The gender pay gap is still wide open. The World Economic Forum’s Global Gender Gap Report 2015 shows that globally, women are still paid on average just above half the average male wage.

While governments and international institutions are making efforts to tackle it, the gender gap has yet to decrease significantly. In fact, according to a UN report, if the pay gap continues to reduce at its current rate, it will take another 70 years to reach wage equality.

A new study from the National Bureau of Economic Research (NBER) looks into the reasons behind the pay gap and finds two key factors play a part.

Where are the women in STEM?

According to researchers Francine D. Blau and Lawrence M. Kahn, the lack of women in certain industries and specific jobs contributes to the gender pay gap.

“Significantly, women continue to lag in the STEM fields, particularly in mathematically-intensive fields.”

Women are significantly underrepresented in the science, technology, engineering and mathematics fields. In the US, women hold less than a quarter of all STEM jobs. That figure is even lower in the UK, where women make up less than 15% of all people working in STEM jobs. On a global level, women make up only 28% of research and development workers.

Why such a big gap? Some say it's because women aren’t studying STEM subjects in the first place. This educational gender gap then results in an imbalance at the company level. The findings of the NBER report confirm this: “Gender difference in college major have been found to be an important determinant of the pay gap between college-educated men and women.”

Perceptions of motherhood

The second reason behind the gender gap, according to the NBER report, is that women’s career choices are being affected by motherhood and family.

“Current research continues to find evidence of a motherhood penalty for women and of a marriage premium for men,” the report notes. Mothers were perceived to be less competent and less committed to work, the report found, and are often offered lower salaries.

This "motherhood penalty" has been shown in a separate study from the Chartered Management Institute. It found that women who return to work following maternity leave experience lower pay and fewer promotions for decades after their return. Mothers are also encouraged into part-time or less challenging roles.

The study primarily focuses on the US, but the researchers have noted that many of the findings are applicable to other countries.

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