From http://www.physorg.com/news99239898.html:
"A popular stereotype that boys are better at mathematics than girls undermines girls’ math performance because it causes worrying that erodes the mental resources needed for problem solving, new research at the University of Chicago shows.
The scholars found that the worrying undermines women’s working memory. Working memory is a short-term memory system involved in the control, regulation and active maintenance of limited information needed immediately to deal with problems at hand.
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“Likewise, our work suggests that if a girl has a mathematics class first thing in the morning and experiences math-related worries in this class, these worries may carry implications for her performance in the class she attends next,” she added.
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Researchers have been aware that stereotypes can undermine achievement in schools in many ways, but little research has focused on the specific mental processes that prompt this response.
In order to examine those mental processes, the team selected a group of college women who performed well in mathematics. They were then randomly assigned to two groups, with one set of women being told that they were being tested to see why men generally do better on math than women, and the other group being told simply that they were part of an experiment on mathematics performance.
The information that men do better in mathematics than women undercut performance drastically. The accuracy of women exposed to the stereotype was reduced from nearly 90 percent in a pretest to about 80 percent after being told men do better in mathematics. Among women not receiving that message, performance actually improved slightly"
