Big discounting... On their fertile days, women are more likely to opt for smaller now than for larger larger later rewards, be it money, food, or sex

Discounting for Money, Food, and Sex, over the Menstrual Cycle. Benjamin T. Vincent, Mariola Sztwiertnia, Rebecca Koomen & Jasmine G. Warren. Evolutionary Psychological Science, Jul 25 2022. https://rd.springer.com/article/10.1007/s40806-022-00334-z

Abstract: Sexual desire, physical activity, economic choices and other behaviours fluctuate over the menstrual cycle. However, we have an incomplete understanding of how preferences for smaller sooner or larger later rewards (known as delay discounting) change over the menstrual cycle. In this pre-registered, cross-sectional study, Bayesian linear and quadratic binomial regression analyses provide compelling evidence that delay discounting does change over the menstrual cycle. Data from 203 naturally cycling women show increased discounting (preference for more immediate rewards) mid-cycle, which is at least partially driven by changes in fertility. This study provides evidence for a robust and broad-spectrum increase in delay discounting (Cohen’s h ranging from 0.1 to 0.4) around the fertile point in the menstrual cycle across multiple commodities (money, food, and sex). We also show, for the first time, that discounting changes over the menstrual cycle in a pseudo-control group of 99 women on hormonal contraception. Interestingly, such women increase their discounting of sex toward the end of the menstrual phase — possibly reflecting a prioritisation of bonding-related sexual activity before menstrual onset.

Discussion

The present study aimed to address three questions. Firstly, we explored whether cycle phase (EF, LF, LP) influences delay discounting across the food, sex, and money commodities in NC women (see Fig. 1). Secondly, by assessing non-linear trends over the cycle for both NC and HC women, we found evidence for a quadratic trend for all commodities and both groups (see Fig. 2). For the NC group, immediate choices increased from the EF to the LF (when most fertile) and then decreased to the LP across all commodities. The same pattern occurred for the HC group for money and food; however, the opposite pattern was shown for sex. Thirdly, we explored whether change in discounting across the cycle could (at least partially) be explained by changes in fertility. This does seem to be the case, as NC women showed a linear increase in immediate choices across all commodities as a function of fertility (see Fig. 3).

The results summarised in Fig. 1 are in line with those of Lucas and Koff (2017). Similar to Lucas and Koff’s (2017) results, the probability of choosing immediate rewards increased from the EF to the LF, followed by a drop again in the LP; however, in contrast to their findings, they remained somewhat elevated compared to the EF phase. Our results go further, however, in that we show this pattern of behaviour generalises across all the commodities tested — money, food, and sex. The present findings did contradict the finding of lower preference for immediate monetary rewards near ovulation (Smith et al., 2014). Although they used hormonal assays to determine cycle phase which is more robust than the count-back method used in the present study, the authors only took measures on 2 days. The findings of the present study match those of Lucas and Koff (2017) which measured day of cycle at multiple timepoints.

Discounting for money, food, and sex changes over the menstrual cycle for NC women, with more immediate choices being made on average around the most fertile LF phase (see Fig. 2), is in line with the ovulatory shift hypothesis. NC individuals would theoretically be expected to optimise resources and immediate choices when conception is at a higher risk (da Matta et al., 2013; Gildersleeve et al., 2014). Additionally, there was support that some of the variation in discounting behaviour in NC women over the menstrual cycle can be attributed to fertility (see Fig. 3). Women who are NC are, on average, more likely to choose immediate rewards at more fertile points in their cycle. NC women should choose immediate rewards at peak fertility, especially sex, as they would be anticipating offspring. Biologically, we know that hormone levels during this point lead to increased impulsivity which is consistent with this result (Diekhof, 2015). The finding of preference for food increasing with fertility contradicts those by Fessler (2003) who reported a decrease in calorie consumption with fertility. However, this could be the result of the present study utilising discounting measures which represent a preference and do not necessarily translate to behaviour. For medium-/high-fertility levels, NC women are more likely to choose immediate lower-quality sex than HC women. This inspires the hypothesis that because HC bodies are more hormonally similar to pregnancy than NC bodies, perhaps higher-quality sex may be preferred as a method of partner bonding over an immediate but low-quality opportunity for intercourse and conception.

Interestingly, HC women discount more for money and food, only matched by NC women at peak fertility. The rise and fall of immediate choices for money and food were even more exaggerated compared to NC women. Most hormonal contraceptives function through simulating pregnancy. Therefore, HC individuals would need resources for offspring. Again, biologically this is plausible as HC hormone levels are most similar to the fertile point of the cycle. The exception to the above was discounting of sex by HC women. The probability of choosing immediate low-quality sex over delayed high-quality sex seemed to be approximately stable but with an increase toward the end of the menstrual cycle. This could be noise that may disappear in a study with more participants or using a longitudinal design. If not, a speculative hypothesis (to be tested in a future study) would be that it represents a desire to have sex before the next menstrual period begins. Given that there is no risk of conception to HC individuals, they may be opportunistically choosing intercourse later in the cycle (LP) to avoid having intercourse during menstruation. Vaginal intercourse during menstruation has medical implications, including an increased risk of sexually transmitted infections (STIs) and endometriosis (Mazokopakis & Samonis, 2018). Another plausible explanation for this result could be that HC can lead to increased libido as it improves PMS and studies have shown increased female-initiated activity later in the pill cycle (LP; Guillebaud, 2017). The quadratic trends seen within the HC group were unexpected, and although hypotheses can be speculated, there is a need for future research to consider the mechanisms underlying these findings, and whether HC type has a role.

Additionally, not comparing different types of HC is problematic as the different types have varying hormonal effects (Hampson, 2020). Further studies should include an HC sample when investigating the menstrual cycle and account for the type and duration of contraceptive use. Both the present study and Lucas and Koff (2017) used cross-sectional designs which is problematic as menstrual cycles vary between females and as such a within-subject design would be beneficial for future research. While the Cohen’s h effect sizes are “small”, this should be interpreted with extreme caution — they are entirely unlike Cohen’s d effect sizes, taking no account of the difference normalised by the degree of variance, nor the sample size. As such, in the context of delay discounting, a change from ~ 7.5 to ~ 20% immediate low-quality over delayed high-quality sex choices (see Fig. 1, top right) could be considered as a behaviourally meaningful change. We do not make strong claims based on our data about the exact shape of this change over the menstrual cycle, nor about the location of the peak of immediate choices. The present study used quadratic regression which has been used before in tracking how variables change over the menstrual cycle (Kuukasjärvi et al., 2004). Future research should continue with this non-linear form of analysis over days (not categorical phases) to encompass changes across the menstrual cycle phases as other analyses may fail to capture its cyclical nature.