A rather controversial area of psychological research is the role of pheromones on human behaviour. A pheromone is a chemical substance produced and released into the environment by an animal affecting the behaviour or physiology of others of its own species. Although pheromones are known to play a significant role in signaling between members of the same species among animals to affect various behaviours, it is not clear that this is also true in humans.
Some psychologists have argued that pheromones may affect the menstrual cycle in groups of women, olfactory recognition of newborn by its mother and that individuals may exude different odors based on mood. Although there is some evidence, nothing is conclusive on whether or not humans have functional pheromones.
In animals, we see two types of pheromones. Primer pheromones that cause slow, long-term physiological changes, such as hormonal effects; and signaling pheromones that produce rapid behavioral effects, such as mating. In humans, there is some evidence of primer pheromones. However, for all the published research that shows these effects, there is an equal number of studies showing that there are no effects. At this stage in the study of psychology, no human pheromone has yet been found.
Research in psychology: McClintock (1971)
One of the early studies of pheromones appeared to show that when women live together in close proximity, their menstrual cycles begin to align - a phenomenon known as menstrual synchronicity. It is rather common knowledge that when women go to university, if they are in an all-female dorm, their menstrual cycles will begin to align.
But is this true?
McClintock studied 135 female residents of a college dormitory. She used questionnaires to determine the onset date of menstruation among roommates and close friends. The data showed a significant increase in synchronization of onset dates over time. McClintock argued that this could be a pheromonal response, or it could be the result of some other process. She did not take any physiological data that would have indicated that here results were due to pheromones.
In spite of the fact that this is a well known "phenomenon", there has been almost no support of the findings of this study. Trevathan et al (1993) carried out a study of 29 lesbian partners to see if their menstrual cycle aligned. They found no evidence of synchronicity in their sample. Yang & Schank (2006) also attempted to replicate McClintock's results; they studied 186 Chinese women living in a dormitory for one year. They found no evidence of synchronicity. In reviewing McClintock's study, they argue that her results were not statistically significant and therefore, were due to chance.
Two potential human pheromones are Androstadienone – found in male semen and sweat – and estratetraenol, which is found in female urine. Zhou et al (2014) carried out a study to see if these substances influenced human sexual behaviour. The sample was made up of 96 participants – 24 heterosexual men, 24 heterosexual women, 24 gay men and 24 lesbian women.
In the experiment, participants were asked to watch stick figures walking on a screen and to determine their gender. While carrying out the task, the participants were exposed to the smell of cloves. In the first condition, the cloves were mixed with androstadienone; in the second condition, the cloves were mixed with estratetraenol; and in the control condition, only cloves were used. The findings showed that smelling androstadienone biased heterosexual females and gay males, but not heterosexual males, toward perceiving the walkers as more masculine. By contrast, smelling estratetraenol systematically biases heterosexual males and lesbian women toward perceiving the walkers as more feminine. The researchers concluded that pheromones influence communication of gender information in a sex-specific manner.
Although the study showed a significant difference in behaviour, there are some concerns with the study. First, the participants were exposed to very high levels of the pheromones; it is unclear that this response would happen in a naturalistic setting. Secondly, although they identified the figure as masculine or feminine, this is not a clear study of sexual odour but rather if participants perceived a person's walk as feminine or masculine. It can be debated whether this is a reliable measure of sexual behaviour. Finally, the study is done on a relatively small sample. The study would need to be replicated on a much larger sample in order to determine whether the results are reliable.
A more promising study was carried out by Doucet et al (2009) on the role of secretion of the areolar glands in suckling behaviour in 3-day-old infants. The areolar glands are located near the nipple. The researchers administered the different secretions to the infants nasally and then measured their behaviour and breathing rate. The researchers compared the infants' reaction to seven different stimuli - including, secretions of areolar glands, human milk, cow milk, formula milk, and vanilla. They found that the infants began sucking only when exposed to the secretions of the areoloar glands. In addition, there was a significant increase in their breathing rate. The researchers argue that this stimulus of the aerolar odor may initiate a chain of behavioural and physiological events that lead to the progressive establishment of attachment between the mother and the infant. However, more research is necessary to definitively draw these conclusions.
There are several problems with the pheromone arguments. First, the human sense of smell is very complex. Richard Axel and Linda Buck shared the Nobel Prize in Medicine in 2004 for their discoveries of odour receptors and how the olfactory system is organized. They found that we have about 400 different kinds of odour receptors - and each of the 400 receptors has genetic variations. This makes it very difficult to see how pheromones would work in humans. Another problem is that many body odours are actually not caused by secretions, but by bacteria that mix with our secretions - for example, in the armpits. However, about 20% of the population does not have this bacteria and thus does not create the same scent. This makes a universal finding of pheromones a bit less likely. Finally, culture plays a key role in our sense of smell - we learn what smells bad and what smells good. This could potentially be a confounding variable when trying to determine the role of pheromones on behaviour.
Checking for understanding
Which of the following statements is true about human pheromones?
Although there is some research that shows that there may be potential pheromones in humans, as the moment there are no true examples of pheromones in humans that have been found.
What is the key limitation of the study by Mcclintock on the role of pheromones on women's menstrual cycles?
Although all of the other points are also potential limitations, the most significant problem with the study is the inability of other researchers to replicate the findings. This has been found to be true in many studies of pheromones.
Which of the following is true about Zhou's study on signalling pheromones in humans?
It is questionable whether asking if stick figures are more masculine or feminine is truly a measure that can be linked to one's potential sexual behaviour.
Which of the following makes the existence of human pheromones less likely?