Previous research has found that women at peak fertility show greater interest in extra-pair sex; however, recent replications have failed to detect this effect

Current Fertility Status Does Not Predict Sociosexual Attitudes and Desires in Normally Ovulating Women. Andrew G. Thomas.  Evolutionary Psychology,January 8, 2021. https://doi.org/10.1177/1474704920976318

Abstract: Previous research has found that women at peak fertility show greater interest in extra-pair sex. However, recent replications have failed to detect this effect. In this study, we add to this ongoing debate by testing whether sociosexuality (the willingness to have sex in the absence of commitment) is higher in women who are at peak fertility. A sample of normally ovulating women (N = 773) completed a measure of sociosexuality and had their current fertility status estimated using the backward counting method. Contrary to our hypothesis, current fertility was unrelated to sociosexual attitudes and desires, even when relationship status was included as a moderator. These findings raise further doubts about the association between fertility and desire for extra-pair sex.

Keywords: ovulatory shift hypothesis, sociosexuality, menstrual cycle, mate preferences, extra-pair mating

Discussion

The purpose of this study was to test the idea, derived from the ovulatory shift and dual-mating hypotheses (Gangestad & Thornhill, 1998; Penton-Voak et al., 1999), that women’s mating strategies change in accordance with fluctuations in fertility across the menstrual cycle. To do so we used a between-subjects sample of normally ovulating women to examine whether current fertility status could predict sociosexual desires and attitudes. Our hypothesis was not supported. No relationship was found between fertility status and attitude or desire subscales of the SOI-R. In fact, the β observed in the regression predicting sociosexual desire was in the opposite direction to that predicted (β = −.04). Exploratory analyses showed that the null effect of fertility persisted when relationship status was added to the models as a moderator. We did find that older women reported higher numbers of past partners and acts of uncommitted sex, but only if they were currently high in fertility. We did not predict this weak association and see no theoretical reason for it. Given that this effect also disappeared when covariates were added to the model (see Footnote 1) we are inclined to believe this to be a Type I error.

It is possible that the null results obtained in this study are due to the methodological issues outlined previously. The ‘gold standard’ in menstrual cycle research is to use within-subjects designs and to establish fertility status using hormonal measurements (Gangestad et al., 2016). Our study, in contrast, used a between-subjects design with self-reported cycle data and non-verified bleed-dates. Nonetheless, there are several reasons to be confident that our results are not due to a Type II error.

First, the sample size is large and surpasses the minimum requirement set out by both Gangestad et al. (2016) and Jones et al. (2019) for a sufficiently-powered between-subjects study. Second, as recommended by Gangestad et al. (2016), fertility status was not determined using high and low fertility windows, but instead measured along a continuous scale of conception risk (Wilcox et al., 2001). Finally, we employed a backwards-counting method to determine day of cycle. Due to the large degree of variability in the follicular phase compared to the comparatively consistent length of the luteal phase (Fehring et al., 2006), the backwards-counting method is the most reliable way to obtain self-report menstrual cycle data.

The results of this study converge with other recent well-powered between-subject investigations into the ovulatory-shift hypothesis (Dixson et al., 2018; Marcinkowska et al., 2018b) and within-subjects studies using hormonal measures (Jünger et al., 2018; Marcinkowska et al., 2018a). However, it should be noted that some studies suggest that fertility-related shifts in mating psychology depend on relationship status and on factors within a relationship, such as the attractiveness of one’s current partner (Haselton & Gangestad, 2006; Pillsworth & Haselton, 2006). We did not have data about perceived partner attractiveness for partnered women in our dataset, but we were able to include their relationship status in our models. Doing so did not qualitatively alter the role of fertility status. It is worth noting, however, that this exploratory analysis was underpowered. A full understanding of this moderation effect would have required us to examine sub-groups of single and pair-bonded women, effectively halving the sample. The two studies that report an effect of partner attractiveness were also both underpowered (see the calculations of Gangestad et al., 2016; Jones et al. (2019). Therefore, future research should investigate the role of relationship moderators on fertility effects in sufficiently large samples to test these hypotheses adequately.

Further consideration should be given to the role of sociosexuality in acquiring extra-pair partners. There is an established relationship between sociosexuality and infidelity (Barta & Kiene, 2005). Thus, it is reasonable to expect that waxing interest in casual sex would facilitate acts of extra-pair infidelity and that this may form part of an ovulatory shift mechanism that functions to shift mating effort away from a primary partner and toward another. However extra-pair liaisons are not exclusively casual and may involve feelings of love and commitment, such as in cases of mate-switching (Buss et al., 2017). To the extent that uncommitted sex is not a strict prerequisite for extra-pair relationships, we cannot rule out the possibility of dual-mating mechanisms in humans based solely on an absence of relationship between within-cycle fertility and SO. Potential future research could consider forgoing sociosexuality for more implicit indicators of relationship commitment, including motivated biases favoring one’s partner (e.g., positive partner illusions) or derogating alternatives (Finkel et al., 2017).

In sum, these null results raise further doubts about the hypothesized association between fertility and desire for extra-pair sex, and more specifically the role of sociosexuality as a potential moderator of this process. Should extra-pair desire change across the menstrual cycle, then this may be context specific and/or facultative. Such changes may be difficult to detect at a general group level, emphasizing the importance of well-powered within-subject designs that both use hormonal verification to reduce measurement error and take relationship context and motivation into account.