No increased inbreeding avoidance during the ovulatory phase of the menstrual cycle

Abstract

Mate preferences and mating-related behaviours are hypothesised to change over the menstrual cycle to increase reproductive fitness. Recent large-scale studies suggest that previously reported hormone-linked behavioural changes are not robust. The proposal that women's preference for associating with male kin is down-regulated during the ovulatory (high-fertility) phase of the menstrual cycle to reduce inbreeding has not been tested in large samples. Consequently, we investigated the relationship between longitudinal changes in women's steroid hormone levels and their perceptions of faces experimentally manipulated to possess kinship cues (Study 1). Women viewed faces displaying kinship cues as more attractive and trustworthy, but this effect was not related to hormonal proxies of conception risk. Study 2 employed a daily diary approach and found no evidence that women spent less time with kin generally or with male kin specifically during the fertile phase of the menstrual cycle. Thus, neither study found evidence that inbreeding avoidance is up-regulated during the ovulatory phase of the menstrual cycle.

Keywords: endocrinology; fertility; inbreeding avoidance; kin affiliation; kinship.

Graphical abstract

Figure 1.

Self-resembling stimulus faces were created by applying 50% of the difference in shape between an individual's face and the female prototype to both female and male prototype faces.

Figure 2.

Treeplot summarising all estimates from Model 1. Plotted uncertainty intervals are 99% CIs. The inbreeding avoidance hypothesis would predict that when estradiol is high and progesterone low (i.e. EPratio high), self-resembling (stim type) male faces (face sex) would be less preferred than control-resembling male faces, and particularly so for ratings of attractiveness compared with ratings of trustworthiness (judgement type). However, credible intervals for both the interaction of EPratio × stimulus type × face sex × judgement type and the lower-order interactions without judgement type included 0. The weakest form of the inbreeding avoidance hypothesis would predict a simple interaction of fertility (EPratio) and avoidance of self-resembling faces (stimulus type), independent of face sex and judgement type; again, we found no evidence for such an interaction.

Figure 3.

Treeplot summarising all estimates from Models 1 (E-to-P ratio, left) and 2 (E ×P interaction, right). Plotted uncertainty intervals are 99% CIs.

Figure 4.

Change in time spent with family over the menstrual cycle. The y-axis range shows the mean ± 1SD (possible responses were 0–4).

Figure 5.

Forest plot of varying slopes of the fertile phase effect on time spent with family. Each line and dot represent the estimate and 99% credible intervals (CI) for the fertile phase effect on time spent with family, ordered by strength of the fertile phase change.

Figure 6.

Robustness checks for fertile window changes in time spent with family.