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PROBLEM: When a woman touches a man's arm during a date, or smiles at a male student during class, how likely is it that she wants to have sex with him? The best way for him to figure that out is probably to ask her.

METHODOLOGY: Researchers at Hendrix College, in Arkansas, recruited 60 college students (31 men and 29 women). The participants rated men and women's general interest in sex, and their intent to have sex, in situations like the ones above. For example: "When a man [or woman] goes out to a bar, how likely is it that he [or she] is interested in finding someone to have sex with that night?" They were also asked how strongly they agreed with generalized statements about both genders' interest in commitment, such as: "A typical woman needs to know that a man loves her before she is willing to have sex with him."

The researchers then had the participants go without sleep for an entire night, and tested them again the following morning.

RESULTS: Overall, the participants rated men as being more interested in sex than woman are, and as more likely to intend to have sex. Once sleep-deprived, however, the male participants' answers changed: they rated women's interest in and intent to have sex as being more or less equivalent to that of men. Sleep-deprivation did not have any effect on the female participants' perceptions.