For many animal species that are incapable of verbal communication, other methods have evolved to be able to alert the opposite sex that one is fertile. It was long believed that humans do not possess these non-verbal cues and that people can only discern information such as which phase of the menstrual cycle a woman is in if she explicitly states it. However, recent research has hinted that this is a major misconception and that human males are able to subconsciously determine whether a woman is ovulating based strictly on odor alone. One study, in particular, conducted by Seppo Kuukasjarv, found that both men and women were more attracted to the odor of women not using contraceptives when they were ovulating than when they were not ovulating, although the results were only significant for men (Kuukasjarv, 2004, p. 1). The study found that this phenomenon was not present in women who were using hormonal birth control, leading the researchers to think that oral contraceptives disrupt the natural cycle of odor attractiveness (Kuukasjarv, 2004, p. 1).
The study participants included 82 female volunteers who were mainly biology and psychology students from the University of Jyvaskyla in Finland (Kuukasjarv, 2004, p. 2). Some of these women were using birth control while others were not. These participants would wear one t-shirt for two days straight in order to infuse the clothing with their natural odors. Then there were 31 male and 12 female raters who would rate the shirts based on how attractive they smelled.
The independent variable in the study was which day of the menstrual cycle each of the 82 women was on. Since menstrual period lengths differ, the researchers normalized the cycles to a 28-day cycle (Kuukasjarv, 2004, p. 2). The dependent variable was how attractive the shirts smelled. This was measured by giving each rater every shirt and asking them to rate it on a scale from 1 to 10 for both attractiveness and intensity. On this scale 5 is neutral and 10 is the highest rating.
The researchers found that there was not a significant correlation between the intensity of the odors and their attractiveness (Kuukasjarv, 2004, p. 2). Furthermore, they did not find a correlation between the intensity of the odors and the day of the menstrual cycle. However, they did find a correlation between the day of the menstrual cycle and the attractiveness of the smell (Kuukasjarv, 2004, p. 3). This correlation was only significant for male raters who were judging the odors of women who were not on birth control. The fact that women were worse at this skill than men hints at an evolutionary purpose for this adaptation.
This study is important because it shows that people (and in particular men) may be able to use non-verbal cues to determine how fertile a woman is. This may be a product of evolution since men that are able to determine which females are ovulating will have a better chance of producing offspring. It is exciting to see that we are more similar to our primate ancestors than we might have thought. It is also important to see the ways in which birth control pills might be affecting our natural methods of reproduction. It begs the question what other subconscious aspects of life are negatively affected by hormonal birth control?
One of the problems pointed out in the study is that not all women ovulate at the same point in their menstrual cycle, and although it can be approximated to around day 12 or 13, discrepancies between different women could account for the insignificant correlation for women raters for example (Kuukasjarv, 2004, p. 4).
One especially important factor that I believe this study overlooked is the fact that humans are not purely a tournament species. This means that many humans find themselves in long-term monogamous relationships. The aforementioned study assumes that men are only interacting with the odors of women they are not familiar with and will have to determine if they are fertile. However, since humans display so many characteristics of a pair-bonding species, it’s important to investigate the effects that long-term relationships can have on this ability to determine fertility through olfactory cues. To examine this theory, we will replicate most of Kuukasjarvi’s study except we will incorporate people in long-term relationships to determine whether boyfriends are better at determining the fertility of their long-term partners than they are for randomly chosen women.
We will select 100 volunteering heterosexual couples who have been dating for over 3 years (to test the effects of monogamy) and who are between the ages of 20 and 30 (peak fertility for women according to the American College of Obstetricians and Gynecologists) for this study (ACOG). The women will be instructed to wear a new t-shirt for an entire day at four randomly chosen points throughout their menstrual cycle. These four random points will be different for each participant in order to have a more complete look at the entire menstrual cycle, but at least one of them will be during menstruation and one of them will be during ovulation. During each 24-hour period, the women will be instructed to not wear any perfume and avoid odorous foods in their diet. They must also avoid sleeping with their boyfriend so as to prevent his odors from being picked up by the shirt. These were the best ways found by Kuukasjarvi to control for other variables that might affect the attractiveness of body odor (Kuukasjarv, 2004, p. 2). Then the shirt which is now soaked in the woman’s pheromones will be kept in a sanitized jar so as to keep the odor constant.
