If birds and bees do it, then surely, Masters and Johnson believed, human beings rely on a sense of smell in sexual selection. Olfaction must play a hidden role in the allure between men and women, the sweet and musky odours that excite the senses and signal the inevitability of love. Smell is one of our most primitive senses, said by some to have the longest recall power. Unlike sight and touch, it travels a direct route to the brain's limbic lobe where it can provoke an emotional reaction that can, quite literally, be a turn-on.

How wonderful it would feel finding that there's an invisible, undetectable chemical force that's powerful enough to override your sense of reason yet draws you to someone with an almost animalistic passion. Have you ever wondered why you meet someone and feel an almost immediate and irresistible attraction towards them? Or may be how a particularly pleasurable scent from a distant past still arouses a rush of emotions when you smell it, reminding you about someone you've been with before but might have even forgotten her face or name. These are remarkable influences, pheromones exact on men and women.

Pheromones are airborne chemical signals that are secreted by an individual into the environment and which affect the physiology and behaviour of other percipient members of the same species. In humans, they are generally linked to sexual attraction between both men and women. In essence, you are attracted to others and they are attracted to you partially due to the pheromones you and they secrete.

Although there is scientific evidence that pheromones exist, little is known about them. It is believed that the way pheromones work is similar to the way hormones in the body send specific chemical signals from one set of cells to another, causing them to perform a certain action. A major difference between pheromones and hormones is that hormones are secreted and circulated inside the body through the bloodstream, while pheromones are secreted outside the body through the sweat glands, skin, and urine.

Pheromones are found throughout the living world and are probably the most ancient form of animal communication. They have been documented to influence sexual behaviour in animals - mammals regularly mark their territorial boundaries with pheromones from specialized glands; female gypsy moths and Japanese beetles each emit a species-specific sexual pheromone to attract males. These odours can be detected by males at enormous distances and can alter male behaviour dramatically. Even mates and offspring of certain mammals often recognize one another by odour. In these animals, pheromones are detected by the vomero nasal organ (VMO) in the nasal cavity.

Just like in these mammals, inside the human nose is a small cavity called the vomero nasal organ (VMO), which is lined with a cell type that is unlike any other cell in the human body. It is far less prominent in humans than in animals which depend more heavily on smell for guidance. The VMO appears to specialise in detecting pheromones without people's conscious awareness. In other words, people do not "smell" pheromones in the same way they smell perfumes. The scent is registered at some brain level and people respond to it emotionally and/or physically. However, we don't have to be aware of pheromones for them to have an effect.

Sexual chemistry is an amazingly complex phenomenon. Biologists describe pheromones as "smellprints" supposedly as unique to each of us as our fingerprints. Sexual chemistry starts with our brains (the biggest sexual organ) and proceeds to be "triggered" through body chemicals such as phenylethylamine, dopamine and oxytocin. Researchers today are of the opinion that the key to sexual chemistry between people is the pheromones they secrete and are particularly looking into the theory that these substances may be the reason you choose or reject a potential partner.

According to a recent Swedish report, men and women respond in different ways to two particular odours that play significant roles in sexual arousal - a testosterone derivative produced when men sweat and an estrogren derivative which is found in women. There is growing belief among most researchers in this field that these chemical compounds are indeed pheromones.

In his book, "He Comes Next", Ian Kerner concluded that "His Smell" was rated highly among other five top factors such as confidence, height, a sense of humour, and a handsome face as factors of sexual attraction of women towards men. Interestingly, most men rated a woman's scent among the least factors in sexual attraction although not necessarily implying that men do not respond to scent without realising it. While there is a peak in pheromone receptivity during ovulation among women, a generality of men's receptivity to pheromones has been shown to remain relatively stable in the course of their lives. Also, women's odour discrimination ability has been shown to be at its peaks during their years of fertility.

Of further interest are women who live together or work in close proximity who tend to find that their cycles begin to coincide. Researchers have discovered that this tendency toward synchrony of cycles among females who spend an extended time together is mediated by pheromones produced by the women themselves that expedite or delay the onset of the ovarian cycle.

While there's no doubt that pheromones underlie sexual and other types of behaviour in animals, the human psyche, is a rather complex and different playing field altogether. There is yet no scientific verdict if these invisible lust signals - pheromones - are all that is needed for love. However there is a body of evidence suggesting that the chemistry of sexual attraction and arousal is more nature than nurture and quite beyond our control.

Few recent studies however suggest that some synthetic pheromones may increase the rates of men's sexual encounters and behaviour. Other studies have shown that adding a particular pheromone (of as yet unknown composition) to a man's shaving cream daily seems to increase his female partner's sexual desire. The use of these substances may have great impact on the future management of low sexual desire. These new fragrances are not billed as "aphrodisiacs" but as "mood-enhancers" designed to help in relaxation and reduction of sexual inhibitors.

Just as women are attracted to male scent, so men are to theirs. A person's scent is individualistic - so an individual may not smell that good at first. However, if you pay attention to his/her scent, you may find it stimulating - this is, after all, the idea behind pheromones.