Pheromones are indeed attractive to all women.

But not all the time. And not everyone's pheromones . Several recent studies have shown that the appeal of pheromones to a specific woman depends on at least two factors

First of all, pheromones convey very individualized information about a man. One of the most important pieces of information conveyed is about his immune system and, from an evolutionary perspective, this is critical to mate selection by women.

Apocrine glands located in the armpits, chest, and groin secrete pheromones. These secretions are actually odorless as they emerge from the gland but, the instant they come into contact with the skin, they're chemically altered, acquiring a characteristic scent in the process.

This chemical alteration occurs as a result of contact with the hordes of bacteria that coat the surface of the skin. Dozens of different bacteria live in harmony on everyone's skin, and the particular mix of bacteria on any one person's skin is largely a function of his immune system. And the mix of bacteria determines the precise nature of the chemical message in his pheromones. One man's immune system tolerates a particular type of bacteria better than another, so his particular pheromones are slightly different from those of the man sitting right beside him.

Without being aware of it, women receive a chemical message from pheromones that includes a bulletin on the state of a man's immune system. And, far below the level of conscious awareness, women are attracted to men whose immune systems are slightly different from their own. This is rather an amazing way of our species propagating itself in the most effective manner possible, because mates with slightly different immune systems confer a broader overall immune competence on their children.

The other factor that helps determine whether or not women find pheromones attractive is the timing of their menstrual cycle. Several studies have attempted to pin down the nature of this relationship, but the results have been contradictory so far. One researcher found that women are most acutely sensitive to the subtle fragrance of pheromones around the time they ovulate. Another found that women in the first part of their menstrual cycle-before ovulation-found one type of male pheromone attractive and another one offensive. Around the time of ovulation, these effects disappeared.

The science of human pheromones is relatively new-less than two decades old-so there simply haven't been enough studies conducted yet to confirm the precise effect of the timing of a woman's fertility cycle on how she perceives pheromones. They strongly suggest that a relationship exists and this, too, makes sense from an evolutionary perspective. We just don't yet know the exact nature of the relationship.

Finally, women perceive different male pheromones differently. Androstenol is almost always perceived as pleasant, while androstenone is either perceived as unpleasant or stimulates negative emotions towards men. This mimics the effect of different pheromones in other species, where they can function to either attract or repel potential mates.