I just read this in the August edition of Allure.

Women can perform well in math – until they are reminded of the stereotype that men are better at it. At Dartmouth College, researchers led by psychologist Anne C. Krendl studied 28 college women who strongly agreed that it was important to them to be good at math. All had their brains scanned while they completed a variety of tasks, including two sets of difficult math problems. Before the second test, the investigators noted to some of the women that “research has shown gender differences in math ability and performance.” The subjects who had been reminded of this performed significantly worse the second time, whereas the others did significantly better. Also, the unprompted group showed activity in parts of the brain that are associated with math learning, but those reminded of the stereotype experienced activation of a brain area that processes negative social information, essentially distracting them from their task. Simply being aware of this effect can help women overcome it, Krendl says.

So, we really are what we think.