Although it certainly isn't the most romantic hormone pumping through our veins, testosterone -- the compound responsible for facial hair and fast driving -- is necessary to warm up our internal love engines. And it isn't just for the guys, either; testosterone also exists in the female body to stoke physical attraction and sexual arousal.
A recent study from the University of Aberdeen identified how the hormone determines who stands out in the crowd. By measuring participants' blood levels of testosterone over a series of weeks and asking them to rate the attractiveness of photographed faces, the researchers charted a distinct pattern. Men with higher levels of testosterone exhibited a stronger preference for more feminine faces, whereas women with higher levels of the stuff were compelled toward more masculine faces [source: Durfee]. Just in case women's testosterone could use a boost, men pass it along in their saliva during passionate kissing [source: Kluger]. That helps explain why men and women spend time kissing as foreplay to sexual intercourse.