15-minute writing exercise closes the gender gap in university-level physics:

Think about the things that are important to you. Perhaps you care about creativity, family relationships, your career, or having a sense of humour. Pick two or three of these values and write a few sentences about why they are important to you. You have fifteen minutes. It could change your life.

This simple writing exercise may not seem like anything ground-breaking, but its effects speak for themselves. In a university physics class, Akira Miyake from the University of Colorado used it to close the gap between male and female performance. In the university’s physics course, men typically do better than women but Miyake’s study shows that this has nothing to do with innate ability. With nothing but his fifteen-minute exercise, performed twice at the beginning of the year, he virtually abolished the gender divide and allowed the female physicists to challenge their male peers.

The exercise is designed to affirm a person’s values, boosting their sense of self-worth and integrity, and reinforcing their belief in themselves. For people who suffer from negative stereotypes, this can make all the difference between success and failure.

The article cites a paper, but it's behind a paywall:
http://www.sciencemag.org/content/330/6008/1234