Scientists have proved that even the most seemingly innocent chat with a woman can be enough to send male sex hormones soaring.
A team from the University of Chicago paid students to come into their lab under the pretence of testing their saliva chemistry.
While there, the students got to chat to a young female research assistant.
Saliva tests showed the brief interaction was enough to raise testosterone levels by as much as 30%.
The more a man's hormone level shot up, the more attractive he later admitted to finding the research assistant.
And perhaps more tellingly, the research assistant herself was able to identify those men who found her attractive.
The men who she judged to be doing the most to try to impress her proved to be those who registered the biggest jump in testosterone levels.
However, little or no change was detected in the saliva of students who chatted with other men.
Testosterone has long been closely linked with the male libido.
The researchers say their work is the first time that hard evidence has been produced in this way.
It is known that the release of testosterone in animals can embolden them, triggering courtship or aggressive behaviour.
The Chicago team believe the same may be true in humans.
However, lead researcher Dr James Roney said it was also possible that the release of the hormone was stimulated by a stress reaction.
Dr Roney told BBC News Online: "The findings are consistent with the existence of brain mechanisms that are specialised for the regulation of courtship behaviour and thus respond to cues from potential mates with coordinated behavioural and hormonal reactions.
"One might call these reactions components of a "mating response" which, if confirmed by future research, could be as basic and significant as, say, the well-known "fight or flight" reaction."
Dr Nick Neave, of the Human Cognitive Neuroscience Unit at Northumbria University, said the study was "very interesting".
"Other researchers have found changes in male hormone levels after watching erotic movies but this seems to be the first that has attempted to assess hormone changes when males meet women on a more 'normal' level."
Dr Benjamin Campbell, an expert in anthropology at Boston University, said it was possible that testosterone made men more bold by suppressing activity in an area of the brain called the amygdala, which controls the stress reaction.
Testosterone levels peak in a man by his early twenties, and then gradually diminish.
Men who are married or in long-term relationships have lower testosterone levels than those still playing the field.
The research is published in the journal Evolution and Human Behavior.