Telling women they can’t do well in math may turn out be a self-fulfilling statement.

In tests in Canada, women who were told that men and women do math equally well did much better than those who were told that a genetic difference in math ability exists between the sexes. And women who heard that differences were caused by environment — such as math teachers giving more attention to boys — outperformed those who were simply reminded that they were females.

The women who did better in the tests got nearly twice as many right answers as those in the other groups, a psychology professor at the University of British Columbia in Vancouver, Steven Heine, said.

Expectations, he said, really do make a difference.”The findings suggest that people tend to accept genetic explanations as if they’re more powerful or irrevocable, which can lead to self-fulfilling prophecies,” Mr. Heine said.

The math study is the latest since Harvard University’s president ignited a scandal last year by suggesting that innate gender differences may partly explain why fewer women than men reach top university science jobs. The comment eventually cost him his job.

Mr. Heine and doctoral student Ilan Dar-Nimrod wanted to see how people are affected by stereotypes about themselves. They divided more than 220 women into four groups and administered math and reading comprehension tests between 2003 and 2006. Their results are reported in Friday’s issue of the journal Science.

“Our study doesn’t explore whether innate sex differences exist,” Mr. Dar-Nimrod said. “Instead, we investigated how the perceived source of stereotypes can influence women’s math performance.”

The women were given a math test, then asked to read an essay, and then given a second math exam.

In two groups the women averaged between five and 10 correct answers out of 25 math questions. In the other two they averaged between 15 and 20 correct.

The women in the lower-scoring groups read essays that either contended that a genetic difference exists between men and women in math ability, or discussed the images of women in art — a reading which did not discuss math but was designed to remind them of being female.

Those two groups not only fell short of the other women, but their performance declined between the two math tests, meaning they scored lower after reading the essays than before.

It’s a process psychologists call a stereotype threat, Heine explained. “If a member of a group for which there is a negative stereotype is in a position to test the stereotype, they are likely to choke under the pressure.”

So reminding them of the stereotype affects them.

On the other hand, reading essays that contend that no natural difference exists between men and women in math skills lets them go ahead and answer the questions without any added pressure.

And that was also the case with those reading essays arguing that any differences aren’t their fault, but exist because of conditions such as teachers giving boys preferential treatment in the early years of learning math.