WASHINGTON - GIRLS can do just as well at mathematics as boys - even at the genius level - if they are given the same opportunities and encouragement, researchers reported on Monday.

Their study, published in the Proceedings of the National Academy of Sciences, contradicts studies showing girls can do as well as boys on average in math - but cannot excel in the way males can.

They also said it is a clear rebuttal to Larry Summers, who as president of Harvard University said in 2005 that biological differences could explain why fewer women became professors of mathematics. Mr Summers is now chairman of the Council of Economic Advisers for President Barack Obama.

They did a statistical analysis comparing various math scores and contests with the World Economic Forum's 2007 Gender Gap Index. This annual report ranks countries according to employment and economic opportunities, education and political opportunities and medical status.

The United States ranks 31 out of 128 nations on the World Economic Forum index.

Janet Mertz of the University of Wisconsin in Madison said no one disputes that at the average level, girls perform as well as boys mathematically.

But at the top levels, disparities persist and some experts have said this is do to the 'greater male variability' theory - the idea that males in general are more likely to score both extremely high and extremely poorly on tests than girls are.

If there were a biological reason for the differences, this would have to hold everywhere, she said. But it does not.

Several different international tests show the same pattern, including the International Math Olympics, Ms Mertz said.

'If girls don't have equal educational opportunities or if they know if they learn the material there won't be jobs available to them, why bother, they seek something else,' she said. -- REUTERS