An international study into why boys tend to perform better at mathematics than girls has concluded that both sexes are equally adept but that cultural factors hold girls back.

The research, led by Professor Paola Sapienza of Northwestern University, found that on average girls perform worse than boys when studying maths subjects.

However, in Scandinavian countries, where female equality is much higher, there is no difference in average peroformance and in some cases girls outperform boys.

By contrast, in societies where inequality is high, the disparity in scores increases markedly.

"The so-called gender gap in maths skills seems to be at least partially related to environmental factors," said Professor Sapienza.

"The gap does not exist in countries in which men and women have access to similar resources and opportunities."

Typical factors holding women back are a lack of resources, and ingrained lower expectations.

For example, Mattel once produced a speaking Barbie which said: 'Maths class is tough.' The company withdrew the product in 1992.

The study also looked at reading differences between the sexes. Overall, girls slightly outperform boys in reading skills. But, where gender inequality is lowest, the girls came out significantly ahead.

"Our research indicates that girls in more gender equal societies will gain an absolute advantage relative to boys," said Professor Sapienza.