Adolescence has emerged as time to learn how to prepare for relationships, make sexual decisions

Teenagers have the reputation of being impossible to understand. One of the first men to try was G. Stanley Hall, whose work in the mid-1800s earned him the reputation as the founder of child psychiatry. His mission grew out of a general panic about what fellow worrier and writer, Frank O. Beck, described as “the tyranny of the special cult of adolescence.”

In those days, young adults were thought to be irrational, impulsive and even dangerous.

But maybe they were just misunderstood.

Hall and his colleagues had little in the way of historical perspective.

In fact, the concept of adolescence, a stopover between childhood and adult life, did not exist before the last two decades of the 19th century, according to a 1969 article in the Journal of Marriage and the Family. Before this time, American children worked alongside their parents on farms, albeit to a lesser extent.

When, in the mid-1800s, people began to move into more urban areas, kids could no longer contribute to the work and finances in the family and became isolated from them.

At the same time, city life meant more people in tighter quarters and easier access to others of their same age.

With more free time, greater professional diversity and more second-generation Americans in their teens, parents’ goals no longer dictated their chilren’s paths. And suddenly, there was a class of young adults who found more in common with each other than with their families. Sound familiar?

Americans may have discovered teenagedom, but they certainly didn’t invent it. Scientists now believe that the adolescent period, which is unique to humans, began as long as 200,000 years ago. While all other species enjoy a period of rapid development during childhood that then steadily tapers, only humans have a dramatic growth spurt during their teen years, according to skeleton and tooth analyses of fossils.

So why does this happen?

Barry Bogin of the University of Michigan-Dearborn suspects that the teenage phase evolved to improve their chances of reproducing, according to a 2005 article in New Scientist.

Because girls develop adult features several years before they reach their fertile peak, about age 18, Bogin believes that “the time between looking fertile and being fertile allows women to practice social, sexual and cultural activities associated with adulthood, with a low risk of having their own children.”

This gives young women time to prepare for motherhood. As evidence, Bogin cites that infant mortality rates for human firstborns are far lower than that of any other species.

Boys, on the other hand, are able to reproduce years before many of them look like men.

Bogin says this period ensures that they don’t become attractive to adult women and are therefore not perceived as a threat by adult males.

In this manner, they are able to reach sexual competency without having to fear their older competitors. Having practiced during adolescence, men entering adulthood are more likely to have success in reproduction.