Thursday, July 12, 2012

It's that time of the month: Hasty Generalizations and Post Hoc reasoning.

A recent press release from Concordia University in Montreal has announced the publication of a paper by lead author Gad Saad linking shopping habits among women to their menstrual cycle. In particular, the study seemed to indicate that women were more likely to buy clothing and beauty related products and spend more time on appearance related activities (manicures, tanning salons, sun-bathing etc.) during the fertile phase of their menstrual cycle, and more likely to consume high-caloric foods during the infertile phase of the menstrual cycle. Of particular interest for me is the linking of these behaviors to evolution, with Saad essentially arguing that women are biologically determined to engage in these sorts of behaviors.
"In ancestral times,” he explains, “women had to focus more time on mating-related activities during the fertile phase of the menstrual cycle, when the likelihood of conception was highest. Those same psychological and physiological mechanisms now lead women to engage in greater consumption of products relevant to reproductive drives during the fertile phase of their cycle."
Now I don't want to get into too much detail about the science behind this article since I want to focus on the press-release, but I do want to highlight one element of the research conducted:
Working with his doctoral student and co-author Eric Stenstrom, Saad recruited hundreds of participants by canvassing classes at Concordia university. Through a careful selection process, 59 female participants were chosen. Over a period of 35 days, the women kept detailed diaries that chronicled beautification behaviours, clothing choices, calorie consumption, and purchases.
So what we have here is a very limited sample size. The study is only looking at 59 women over a period of 35 days. All these women are college students, and given that Concordia is a public university they are all likely Canadian citizens. In addition, Montreal is about 75% Caucasian, so it is likely that a majority of the participants in this study are college-age white women (Let me be clear that I am just making estimates based on the press release and some quick Google searches). Given this small, limited sample size, it is clear that the conclusions drawn about the research by the author of the press release is a good example of a Hasty Generalization. A Hasty Generalization occurs when one draws a general or universal conclusion from a small or limited set of data, and this is exactly what we see here in drawing a conclusion about the evolutionary origins of female behavior from a sample size of 59. There are roughly 7 billion people on earth and about half of them are women. That amounts to about 3.5 billion women in the world, so a sample size of 59 would equal about .00000000842% of the population (please check my math and leave a comment if I got ti wrong!). This is really too small a percentage to draw a general conclusion about the evolutionary origins of this behavior.

Beyond this, the authors also appear to be guilty of a Post Hoc fallacy. Here I am less confident about whether or not a fallacy has been committed. Having not read the original paper, perhaps the authors of the study do correct for this, but given the limited information in the press release, I think we can reasonably question whether or not there is a causal link between the behaviors identified and the menstrual cycle. First, the study had a very short duration, just 35 days. The average menstrual cycle is about 28 days, but there is enormous variation among women, and even the same woman can have significant differences between the lengths of her menstrual cycles. Given this, a 35 day study is far too short to draw a causal inference between the cycle and the behavior. Furthermore, given that all of these women were in essentially the same environment (college students at a public university) it seems that there could be numerous other factors causing or contributing to the behavior of the women. Given a study performed under these constraints, the authors have at best identified a correlation, but one can't assume this correlation to be equivalent to causation. This is a Post
Hoc fallacy.

Finally, even if the authors have discovered a causal connection between menstruation and consumption behaviors, it doesn't seem that one can assume that the latter causes the former. For example, it might well be the case that cultural factors play a much larger role in determining behavior than genetic or biological factors. It might be that women are told by their cultures to engage in certain behaviors at certain times of their menstrual cycle. If this were true, then it wouldn't be biology causing behavior, but rather culture causing behavior during certain times of a woman's menstrual cycle. Given that all the participants in the study were members of a similar culture group (women at a large, Canadian public university), this hypothesis seems as reasonable as the conclusion drawn by the authors. I am not claiming that this hypothesis is correct, but merely noting that it is another possible explanation that appears to be neglected, and when one assumes a particular causal relation beyond the available evidence, one is is committing a Post Hoc or False Cause fallacy.

For more on the relation between genetics, culture and behavior generally, I recommend this podcast.

h/t to Pharyngula

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