Fertility, hormonal contraceptives and competitiveness: why can’t we agree?

– by Lindsie Arthur

Competition is woven into nearly every aspect of human life, often in ways we don’t consciously notice. Whether striving for promotions, signalling status in social groups, or competing in dating markets, competition shapes our interactions and decisions. But what fuels these competitive processes?

One possibility is that hormonal fluctuations influence competition in women, particularly hormone change associated with fertility and hormonal contraceptive use. Researchers expect hormones to influence behavior because they are chemical messengers that regulate brain activity related to emotion, motivation, and decision-making. Hormone levels also change in response to internal and external conditions so that organisms can adapt to changing environmental and physiological needs.

Some researchers argue that women may exhibit greater competition for status and mates during the fertile phase, when estradiol is elevated relative to other times in the cycle. Supporting this idea, some studies find that during the fertile phase women are more likely to degrade other women, enhance their appearance, behave dominantly in economic games, and to experience peak motivation for prestige and achievement. However, not all research supports this position, as many studies have failed to find an association between fertility and a range of competitive outcomes, including appearance enhancement, competition in economic games, or self-reported intrasexual competitiveness.

Researchers have also investigated how hormonal contraceptive use may influence competitive motivation and behavior. Hormonal contraceptives prevent pregnancy by introducing synthetic hormones into the body which disrupt ovulation and the implantation of a fertilized egg. A recent review proposed that because hormonal contraceptives disrupt hormone changes across the menstrual cycle, then behaviors associated with certain times in the cycle may also be disrupted. Support for this theory is mixed. Some studies report less competitiveness compared to naturally cycling participants overall, or when comparing competitive motivation during the fertile phase. However, other work finds no differences between hormonal contraceptive users and non-users.

Given mixed results regarding the effect of fertility and hormonal contraceptive use on competitiveness, we conducted a large longitudinal diary study with 302 women (5,600 daily observations) from 22 countries. Participants completed daily surveys for at least one full menstrual cycle. Regarding competitive motivation, daily surveys assessed achievement motivation (reflecting a drive for success and self-improvement) and a general disinterest in competition. We also examined six competitive behaviors: gossip, appearance enhancement, social comparison, taking selfies, giving advice, and negatively evaluating others. These behaviors were chosen because previous research suggests that these are common strategies used by women to compete for mates and status. To estimate fertility, we used each participant’s individual menstrual cycle data to calculate daily fertility probability estimates. In addition to looking for fertility effects, our analyses allowed us to test for different patterns of responding between naturally cycling participants and hormonal contraceptive users.

Contrary to some previous research, we found no evidence that naturally cycling women became more competitive during high-fertility days. That is, we observed no mid-cycle increase in achievement motivation or any of our self-report competitive behaviors. While it is possible that there is no true association between fertility and competitiveness, we wanted to consider reasons why we failed to replicate previous findings, as well as theorise about why replication is relatively uncommon in menstrual cycle studies. Replication challenges in psychology are often attributed to researcher degrees of freedom and methodological issues. Although we agree that these are important factors to consider, differences in sample populations, study design, and environmental factors can also significantly influence findings. While diary studies provide a detailed view of individual behavior over time, the richness of this method also introduces countless contextual variables that may affect replicability. Context dependency may therefore contribute to non-replication, where psychological effects are shaped by specific situational factors that are not currently accounted for in our analyses.

Unlike fertility effects, we did replicate existing contraceptive effects, finding that women using hormonal contraceptives reported less interest in competition compared to naturally cycling participants overall. This suggests that synthetic hormones found in hormonal contraceptives may influence psychological processes related to competitiveness, though further research is required to understand the practical implications of this result. In any case, by better understanding the psychosocial effects of hormonal contraceptives, women and others who rely on this critical medicine can be empowered to make informed decisions about what medical interventions are appropriate for them.

To conclude, we believe our study highlights the need to account for contextual factors in menstrual cycle research. Although context may not be crucial in all research areas, it plays a vital role in dynamic fields like social and evolutionary psychology, where decisions and behaviors are shaped by situational factors. More research is therefore required to reasonably understand how contextual factors and methodological choices may enhance disagreement in the literature. In other words, we know too little about the ways that the social and cultural environment influences the many ways that competitiveness has been measured in previous menstrual cycle research. We encourage future research to combine methods, such as self-reports, diary designs, behavioral observations, and hormonal sampling, to better understand these processes in a range of contexts. By continuing to refine our methods and theories, we move closer to uncovering the full story of how hormones influence our motivations, behavior, and social interactions.

Read the original article: Arthur, L. C., Bastian, B., & Blake, K. R. (2024). Hormonal contraceptive use, not menstrual cycle phase, is associated with reduced interest in competition. Evolution and Human Behavior, 45(6), 106616.