The ideas for new treatments are often generated in the laboratory, where gender bias in basic biomedical research and neuroscience is ingrained.

Bias in mammalian test subjects was evident in eight of 10 scientific disciplines in an analysis of published research conducted by Irving Zucker, a professor of psychology and integrative biology at the University of California, Berkeley. The most lopsided was neuroscience, where single-sex studies of male animals outnumbered those of females by 5.5 to 1.

Contrary to the conventional wisdom in laboratories, there is far more variability among males than among females on a number of traits and behaviors, Dr. Zucker has found. Yet even when researchers study diseases that are more prevalent in women — anxiety, depression, thyroid disease and multiple sclerosis among them — they often rely on male animals, according to another analysis led by Dr. Zucker, who has written extensively on gender bias in scientific research.

Jill Becker, a senior research scientist at the University of Michigan who studies gender differences in addiction, has found that women increase their drug use much more rapidly than men and that the hormone estradiol plays a critical role in the escalation, especially during ovulation.

Nonetheless, researchers studying escalating drug use in rats and mice rely almost entirely on males, she said.

“One of the underlying assumptions has been that females are simply a variation on a theme, that it isn’t a fundamentally different mechanism, that if you’ve learned about the male you’ve learned enough to deal with both males and females,” she said. “We’ve discovered that’s not always the case.”

The N.I.H. is directing scientists to perform their experiments with both female and male animals, and grant reviewers will take the balance of each study design into account when awarding grants. (If the subject is gender-specific, like ovarian cancer or prostate cancer, then the rule will probably not apply, Dr. Clayton said.)