In an unusual scientific about-face, researchers at Northwestern University have found evidence that at least some men who identify themselves as bisexual are, in fact, sexually aroused by both women and men.

The finding is not likely to surprise bisexuals, who have long asserted that attraction often is not limited to one sex. But for many years the question of bisexuality has bedeviled scientists. A widely publicized study published in 2005, also by researchers at Northwestern, reported that “with respect to sexual arousal and attraction, it remains to be shown that male bisexuality exists.”

That conclusion outraged bisexual men and women, who said it appeared to support a stereotype of bisexual men as closeted homosexuals.