Above and beyond looks and bank account size, women rank how a man smells as the number one determinant for whether she'll be sexually attracted to him. Moreover, what men each woman finds most sexy smelling varies widely and is tied to immune system genetics. Everyone (except identical twins) has a genetically unique immune system, and the specific genetic fingerprint of your immune system is outwardly represented by your body odor. Research shows that naturally cycling women prefer the body-odor of men whose immune system genes are relatively different from their own. This "opposites attract" phenomenon is what evolutionary matchmaking aims for, as it is adaptive for fecundity, infant survival and reproductive success.
When it comes to men, the story has been that how a woman looks-her hourglass figure, full lips, lustrous hair and sparkling eyes are what appeal most. This is not superficial or sexist; it makes good evolutionary sense because these physical attributes are in fact signals to youth and health and therefore probabilistic fertility. By contrast, though a few studies have shown that men find a woman's natural body-odor to be most pleasant when she's ovulating, there has been little else to suggest that a man's biology is at all influenced by scent. However, other male mammals use odor as the dominant cue for the initiation of sexual behavior. A male rhesus monkey with a blocked nose will ignore a female in heat. Now new research from Florida State University has revealed that human males may be driven more by the scent of a woman's "heat" than has ever been realized before.
Saul Miller and Jon Manner tested college men for their responses to T-shirts that had been (1) worn to bed by college women who were ovulating, or (2) worn to bed by college women who were not ovulating, or (3)T-shirts that hadn't been worn by anyone (unscented). Regardless of the condition, all men were told that the shirts "had been worn by a woman" and they were asked to take big sniffs of it three times over a 15 minute session. Testosterone levels were measured before they sniffed and then after the 15 minute T-shirt session. Testosterone, the male sex hormone, is directly influenced by external cues- when heterosexual men interact with an attractive woman or watch pornography their testosterone levels rise.
Miller and Mann's study revealed that the men who sniffed T-shirts from ovulating women had higher testosterone levels than the men who sniffed T-shirts that didn't indicate fertility; either worn by non-ovulating women or unworn. But the testosterone levels of the men who smelled the shirts signaling fertility didn't actually increase from their pre-sniff levels, as happens when men are exposed to other overt sexual signals. They just didn't drop-- which is what happened to the men in the other conditions-- who sniffed T-shirts that didn't indicate fertility. There are various possible explanations for this finding. My speculation is that it reflects the biological response to either a thwarted or successful sexual ‘match to expectation'.
If you tell a heterosexual man that he's going to be smelling a T-shirt from a young woman, evolutionary theory might suggest that he would become interested both mentally and physiologically-- as a possible indication of mating to follow. However, if the actual act of sniffing does not in fact signal the "presence" of a woman with immediate reproductive value (as in the case of T-shirts from non-ovulating women or not worn), this unconscious biological disappointment may manifest as a drop in testosterone. By contrast, the constant level of testosterone observed in the men who smelled T-shirts from ovulating women may indicate a match to expectation. In order to test whether this explanation has any merit we would need to know what the level of testosterone was in the men before they were told anything about the T-shirts. Or we might conduct a new study where men are given various types of reproductive information (false or not) about the women whose T-shirts they may be smelling. It would also be fascinating to know whether any testosterone changes occur when heterosexual men smell T-shirts worn by other men (under various "informational" conditions). I could go on. Suffice it to say that this is a fertile field for future research!
Miller, S. L. & Maner J. K. (2010). Scent of a woman: Men's testosterone responses to olfactory
ovulation cues. Psychological Science, 21, 276-283.