Dwayne Bent/Flickr
Source: Dwayne Bent/Flickr

Women are more competitive during the fertile phase of their menstrual cycles, and are less motivated to cooperate with their female rivals, according to psychologists from the University of California.

Research suggests that there is a link between female fertility and competitiveness. At the time of the month when women are most likely to conceive, they are less willing to cooperate with other women and more willing to dish out punishments for perceived slights.

Ultimatum

Adar Eisenbruch and James Roney of the University of California, Santa Barbara’s Department of Psychological and Brain Science decided to test for an effect of the menstrual cycle on behavior in the Ultimatum Game. The Ultimatum Game is a standard game in behavioral economics. The rules are as follows:

There are two players. One of the players acts as the Proposer. She is told that there is a monetary reward that she can offer to her opponent, the Responder. She can offer as much or as little of the reward as she wants. The Responder then gets to decide whether she wishes to accept or reject the offer. If she accepts, she gets to keep the reward and the Proposer takes whatever’s left. But if the Responder rejects the offer, both players get nothing.

Now, in purely economic terms, it makes sense for the Responder to always accept the offer. If they are offered 90% or 9% of the reward, that’s always going to be better than nothing. But humans don’t think in purely economic terms. Our concept of fairness leads us to feel that a stingy offer is inequitable. If either player could have been chosen as Proposer, why is it right for the Responder to agree to a low-ball offer? An ungenerous offer provokes spite: the desire to punish others, even at our own expense, when we feel cheated. A feeling that, if we are not offered a fair portion, we would rather go without than see our rival take the lion’s share of the reward.

In practice, most offers hover around 50%, and Responders are more or less willing to accept offers that are low but not insultingly minuscule.

In Eisenbruch and Roney’s version of the task, ~70 women played a series of online Ultimatum Games against unknown female competitors, photographs of whom appeared on screen. Women played two games with each competitor, acting as both Proposer and Responder.

Ultimatum Game Results

The average offer made by Proposers in Eisenbruch and Roney’s Ultimatum Game was $4.93 of a possible $10. The average level at which the Responders accepted the offer (what the researchers call their ‘demand’) was $4.03.

The researchers found that there was a significant and positive correlation between women’s mean demand and their conception risk. That is, women who were more fertile were also more demanding of a fair share of the reward. Women who were less fertile were less demanding of a fair share. On average, Responders accepted offers of ~$4.50 when they were most fertile, but were happy with ~$3.00 when least fertile.

Remember that we’re not talking about fertility in general, but rather fertility as estimated from where a woman is in her cycle. So we would expect women to change in their demands across the month, with higher demands near ovulation and lower demands earlier and later in the cycle when it is less likely for them to conceive.

Conception risk and mean demand in $ plotted against days since the start of the period. Conception risk (likelihood of becoming pregnant after intercourse) is higher mid-cycle. There is a cyclical effect on mean demand, such that women are more demanding of a fair share of the reward when they are more fertile, and less demanding when they are less fertile. Graph taken from Eisenbruch & Roney (2016).

There was no effect of cycle on the size of offers, however, which contradicted the findings of previous researchers. This implies that the cycle affects women’s cooperativeness with other women in a specific way. Fertile women are no more or less generous with their opponents, but they do seem to be more susceptible to spite or a willingness to punish ungenerous rivals. However, Eisenbruch and Roney are hesitant to place too much weight on these findings, given that the number of women they tested was rather small.

Fertile rivals

In a weird coincidence, another research paper was published recently on this very specific topic of female fertility and competition, a paper that examines the question from a different perspective. We’ve already seen that fertile women compete more strongly with their rivals. But do women compete more strongly with fertile rivals?

Elizabeth Necka, Stephanie Dimitroff, and Greg Norman of the University of Chicago, along with my former boss, David Puts of the Pennsylvania State University, had women take part in an economic game. Not the Ultimatum Game used by Eisenbruch and Roney, but a simpler version called the Dictator Game. Severely stretching the definition of the word ‘game’, it works like this: the Dictator, like the Proposer in the Ultimatum Game, splits a reward with their opponent. However, unlike the Ultimatum Game, the opponent has no active role to play. They can’t refuse the split. They get whatever the Dictator decides to give them, and the Dictator keeps whatever she wants.

~150 women were photographed and were then asked to play a computer-version of the Dictator game against another woman, who was pictured on screen. They were told that this other woman would be able to see their photo. The research participants were then informed that they had been randomly selected to act as the Dictator. This was, of course, not strictly true because all participants acted as Dictators: the women pictured on screen were participants in an earlier study of the menstrual cycle. They had been photographed when they were at peak fertility and also later in their cycles when their fertility was low. Participants in Necka and colleagues’ new study were shown a photo of their opponent — a high fertility or a low fertility opponent chosen at random — and offered her a split of the total reward, which in this case was $5.

There is good evidence that women appear more attractive when most fertile, and that both men and women can pick up on this difference. But are women more generous with rivals who are high or low in fertility?

Dictator Game Results

The results of the study showed that there was an interaction between the fertility of the Dictators and their opponents. This means that the amount of money the Dictators offered depended both on their own fertility and their opponents’ fertility.

When Dictators were high in fertility they offered, on average, $2.30 (46%) to opponents at low fertility but only $1.67 (33%) to opponents at high fertility. When Dictators were low in fertility, the share they offered did not vary according to their opponents’ fertility.

So, yes, women are more generous with rivals who are low in fertility, and less generous with rivals who are high in fertility, but only when they themselves are fertile.

Echoing Eisenbruch and Roney, the authors of this paper admit that their results should be considered provisional until they are replicated. Some scientists have suggested that research on psychology and the menstrual cycle is often underpowered because too few women are recruited as volunteers. We may need sample sizes approaching 1000 rather than the low hundreds to detect genuine effects of the cycle on social behavior.

That Time of the Month

Why does fertility affect women’s competitiveness, generosity, and desire to punish their rivals?

One possibility is that, at times when they are most likely to conceive, straight women are in competition for access to the best males. That is, the males who would make the best genetic fathers for their offspring. We know that women are more motivated to appear attractive and to spend money on attractiveness-enhancing products when most fertile, so a desire to keep resources for oneself (and out of the hands of a potential love rival) could increase a woman’s buying power on the mating market.

Another possibility, floated by Eisenbruch and Roney, is that the effects of the cycle are evolutionary by-products. Perhaps in our species’ distant past, women spent most of their reproductive lifespan gestating, lactating, or under nutritional stress. During these times, their primary motivation would have been to nurture offspring. However, when their fertility was high — perhaps after weaning their most recent child or when their nutrition was relatively good — they would have been more motivated to seek a mate, and to out-compete rivals for the attention of potential mates. Changes in these circumstances would be associated with changes in estrogen levels, which also vary on the smaller time-scale of the menstrual cycle. The reasoning goes that the link between estrogen and competitiveness, adaptive over a long time-scale, is not so important on a month-to-month basis, but that we can detect that link in modern Western women whose life history is so different to that of our ancestors.

I think one important point to keep in mind is that the effect of the cycle is not huge, and that we could just as easily say that women are more cooperative during the less fertile phase of their cycle instead of less cooperative when more fertile. I shudder to think how generous men are when playing the Dictator Game…

References

Eisenbruch, A. B., & Roney, J. R. (2016). Conception risk and the ultimatum game: When fertility is high, women demand more. Personality and Individual Differences, 98, 272–274. Read summary

Necka, E. A., Puts, D. A., Dimitroff, S. J., & Norman, G. J. (in press). Other women’s fertility moderates female resource distribution across the menstrual cycle. Evolution and Human Behavior. Read summary

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