The 'maths gender gap' was eliminated in the United States during the Bush administration under the No Child Left Behind program, and it has closed substantially in European countries and parts of Asia as well. Where do young women still lag in math? In societies with poor rates of gender equality, according to the American Economic Review.

The authors highlight that even Turkey, the only secular Muslim country, lags in math scores. Not being forced to wear a head scarf does not make women equal, and Turkey scores just 0.58  on the Gender Gap Index (GGI), an equality scale used by Western humanities scholars to reflect how well the rest of the world adopts progressive (Western) values.   Natalia Nollenberger, co-author and scholar with the IE University, said. "Use Turkey as an example of a country that has a poor rate of gender equality (0.58). Girls of Turkish ancestry underperform boys of Turkish ancestry by an average of 13.8 points. Our data show that if Turkey had a higher rate of gender equality closer to the average country of ancestry in our sample (0.69), then the maths gender gap between boys and girls of Turkish ancestry would disappear." 

The data were a sample of 11,527 15-year-olds who came from 35 different countries of ancestry and live in 9 different countries,  along with the Gender Gap Index in their country of ancestry. Maths scores come from Program for International Student Assessment (PISA), and the GGI comes from the 2009 World Economic Forum. They found that the more gender equality in the country of ancestry, the higher the maths scores of girls relative to boys living in the same country. The findings were significant and robust even when the researchers controlled for other individual factors that may affect youths' maths performance. In particular, the results show that an increase of 0.05 points (or one standard deviation) in the GGI is associated with an increase in the performance of girls in maths, relative to boys, of 7.47 points - equivalent to about one and a half months of schooling.  Their maths gender gap averages 16 points, the equivalent to four and a half months of schooling.  

Almudena Sevilla, co-author and Professor of Labour Economics at Queen Mary University of London explains, "These boys and girls grew up and live in the same country, but their parents came from elsewhere. So, differences in how well girls perform compared to boys - given that they are in the same environment and exposed to the same institutions - is likely to be the result of parents (or their social network) transmitting values about gender equality to their kids."

Previous evidence correlated societal factors and the maths gender gap, but establishing a firm cause. These authors go for it, suggesting that the performance gap is because girls raised in Western countries are taught to expect lower returns from maths in terms of the labor market - perhaps they face more discrimination in places like Europe so they don't expect to get jobs where math is needed anyway.