The teenager is a uniquely human phenomenon.
Adolescents are known to be moody, insecure, argumentative, angst-ridden, impulsive, impressionable, reckless and rebellious. Teenagers are also characterised by odd sleeping patterns, awkward growth spurts, bullying, acne and slobbish behavior. So what could be the possible benefit of the teenage phase?
Most other animals - apes and human ancestors included - skip that stage altogether, developing rapidly from infancy to full adulthood. Humans, in contrast, have a very puzzling four-year gap between sexual maturity and prime reproductive age. Anthropologists disagree on when the teenage phase first evolved, but pinpointing that date could help define its purpose.
There are a variety of current explanations for the existence of teenagers. Some believe that we need longer for our large brains to develop. Other explanations suggest that a teenage phase allows kids to learn about complex social behaviour and other difficult skills, or that it is even required to develop coordinated bipedal bodies adapted to travelling long distances.
Raging hormones
Scientists once thought that the brain's internal structure was fixed at the end of childhood, and teenage behaviour was blamed on raging hormones and a lack of experience. Then researchers discovered that the brain undergoes significant changes during adolescence.
According to many recent studies, teen brains really are unique (see interactive graphic). Though many brain areas mature during childhood, others mature later - such as the frontal and parietal lobes, responsible for planning and self-control.
Other studies have shown that teens fail to see the consequences of their actions, and that sudden increases in nerve connectivity in teen brains may make it difficult for teenagers to read social situations and other people's emotions.
Risky behaviour
One study in 2004 showed that teens have less brain activity in areas responsible for motivation and risk assessment, perhaps explaining why they are more likely to take part in risky activities such as abusing drugs and alcohol, develop a hard-to-kick smoking habit or indulge in under-age sex.
Teenage pregnancies and rising rates of sexually transmitted diseases among teens are big problems - especially because today's teen generation is the biggest the world has seen: a 2003 UN report revealed that 1 in 5 people were between 10 and 19, a total of 1.2 billion people.
But not everyone agrees on the best way to tackle the problem. Some believe that comprehensive sex education is the key, while others argue for abstinence only education courses.
