Study on hormone fluctuations finds women feel more assertive when they are fertile

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Female hormones impact women’s assertiveness and sexual availability, according to a study recently published in Psychoneuroendocrinology.

Previous studies of female sexual behavior have found that women are the fussier sex when it comes to mate selection. This is thought to be because women put in the greater parental investment due to pregnancy, giving birth, breast feeding and historically being the primary carer for the offspring, whilst the male took on a hunter-gatherer role. Therefore, females are more selective in choosing a high quality mate that compensates for their parental investment.

Female reproduction is limited by windows of fertility, as well as the duration and physical costs of gestation. During windows of fertility it is important for women to be able to discern good from bad quality mates as this is when genes will be passed on to their offspring. Therefore, it would make sense that women should be more selective about mates during times when conception is likely. However, little is known about whether a woman’s ability to choose the most beneficial mate changes when it is most important i.e. when she is fertile.

The study conducted by Khandis Blake, Siobhan O’Dean, Thomas Denson (University of New South Wales) and Brock Bastian (University of Melbourne) recruited a total of 98 women and measured their ovarian hormone levels during fertile and non-fertile points in their menstrual cycles, to investigate whether level of assertiveness during windows of fertility was linked to increased sexual motivation. The participants were also measured for interest in buying sexy clothing.

The results found that women are more assertive when levels of estradiol are high and when progesterone levels are low; this is the time when women are fertile. The authors state ‘high assertiveness in women is influenced by ovarian hormones and associated with high sexual motivation’. It was also found that women are more interested in buying sexy clothes when they are fertile. The results of this study are consistent with previous research that has found women are more interested in attracting a mate when they are fertile compared to when they are not.

The findings suggest that being more assertive and sexually motivated during windows of fertility could be an evolutionary advantage for women as it increases the chances attracting a suitable mate and protecting themselves against unwanted sexual advances.

PsyPost interviewed Blake about the research. Read her responses below:

PsyPost: Why were you interested in this topic?

Blake: I was interested in the topic because of the evolutionary literature on female mate choice, and whether women are the most ‘choosy’ sex. If this was the case, you’d expect that women would be more likely to demonstrate choosy abilities during the period where they were fertile. This is the only time when choosing a sexual partner really ‘matters’, in biological terms. Assertiveness is important to women for various reasons, especially regarding sexual assault (I’ve written about it here: https://www.ncbi.nlm.nih.gov/pubmed/26848102). Thus, I wanted to look at this area.

What should the average person take away from your study?

The average person should take away that women feel more assertive when they are fertile. This probably has adaptive significance – it allows them to negotiate male-female relations and make their preferences regarding mates clear. It might also provide some protection against unwanted sexual advances – findings from some of my other work point to the notion that men are more likely to try and sexually victimize non-assertive women (the paper above).

However, it probably has implications for other areas of life, too. If women are more assertive when they are fertile, those effects may transfer to other domains. In fact, we didn’t measure mating specific assertiveness, but assertiveness generally. So the results suggest that during the brief period of fertility each month, women’s assertiveness boosts. This has implications for understanding hormonal birth control, hormonal replacement therapy, and other medications which influence estradiol and progesterone. Because our effects were hormonally mediated, the implication is that other medication which influence hormones may be increasing or decreasing women’s assertiveness.

Women who are on birth control all throughout their 20s, for example, may not experience the natural peaks and troughs of assertiveness that we find in our study because their estradiol and progesterone is being constrained by the medication.

Are there any major caveats? What questions still need to be addressed?

I think the main thing that needs to be addressed is whether HBC and HRT also affect assertiveness. This has major implications for understanding the social impact of these medications which are so widely used. Also, it would be useful to see how hormonally-mediated increases in assertiveness support women’s intrasexual competition. High assertiveness when fertile probably helps women compete against other women , too.

Is there anything else you would like to add?

I’d like to add that we don’t know much about how female sex hormones affect women’s social behaviour. In contrast to work on testosterone, for example, much less is known about estradiol and progesterone. I think this is an important area for future research. Progesterone can increase x500 fold over the course of the menstrual cycle, and estradiol can increase ~30 fold. With such great fluctuations it is important to understand how these hormones can encourage certain behaviors to manifest. That’s not to say that our behaviour is completely determined by hormones – but that these things can have an influence. I look forward to more work in this field.

The study was titled: “High estradiol and low progesterone are associated with high assertiveness in women“.



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