From Hollywood stars demanding equal pay to the BBC cutting male journalists’ salaries, the gender pay gap has featured prominently in the news in recent months. We have all heard the typical explanations for it: “women have babies”, “women do more housework”, “they don’t negotiate”, “they opt out of the workforce”.

Many studies have tried to explain why men are paid more. The first set of explanations explain these pay discrepancies through individual differences. They argue that women’s pay is lower because they have fewer years of education or they are not studying the right topics, or because they have fewer years of training and on-the-job experience. This is mostly explained through work interruptions due to parental leave, childcare duties and unequal distributions of housework.

The second set of explanations focus on occupational differences and argue that men and women work in different jobs and sectors: women are concentrated in positions with less money, less authority and less autonomy. When a job becomes a woman’s job, overall pay for the job declines. To make things worse, women also face a motherhood penalty: where mothers are paid less than their non-mother counterparts.

Even though these explanations look very different, almost every study on the pay gap has one thing in common: they all focus on adult workers. However, according to the Department of Labor’s Report on the Youth Labor Force, many teenagers work part-time while still in school and work starts as early as 12. Therefore, the work experience – and potentially the pay inequality – starts long before the completion of formal education and the start of real jobs.

In my new book on the pay gap, The Cost of Being a Girl: Working Teens and the Origins of the Gender Wage Gap, I focus on this substantial yet often neglected part of our economy: tween and teenage workers. Focusing on younger workers is like a social laboratory: many commonsense explanations used to dismiss discrimination claims are simply not applicable. Most 12-year-olds are not married; they don’t have children. And 12- to 16-year-olds also have the same education and same skills.

Using data from the National Longitudinal Study of Youth, when you look at 12- and 13-year-olds, they have no gender inequality in pay. However, when they reach 14 and 15, we see the emergence of the first wage gap. Some individual factors matter: the gap is wider with age. Also, race matters. African American girls and young women experience a wider gap. Types of jobs also matter. Boys and girls start out in freelance jobs (babysitting, snow removal and yard work), but as more employee-type jobs become available, boys move into them and girls stay longer in freelance jobs. The rest is the cost of being a girl.

The pay gap starts much earlier than previously reported

Even within freelance jobs, male babysitters are paid more than their female counterparts. Babysitting girls have more unpaid hours, do more extra chores and have lower pay. Male babysitters tend to have few unpaid hours, are not asked to run errands and are more likely to be asked how much they charge. When girls ask for more money they are less likely to get it than boys.

Within service and retail jobs, things are not much better for girls. It may look like girls are doing similar jobs to boys, but they are more likely to be asked to deal with challenging customers, and more likely to be asked to get in debt to fit the brand’s image. When they ask for more money, they are more likely to be told: “You’re here for the discounts.”

Overall, the pay gap starts much earlier than previously reported. Our commonsense explanations and trusted theories fail us because they fail to explain pay inequality among very young workers. If we want to solve this persistent problem, we need to better identify the origins and the mechanisms through which these early gender differences are created and reinforced.