In the past few weeks, we have been bombarded with news from all over the world on gender inequality in the workplace.

From Hollywood to media to politics, many sectors point to unequal pay in the workplace as well as other problems such as sexual harassment. Unequal pay has been a systematic problem of workplaces and women's lives. A wide range of discipline and approaches have offered explanations to this persistent problem. Some have focused on the women and have argued the women have lower pay because of their own characteristics: They study different topics, have lower education, less job experience because they leave the workforce due to childcare and parental leave. Some have focused on occupational characteristics: Women and men are concentrated in different jobs, different sectors and different positions.

Women's positions tend to pay less and have less authority. No matter how they looked at the pay, there always remained an unexplained portion: the cost of being a woman.

When I studied these dominant theories while researching my book, I sat at a coffee shop, where a teenage barista brought my coffee. It occurred to me at that coffee shop that we were looking at this problem all wrong. Even though the focus of the theories seemed different (workers vs. jobs), almost all the studies on the wage gap studied the same population: the adult workforce.

However, work experience does not begin with the completion of formal education. Many teenagers work while still in school, as working part-time while still school is a quintessentially American phenomenon. Therefore, work experience, and potentially the wage gap, starts long before the start of "real" jobs.

In The Cost of Being a Girl, I look at a substantial yet previously neglected portion of the workforce: teenage workers. Focusing on this group includes a previously understudied portion of our workforce to offer a more comprehensive understanding.

More importantly, the teenage workforce is like a social laboratory: At these early ages these typical explanations of the wage gap — "women have babies," "women leave the workforce," "women do more housework" — are not relevant.

If we look at 12- and 13-year-olds: They do not have spouses. They don't have children. Boys and girls are at the same education and skill level. What happens when we look at the wage gap?

In my study of teenage workers, I found that all else being equal, 12- and 13-year-old boys and girls make the same amount of money, doing mainly freelance jobs like yard work and snow removal.

Once they reach 14 and 15, we see the emergence of the first wage gap, which widens with age. Especially for young girls of color, the wage gap is larger. One contributing factor is that as employee-type jobs become available, boys move into them, whereas girls stay longer in freelance jobs.

Freelance jobs like babysitting are easy to find through personal contacts, but they also make it harder to ask for more money. In such jobs, workers internalize not negotiating and asking for more money because when they do they are not likely to get the raise, and fear being seen as manipulative. The boys I studied, on the other hand, were considered more professional, get more respect, have fewer unpaid hours and are more likely to get raises.

Even in retail and service jobs, it is not much easier for girls. They are more likely to be in positions where they deal with demanding customers and are more likely to change their looks to fit the look of the company. These aesthetic requirements for girls put pressure on them to buy the products they sell, which can result in substantial credit card debt. Many experience sexual harassment, yet do not report it because they say  the positions are not their ultimate career.

These early work experiences have important lasting effects. Many of our teenagers are given positive messages at school and at home: you can be anything you want. However, these messages may fall flat because of their first-hand experience at work. When we send our teenagers to work, they see the systematic biases and experience firsthand the problems of the workplace. We need to change the workplaces: make pay information accessible and make job descriptions clearer in the workplace.

Yasemin Besen-Cassino is a Professor of Sociology at Montclair State University. Her new book, 'The Cost of Being a Girl: Working Teens and the Origins of the Gender Wage Gap' is published by Temple University Press. @besencassino