanning experiment, scientists at Dartmouth College showed that adolescents use a more limited brain region and take more time than adults - about a sixth of a second more - to respond to questions about whether certain activities, like swimming with sharks, setting your hair on fire, and jumping off a roof, were "good" ideas or not.
Adults in the experiment appeared to rely on nearly automatic mental images and a visceral response to answer the questions. Adolescents, on the other hand, relied more on their ability to "reason" an answer.
The ability to quickly grasp the general contours of a situation and make a good judgment about costs versus benefits arises from activity in the frontal cortex, the parts of the brain that are still under construction during adolescence.
Adults are also better at learning from their mistakes, courtesy of areas in and around the frontal lobes including their developed anterior cingulate cortex, which can act as a kind of behavioral monitor and help detect mistakes.
During MRI experiments, when adult subjects make an error, their cingulate cortex lights up as if to say, "Oh boy, I'd better make sure not to do that again." This part of the brain is still being wired in teenagers, making it more difficult for them, even when they recognise a mistake, to learn from it.
The chief predictor of adolescent behaviour, studies show, is not the perception of the risk, but the anticipation of the reward despite the risk. In other words, gratification is at the heart of an adolescent's impulsivity, and adolescents who engage in risky behaviour and who have never, or rarely, experienced negative consequences are more likely to keep repeating that reckless behaviour in search of further gratification.
This reward-seeking impulse is located deep in the brain in two areas, the nucleus accumbens and the ventral tegmental area (VTA). These structures belong to the brain's pleasure centre because they are responsible for releasing dopamine when a person contemplates or anticipates a reward (eating food, obtaining money, taking drugs, etc.)
In effect, the nucleus accumbens alerts us to the possibility of pleasure and motivates us when we are in a position to experience that pleasure. It turns out that this area is much more susceptible to the powers of addiction in the adolescent brain compared with the adult brain. Addiction, therefore, is more strongly "hardwired" into the adolescent brain and, as rehabilitation centres well know, detox is much harder and fails more often in adolescents, too.
One thing you can do as a parent, guardian, or educator to help adolescents avoid giving in to the immediacy and the emotion of rewards is to talk to them about different kinds of risky behaviour. Whether it's drug experimentation or car racing, help them visualise the costs versus benefits through an analogy.
Let's say you want to underscore to your teenage son or daughter that no possible payoff of a risky behaviour is worth the chance of death; then ask something like, "Would you pick up a gun and play Russian roulette, even once, just for a million dollars?"
The dual motivation of immediacy and emotion is perhaps nowhere more engaged in adolescent behaviour than around sexual activity. This was especially evident in one infamous episode about 10 years ago at a New England prep school. The headline in the Boston Globe on Sunday, February 20, 2005, appeared on page one: "Milton Academy rocked by expulsions." The 200-year-old alma mater of T. S. Eliot was the scene of a teenage sex scandal involving a 15-year-old girl who, a month earlier, had performed oral sex on five varsity hockey players, ages 16-18, in a high school locker room.
A three-day investigation was followed by the expulsions of all five boys, a leave of absence and eventual transfer of the girl, and months of media scrutiny for the prestigious old boarding school outside Boston.
The school's spokeswoman, Cathleen Everett, told reporters the boys' actions were "outside common norms". At the same time, she also asserted, "Unfortunately, adolescents make big mistakes."
The Milton Academy incident was hardly the first at an elite private boarding school - it wasn't even the first at Milton Academy - but it did spawn a best-selling nonfiction book based on the case, Restless Virgins: Love, Sex, and Survival at a New England Prep School, written by two recent Milton Academy graduates.
In the book, Abigail Jones and Marissa Miley write how teenagers today no longer regard oral sex as "an intimate act between two established partners" but rather consider it "part of a larger high school culture in which sex and girls' deference to boys reigned".
Despite adolescents' acceptance, even expectation, of sexual activity - close to two-thirds of all high school students report having sex before they graduate, according to the Centers for Disease Control - it remains a high-risk venture precisely because adolescents disregard the risks associated with sex.
Although 80-90 per cent of teenagers report using contraception, nearly a third of girls aged 15-19 who rely on oral contraceptives admit they don't take the Pill every day.
And among men of the same age, only about half reported they always used a condom. It should come as no surprise, then, that risks to teens from sexually transmitted diseases are considerable.
The role of peers should not be underestimated when it comes to risk-taking behaviour in teens. The risk-reward system in the adolescent's limbic region works closely with nearby brain structures involved in processing not only emotions but also social information.
In her 2009 dissertation for a PhD in educational psychology from Temple University, Kathryn Stamoulis studied adolescent online risk-taking. The basis of her research was a survey of 934 American teens conducted by the Pew Internet and American Life Project.
Stamoulis found that social isolation for girls and a lack of extracurricular activities for boys increased risk-taking behaviour. In other words, socialising with friends or playing team sports appeared to have protective value in keeping teens out of risk-taking trouble.
What is key is that the brain areas involved in the perception of risk and the evaluation of rewards are closely related to the region that regulates behaviour and emotion. So here's the paradox: Adolescence is a stage of development in which teens are capable of superb cognitive abilities and high rates of learning and memory because they are still riding on the heightened synaptic plasticity of childhood.
These abilities give them a distinct advantage over adults, but because they are so primed to learn, they are also exceedingly vulnerable to learning the wrong things.
How does this happen? It all goes back to the brain's craving for rewards and the fact that anything that is learned, good or bad, that stimulates the production of dopamine is construed by the brain as a reward.
This means a little stimulation to a teenage brain whose synapses are firing all over the place leads to a craving for more stimulation that can, in certain situations, result in a kind of overlearning. The more commonly known name for this overlearning, when it is in the brain's reward system, is addiction.
• Edited extract from The Teenage Brain by Frances E Jensen, rrp $32.99. Available now.
