Smell and Attraction

Are there Human Pheromones?

For many years it has been known by scientists that pheromones exist in most animal species. Pheromones are odorless chemicals that can signal an individual's identity, arousal or sexual receptivity (Wright 1994). When emitted they have some effect on members of the same species. For example, pheromones produced by queen bees stall sexual development in other females, who then become workers. Male mice emit pheromones that can promote the sexual development of nearby females. When a pregnant female mouse detects the pheromones of an unfamiliar male, a spontaneous abortion occurs (Cowley and Springen 1997).

In order for pheromones to be detected, a special organ, called the vomeronasal organ must be present. This organ is composed of a pair of small pits on the membranous skin inside the nostrils. Recently there has been controversy as to whether or not this organ exists in humans (Lawton 1997). Several scientists have identified what they believe to be the human vomeronasal organ, located near the bottom of the septal wall dividing the nose (Wright 1994). However, other researchers claim that this organ is not present in human adults even though it is present in human fetuses and virtually all other animal species (Lawton 1997). To see a picture of the vomeronasal organ click here.

Other scientists who believe that they isolated human pheromones from a sample of human skin tested the human vomeronasal organ (VNO) to see if it is functional. They designed a special electrode that could be inserted into the pits of the VNO which could detect any electrical activity that would be assumed to occur following a chemical interaction. The electrode was first tested in the olfactory cleft and responses were recorded for substances such as mint and clove oil. When the suspected pheromones were tested, no responses were recorded for the olfactory cleft. But when the electrode was placed in the VNO, responses were recorded, suggesting that neurons in the VNO were firing in response to the suspected pheromones (Wright 1994). Hormone levels in the blood also changed, suggesting that the VNO is somehow connected to the brain (Medical Industry Today 1996). The responses varied among male and female subjects, which would be expected as in the animal world, pheromones usually have an effect on only one sex. The behavioral effects are still unproven as the uncomfortable experimental apparatus makes it difficult to tell if one feels significantly different (Wright 1994).

Other researchers are trying to figure out the wiring of the VNO to the brain. The region surrounding the VNO is filled with neurons that connect to the brain, however a primary structure that is involved in the processing of information from the VNO, the accessory olfactory bulb, is not present in humans. These researchers believe that the accessory olfactory bulb does exist, but because of the enlarged frontal lobes in humans, it has become flattened and stretched thus making it difficult to see (Wright 1994). The researchers believe that the cells in the VNO send electrical impulses to the hypothalamus which stimulates the pituitary gland to release or stop releasing certain hormones (Lawton 1997). In other mammals, information from the VNO is transferred via nerve fibers to the accessory olfactory bulb and other regions of the brain such as the hypothalamus (the hypothalamus is involved in basic body functions like sleeping, eating and mating). The anterior region of the hypothalamus then alters the activity of the neuroendocrine system which is involved in reproductive physiology and behavior. Some nerve fibers may also be linked to the limbic system, which is involved in emotions.

So where is it that these pheromones come from anyway?

The most likely answer is our apocrine glands. The three types of glands that are present in humans are sebaceous glands, sweat glands and apocrine glands. Sebaceous glands exist around the body’s openings and secrete substances that kill potentially dangerous microorganisms. Sweat glands help regulate our body temperature and release water and salt. Apocrine glands in humans do not regulate body temperature as they do in other animals. They are found in large concentrations on the face, chest and wherever body hair exists. Interestingly, apocrine glands become functional after puberty which is when we would most likely be searching for a mate. In other animals apocrine glands are known to release substances which effect sexual behavior, so it seems likely that if human pheromones do exist this is where they would come from (Furlow 1996).

More evidence for the existence of pheromones comes from research that was done almost thirty years ago and a carefully controlled experiment that was just recently published. The experiment was based on the research that showed that women living in close proximity to one another often found that their menstrual cycles would converge over a period of time. It was suspected that pheromones might be the possible cause of this phenomena. In order to test this hypothesis women in the study were exposed to the body odors of other women (Weller 1998). The odor was masked by mixing it with isopropyl alcohol, and was then swabbed onto the upper lip (Seppa 1998). The results showed that the menstrual cycles of the women either slowed down or sped up, depending on what phase the donor woman was in her cycle (Weller 1998). A control group of women who received only isopropyl alcohol showed no change in their cycle (Seppa 1998). This experiment shows that pheromones may be responsible for the convergence of menstrual cycles but it also raises a lot of questions. If pheromones are causing this change their chemical structure has yet to be identified, the reason for why they cause menstrual synchronicity is unclear and it has yet to be tested whether or not men release pheromones that can affect menstrual cycles as well. To further clarify the existence of pheromones experiments also need to be carried out on women who have no sense of smell, but a working VNO (Weller 1998).

If pheromones do exist, the synthesis of pheromones for medical conditions could be one of the first applications as the VNO provides a new pathway to the brain which could be used to treat mental disorders or even possibly a new form of birth control. In fact experiments are already underway testing synthetic pheromones that may provide relief from PMS. Pheromones might also explain why we may feel an instant attraction or dislike for someone upon first meeting them (Lawton 1997).

Evidence for attraction based on our immune system?

Sounds odd, but it seems to be true. Researchers studying the immune systems of mice found that female mice would choose a mate whose MHC genes were the least similar to her own. MHC stands for major histocompatibility complex. These genes code for special protein markers that are attached to the surface of cells and help the body recognize whether a cell belongs to an organism or if it is an invader such as a bacteria or virus. If a cell or bacteria is identified as an invader, the body’s immune system mounts a defense to kill the intruder. Different MHC molecules are good at recognizing different invaders. By a choosing a mate whose MHC molecules are different, the female mouse is ensuring that her offspring will have a wide variety of MHC molecules that which can identify a large array of invaders and thus promote survival (Furlow 1996).

Research done on human females shows that they too prefer men whose MHC genes are the least similar to their own (Richardson 1996). In an experiment, men were given an unscented T-shirt and were asked to wear it for two nights in a row. During this time they were not to use deodorants or scented soaps. Women were then presented with six shirts - three from men with similar MHC genes, and three from men with different MHC genes from their own. The results showed that the women preferred the scents of men whose MHC genes were different from their own. The scent of men with similar MHC genes often remind the women of a relative’s odor, such as a brother or father while the smells of MHC dissimilar men would often remind them of a past or current boyfriend. This suggests that body odor might have influenced past and current decisions on who to date.

It was also found that women who were taking birth control pills would often choose the T-shirts of men with similar MHC genes as smelling better. A possible explanation for this is that birth control pills trick the body into thinking its pregnant, and women on the pill often report that they prefer smells that remind them of home and relatives. Since natural preferences are reversed a woman might then be attracted to men she normally wouldn’t be - namely men with similar MHC genes (Furlow 1996).