"Walking into a situation in which you sense the possibility of being ostracised or isolated can be quite threatening," said the study, which looked at the effect of so-called situational cues, such as being outnumbered, on women's performance in math, science and engineering courses.

The gender ratio in those classes is "approximately three men to every one woman," said the study, led by Mary Murphy, a psychologist at Stanford University in California.

Published in the October issue of Psychological Science, the study monitored the reactions of a group of advanced students in traditionally male-dominated fields while they watched videos depicting a conference.

In one of the videos, men outnumbered women, while in the other the sexes were equally represented.

The women's heart rates accelerated and perspiration increased when they watched the video in which their gender was outnumbered.

They also reported "a lower sense of belonging and less desire to participate in the conference," the study showed.

And they paid more attention to their surroundings when they watched the video in which women were in minority.