If the consciousness of the American male sometimes seems intractably hard to raise, the reason may be evolution, according to a study of college students' sex fantasies. Anthropologist Donald Symons of the University of California at Santa Barbara and psychology graduate student Bruce Ellis of the University of Michigan queried 307 undergraduates and found that males had sex fantasies that were 50 percent more frequent than females, much more graphic, involved twice as many imagined partners and showed less concern for "emotional setting." Women's fantasies placed higher importance on ambience, mood and nonsexual caressing.

Such disparities are usually attributed to experience. But in the November Journal of Sex Research, the researchers suggest the differences reflect divergent reproductive strategies developed millions of years ago. Because of the greater investment females had to make -- gestation and nursing -- in offspring, natural selection favored women who were not prone to easy sexual arousal or random copulation and chose dependable protectors as mates.

But a male who mated as often as possible could pass on his genes with minimal investment, so "males would have benefited from relatively fast and frequent sexual arousal," the authors wrote. Modern sex fantasies may simply embody those evolutionary forces.