Then at the end of a month, each man will be required to evaluate the odors of their girlfriend’s shirts as well as the shirts of 5 randomly selected other girlfriends who are already participating in the study. The jars will be placed in a random order, so he will not know which time of the menstrual cycle or what person they came from. He will be asked to rate each shirt on a scale from 1 to 10 for attractiveness where 5 is neutral and 10 is the highest rating. This is a rating scale borrowed from Kuukasjarvi.
Based on the findings from Kuukasjarvi’s study, we expect to see a correlation between the attractiveness of body odor and the part of the menstrual cycle a woman is experiencing. We expect that while women are ovulating, their odor will be rated as most attractive, and when they are menstruating, it will be rated least attractive. We hypothesize that, overall, men will rate their significant other’s smell as more attractive than the 5 randomly chosen participants because men are more likely to date women who smell nice to them, and also they have grown accustomed to their smell after so much time. Therefore, we must only look at the changes in smell attractiveness and not the absolute magnitude of the smells themselves. When looking at these relative values, we predict that men in long-term relationships will notice a greater difference between times of the menstrual cycle when evaluating their girlfriend’s odor than other women’s. Therefore, we hypothesize that there will be a higher correlation between the attractiveness of body odor and the part of the menstrual cycle when men are rating the odors of their significant other. However, it is also possible that since men are usually more attracted to the pheromones of their girlfriend than to the average girl, we might also see that the trend disappears. Another explanation could be that this pheromonal signaling is only relevant for polygamous mating, and since monogamous humans have sex throughout the menstrual cycle anyway, there is no need for this process.
Our proposed study has many strengths. Firstly, it can serve as a duplication experiment to confirm the findings from Kuukasjarvi’s study by simply ignoring the data we collect about long-term couples. But since we have added this extra component, we also are able to tell how familiarity and monogamy affect our subconscious biological and physiological processes. This is important not only in the field of psychology but depending on the results, this can either support or contradict the belief that humans are more monogamous than polygamous. Another strength is that we are collecting numerous shirt samples from each female participant. Comparing the scent of women to their own smell at different times during the menstrual cycle is inherently more causal than comparing it to other women experiencing a different part. This essentially eliminates the chance that, by coincidence, Kuukasjarvi could have selected more overall attractive-smelling women who were ovulating than those who were menstruating.
There are also a few unavoidable weaknesses present in this proposed study. Firstly, since we must collect t-shirt samples from each participant over the span of 28 days, the odor might fade in the ones collected earlier in the study. This shouldn’t affect the results too much because the jars are fairly adept at maintaining odors and unlike in Kuukasjarvi’s study, we are not rating the shirts based on the intensity of odor in addition to attractiveness. Another weakness is that we only collect shirt samples at four points throughout each woman’s menstrual cycle. This number was selected for ethical and practical reasons. It is a lot to ask someone to wear a shirt for 24 hours straight and to avoid their boyfriend the entire time. It would be ideal if we could collect one t-shirt each day to fill in the missing data points, but since we are selecting random days for each participant, the collective data should end up being fairly continuous. We are also not investigating as many variables as Kuukasjarvi’s study did. We are not comparing male raters and female raters, and we are not comparing women on birth control to women not on birth control. However, this was intentional because we are merely focussing on the topic of interest and Kuukasjarvi already showed there was no significant result from these other variables.
Throughout the entire study, we will ensure that all of our practices and methods follow ethical guidelines and are as minimally invasive as possible while still minimizing the effects of uncontrolled variables.
There are still many more aspects of this biological phenomenon that will be unanswered by our study. There are probably a plethora of confounding variables that improve this ability to determine through smell what part of the menstrual cycle a woman is on. For instance, there may be certain genetic factors, it may be affected by the degree of relatedness, it might also be affected by the overall sex drive of the woman or the male rater. It would be fascinating to investigate the physiological effects on a man when he is smelling different odors. For instance, how are a man’s levels of testosterone affected by the ovulation statuses of the women he lives with? This would be a really interesting starting point for studying male behavior and how it changes over time in male-dominated spaces compared to home life or co-ed spaces.
ACOG, “Having a Baby after Age 35: How Aging Affects Fertility and Pregnancy.” ACOG, ACOG,https://www.acog.org/womens-health/faqs/having-a-baby-after-age-35-how-aging-affects-fertility-and-pregnancy#:~:text=A%20woman's%20peak%20reproductive%20years,is%20unlikely%20for%20most%20women. Kuukasjarvi, S. “Attractiveness of Women's Body Odors over the Menstrual Cycle: The Role of Oral Contraceptives and Receiver Sex.” Behavioral Ecology, vol. 15, no. 4, 2004, pp. 579–584., https://doi.org/10.1093/beheco/arh050.