Pheromones are kind of the slightly more grown up version of asking Jenny to tell Sam that you fancy him. They are chemicals that trigger a change in behaviour or certain response from someone else. Pheromones are thought to be released through your sweat and then others pick up on their scent. Animals use pheromones to communicate in lots of different ways but, in humans, research so far suggests they are used most commonly to send signals to the opposite sex.
The smart guys are still a little unsure whether us humans actually use pheromones or not but they think we do. There are definitely responses in humans that are present in other animals that use pheromones, but research has yet to find an actual human pheromone chemical. However, when human ancestors gained their ability to see in colour, they lost the organ used by many animals to detect pheromones (vomeronasal organ). If we do use pheromones they would work by communicating information about you to other humans. Which (when they smell them on your sweat) they pick up on sub-consciously.
There are three main demonstrations of pheromones affecting humans:
There is also some suggestion that women may be able to smell a guy’s healthiness and suitability through his pheromones. In mice the pheromones of male mice communicate information about their immune system and health to female mice and females tend to pick males with an immune system that provides different immunity to their own. It has been proposed this is the reason humans kiss, as through kissing they get close enough to a possible mate to have a real good sniff and find out if he’s really suitable.