What does the nose know? Recent research suggests a lot more than we might think. The sense of olfaction is perhaps the oldest sense in evolutionary terms and the only sensory channel that has direct inputs to the brain. Unlike other senses like vision and hearing which transmit signals to the brain via the spinal cord which are then routed through the brain's relay station, the thalamus, olfactory stimuli pass directly through the olfactory nerve to the olfactory bulb in the brain, where they processed to produce sensations of smell.
Social behaviors of other animals are largely dependent on the sense of smell, including mating behaviors and dominance hierarchies. When a male and female dog sniff each other, they are responding to chemical scents that guide their mating behaviors. They don’t require introductions from mutual friends, scanning photos on an online dating site, or checking out each other's Facebook profiles before making an acquaintance of the most natural kind. Human behavior is more complex and depends more on learning and cultural mores than bodily bouquets, but our sensory systems retain vestiges of olfactory signaling that may influence our behavior in some subtle ways.
Scientists have discovered that the human nose possesses receptors for chemical scents called pheromones, which are bodily chemicals that regulate mating and other social behaviors in insects and (nonhuman) mammalian species. But having a biological capacity to process these stimuli does not mean we are led around by our noses, let alone in the way Hollywood envisioned when Cameron Diaz swooned in Ocean's Eleven (to the soundtrack of Bolero, naturally) when Matt Damon opened the door and she got a whiff of a “Love Potion No. 9” he had dabbed behind his ears. The proverbial pursuit of the elixir of love may not have yielded any viable consumer product–not yet at least–capable of inducing sexual desire, but evidence from the lab points to more subtle ways in which scent signals influence our behavior outside the range of conscious awareness. We may respond automatically to these stimuli even if we are not aware of them.
Take, for example, the game of love. The results of a recent study suggest that sniffing a woman’s tears may be a sexual turn-off for men, even if a crying woman is not physically present (Gelstein et al., 2011). Tears are odorless, but they contain chemicals that may signal a message that says something like "Not tonight, dear.” In another study, men were exposed to female scents by sniffing unlaundered T-shirts previously worn by women who were ovulating (Miller & Maner, 2010). Ovulation is the time of the month of a woman's greatest fertility. Men who sniffed an ovulating woman's T-shirt showed higher levels of the male sex hormone testosterone than did men exposed to control T-shirts that had been worn by non-ovulating women. Still other research studies show that women exposed to male sweat tend to feel more relaxed afterwards and report greater sexual arousal than those exposed to control substances (Pilcher, 2003; Preti et al., 2003; Wyart et al,. 2007).
To what extent are we influenced by these scent signals? It's hard to say, as we learn to follow cultural scripts that teach us that bodily odors are offensive and to adopt daily rituals of bathing and applying deodorant and other chemicals to erase these natural scents from our bodies. Rather than shielding us from our natural scents, the science of scent research is now opening a window that may reveal just how the automatic mind processes scent signals. The scientific study of scent signals may teach us what the nose may already know.
Gelstein, S., Yeshurun, Y., Rozenkrantz, L., Shushan,S., Frumin, I., Roth, Y., . . . Sobel, N. (2011). Human tears contain a chemosignal. Science, 331, 226-230.
Miller, S. L., & Maner, J. K. (2010). Scent of a woman: Men’s testosterone responses to olfactory ovulation cues. Psychological Science, 21, 276-283.
Preti, G., Wysocki, C. J., Barnhart, K. T., Sondheimer, S. J., & Leyden, J. J. (2003). Male axillary extracts contain pheromones that affect pulsatile secretion of luteinizing hormone and mood in women recipients. Biology of Reproduction, 68, 2107–2103.
Wyart, C., Webster, W. W., Chen, J. H., Wilson, S. R., McClary, A., Khan, R. M., . . .Sobel, N. (2007). Male sweat as a possible chemosignal. The Journal of Neuroscience, 27, 1261-1265.