Decreased Sexual Motivation during the Human Implantation Window
– by James R. Roney
A lot of research has investigated shifts in women’s psychology and behavior across different phases of the menstrual cycle. One pattern that has emerged in multiple studies is an increase in sexual desire during the “fertile window,” which is defined as the cycle days when unprotected intercourse can lead to conception. In humans, the fertile window extends from about 5 days before ovulation through the day of ovulation itself, with the ovulation day usually falling near the mid-point of a typical menstrual cycle.
Why should desire go up during the fertile window? Motivational priorities theory argues that hormonal signals tend to increase sexual motivation when the net fitness benefits of sex were highest during human evolution. Fitness benefits were outcomes associated with increased gene replication, whereas fitness costs were outcomes associated with decreased gene replication; net benefits were benefits minus costs. Sex would have had fitness costs for our female ancestors: it could have caused injury, infection, social disapproval from competitors, or just distraction from other behaviors. Conception would have been a fitness benefit from sex: indeed, gene replication is not possible without it. Because conception was possible only during the fertile window, however, there was an increase in the benefit side of the fitness equation during the fertile window that may have acted as a selection pressure to increase sexual motivation at that time.
This simple cost-benefit analysis may explain why most of the evolutionary literature on menstrual cycle shifts—including my own research—has focused primarily on the fertile window. But what if certain fitness costs of sex were not constant across the cycle, as perhaps generally assumed, but instead increased during specific time windows? My co-authors and I explored that possibility, reasoning that increased infection risk during the “implantation window” may have selected for reduced sexual motivation at that time.
The implantation window refers to those days when the endometrium (the uterine lining) is receptive to attachment by an embryo. (The image at the top of this post illustrates the attachment process.) Part of being receptive entails a reduction of immune responses within the uterus to prevent the immune system from attacking an embryo if conception has occurred. However, that same immunosuppression may make women more vulnerable to sexually transmitted infections (STIs) at this time. As reviewed in our article, progesterone appears to be a key signal that helps to cause such immunosuppression, and both experiments in nonhuman species and correlational research in humans supports increased susceptibility to some types of STIs when progesterone is elevated. That increased infection risk should have increased the cost side of the benefits minus costs fitness equation during the implantation window, thus acting as a selection pressure to reduce sexual motivation at that time.
Based on the above reasoning, we tested whether measures of women’s sexual motivation were lower during the estimated implantation window. The implantation window is estimated to run from about 5 to 9 days after ovulation in humans, during an infertile region of the cycle called the luteal phase. We used data from three daily diary studies that had previously been conducted in my lab. In each study, women responded to daily online surveys across full menstrual cycles. The surveys included measures of subjective sexual desire and reports of whether women masturbated, which were chosen as our primary measures of sexual motivation. Hormonal estimates of the day of ovulation were also available in each study; by counting forward from the day of ovulation, we could estimate the implantation window days within each cycle.
Across the three studies, there were 2576 survey responses across 102 ovulatory menstrual cycles sampled from 83 women. Our data analyses used statistical techniques to estimate, within the same women, whether responses differed on implantation window days relative to other cycle regions. (Imagine, for instance, for one woman, subtracting her average desire during the implantation window from her desire on all other days, and then averaging that difference across all women; our multi-level regression models made conceptually analogous comparisons.)
Our results provided evidence for lower self-reported sexual desire during the implantation window relative to the remaining days of the cycle. This was true in each of the three studies and also in the combined sample. Importantly, desire in the combined sample was lower in the implantation window relative to cycle regions that excluded days within the fertile window. That result demonstrates that lower desire during the implantation window was not simply an artifact of comparisons with elevated desire during the fertile window. Results for our behavioral measure of sexual motivation were slightly less clear, perhaps because about 40% of our sample reported no incidents of masturbation. Nonetheless, in the combined sample, there were one-third lower odds of masturbation during the implantation window relative to all other cycle days.
These findings add a new wrinkle to research on cycle phase shifts in women’s sexuality. Hormonal mechanisms may generate changes in sexual motivation within two conceptually significant windows of time: increased desire within the fertile window when conception is possible, and decreased desire during the implantation window when infection risk is elevated. There may be alternative explanations for why desire is reduced during the implantation window—some of which we considered in our article—and we hope these findings encourage further research into whether there are adaptations that modulate sexual motivation based on implantation window timing.
