New research questions the role of fertility in women’s indirect aggression

A new study finds that a woman’s competitive behavior toward other women is driven by her individual personality traits rather than changes in her menstrual cycle. The research suggests that fertility levels do not influence whether women act competitively in mating or social contexts. The findings were published in the journal Evolution and Human Behavior.

Evolutionary psychologists study how human behavior adapts to ensure survival and reproduction. For women, competing for romantic partners and social alliances is an important part of life. Historically, direct physical fights posed severe risks, as injuries could limit a mother’s ability to raise offspring.

To avoid physical harm, female competition often takes the form of indirect aggression. Indirect aggression includes covert strategies like spreading rumors, social exclusion, and subtle character assassination. These tactics allow individuals to damage a rival’s reputation without initiating a physical confrontation.

Past research has suggested that hormone fluctuations during the menstrual cycle might dictate when and how women use these strategies. Estrogen reaches its peak just before ovulation. This is the phase of the menstrual cycle when women are most fertile and physically capable of becoming pregnant.

Because the immediate reproductive benefits of mating are highest during ovulation, some researchers hypothesized that this estrogen peak would trigger increased competition for romantic partners. Progesterone, another hormone, follows a different pattern. It remains low during ovulation but peaks later in the luteal phase as the body prepares for a potential pregnancy.

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High progesterone levels have been linked to affiliative behaviors and a desire for social bonding. During a pregnancy, social support from other women would be highly beneficial for acquiring resources and childcare. Researchers theorized that during this high-progesterone phase, women might compete more fiercely to protect their existing social alliances from potential threats.

Lead author Jaime L. Palmer-Hague, a psychology researcher at Trinity Western University, and her colleagues set out to test these hormone-based theories. They wanted to see if women target their competitive behaviors toward strong rivals at specific points in their menstrual cycles.

The research team recruited 464 naturally cycling women for an online experiment. The participants did not use hormonal birth control, which alters natural hormone fluctuations. The researchers estimated each participant’s current menstrual cycle phase using a technique known as the backward counting method.

This method involves tracking exactly when a participant’s next period begins and counting backward to estimate the day ovulation likely occurred. Based on these estimates, participants were categorized into groups reflecting high or low fertility phases.

In the first part of the experiment, participants viewed computer-generated faces of hypothetical romantic rivals. The researchers used special software to blend existing photos into composite faces. One face was designed to be highly attractive, representing a strong mating threat, while the other face was less attractive, representing a weak mating threat.

Participants answered questions about how they would react to each woman in a romantic context. They rated their likelihood of introducing the rival to their boyfriend, their comfort leaving the two alone, and their likelihood of gossiping about the rival. The researchers then compared these behavioral responses across the different menstrual cycle phases.

Women reported a much higher likelihood of acting competitively toward the highly attractive rival than the less attractive one. However, the menstrual cycle phase had no effect on these behaviors. The statistical tests comparing competitive behavior across the different hormonal phases were not statistically significant.

Instead of hormones, the researchers found that an individual’s baseline personality predicted their responses. Women who scored high on a separate questionnaire measuring intrasexual competitiveness were much more likely to act competitively toward the attractive face. Intrasexual competitiveness is a psychological trait reflecting a general tendency to view same-sex peers as rivals.

Participant age also played a predictive role in the mating context. Younger participants displayed more competitive responses toward the highly attractive face than older participants. Estimated hormone levels played no role in predicting these behaviors.

In a second set of tasks, the researchers explored competition for social alliances. They showed the participants a different pair of computer-generated faces representing potential social rivals. One face represented a strong social threat, designed to look socially dominant and prone to indirect aggression, while the other represented a weak social threat.

Participants answered questions about how they would interact with these women in relation to their own best friend. They rated their comfort with the rival spending time with their best friend and their likelihood of gossiping about the rival. Once again, participants exhibited more competitive behavior toward the strong social threat compared to the weak one.

Just as in the mating context, the menstrual cycle phase did not influence the social competition results. Variations in estimated estrogen and progesterone levels played no role in how women reacted to the social rivals. Personality traits were the only factors that predicted social competition.

Women with higher levels of self-reported dominance and intrasexual competitiveness were more likely to act competitively toward the social rivals. Conversely, women who scored high in friendliness and self-esteem were less likely to engage in competitive behaviors in the social scenario.

The study relied on a mathematical estimation method to determine menstrual cycle phases rather than biological tests. The researchers did not use ovulation kits or blood tests to verify exact hormone levels. This limitation means that subtle hormonal effects might have been missed due to natural variations in cycle length across individuals.

Additionally, the experiment used hypothetical scenarios and computer-generated faces to gauge reactions. Self-reported responses in an online survey might not perfectly reflect how people behave in real-world situations. People are not always fully aware of their own competitive behaviors, which could lead to underreporting.

Future studies might benefit from observing actual social interactions to verify these self-reported tendencies. Larger samples and direct hormone tracking could also help evaluate whether any minor hormonal influences exist.

The study, “Women’s intrasexual competitiveness, but not fertility, predicts greater competitive behavior toward attractive women across the menstrual cycle,” was authored by Jaime L. Palmer-Hague, Jade S. Stobbart, and Benjamin J. Zubaly.