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Do Body Scents Reveal Women’s Ovulatory Timing?

– by James R. Roney

In many nonhuman mammals, females emit clear cues of time periods when they are able to conceive: from large sexual swellings in some primates, to odors that reveal ovulatory timing. In humans, overt signaling of this kind appears absent, and scientists like Don Symons and Beverly Strassmann theorized that concealment of ovulatory timing may have facilitated the evolution of human pair bonding. Nonetheless, a series of studies have provided evidence that women smell more attractive near ovulation (reviewed here), raising the possibility that although the cues are more subtle than in nonhuman species, perhaps human ovulatory timing is not completely concealed. So, which is it, concealed or not?

In prior research, my lab used signal detection theory to test whether attractiveness shifts in women’s odors were substantial enough for perceivers to accurately diagnose ovulatory timing. The intuition is this: even if odors collected near ovulation smell better than odors collected at other times on average, is there still enough overlap in fertile vs. non-fertile odor attractiveness that perceivers cannot reliably categorize whether an odor sample was produced during the fertile window (the fertile window is defined as cycle days when conception is possible)? Our findings provided evidence for just such a conclusion: even for samples from the same woman, the signal detection analyses showed so much overlap in fertile vs. non-fertile sample ratings that attractiveness ratings could not diagnose fertile window odors with any appreciable accuracy.

A limitation of the above study was that its findings apply only to conscious perceptions of odor attractiveness. But adaptations designed to respond to odor cues of ovulatory timing do not have to produce subjective impressions of odor attractiveness. As long as those mechanisms respond to such cues in functional ways (for instance, by priming sexual desire or courtship efforts), then human ovulatory timing may be effectively unconcealed. And indeed, some prior research had provided evidence that men may respond to women’s ovulatory scents with increases in hormones and sexual thoughts. Perhaps, then, something really subtle is going on: men accurately diagnose ovulatory timing through subconscious hormonal and psychological responses.

We sought to test a replication of such hormonal and psychological responses in a recent study. Women collected both underarm and genital scent samples on 6 nights spaced five days apart over a period of 30 days. We chose 28 women odor donors with confirmed ovulation in their collection cycles and drew samples from three cycle regions from each woman: during the fertile window, before the fertile window (the follicular phase), and after the fertile window (the luteal phase). One hundred eighty-two male raters were randomly assigned to each smell one of the odor samples (or plain water as a control) and each odor sample was smelled by one man. We collected saliva samples before and after the smelling in order to test for reactive changes in men’s testosterone and cortisol, and we also collected a series of post-stimulus survey measures that assessed the priming of sexual concepts, attribution of sexual arousal to the women scent donors, sexual desire, and motivation to approach others.

The statistical tests most directly relevant to the question of concealed ovulation are those that compare responses to the fertile window stimuli to the responses to all other stimuli. None of those tests produced statistically significant results in this study. Thus, we found no evidence that men respond differently to ovulatory scent cues in terms of hormonal or psychological responses. Bayesian follow-up analyses also suggested strong evidence in favor of the null hypothesis for all dependent variables. These findings occurred despite our study generally having greater statistical power than prior studies that reported positive findings.

Our results differed from some reported in prior studies, and so further research on the question of adaptive responses to ovulatory scent cues could provide important additional evidence. Nonetheless, when considered together with our prior work using signal detection analyses to assess cycle phase shifts in scent attractiveness, the findings from this study suggest that body odors do not reveal women’s ovulatory timing.

Of course, scents are just one sensory modality through which ovulatory timing might be perceived. Cycle phase shifts in faces, voices, and behaviors also have some empirical support, although prior studies have not always rigorously determined ovulatory timing in such studies via use of biological markers (such as urinary luteinizing hormone tests, which were used in the research profiled in this blog post). Future research that both precisely determines ovulatory timing and applies signal detection analyses to any observed cycle phase shifts may lead to a more rigorous set of conclusions regarding the concealment of human ovulatory timing.

Read the original article: Roney, J. R., Mei, M., Grillot, R. L., & Emery Thompson, M. (2023). No effects of exposure to women’s fertile window body scents on men’s hormonal and psychological responses. Evolution and Human Behavior, 44(4), 305-314.