Eggs, cancer, and motivated reasoning

The following headline in the The Daily Express caught my attention this week:

“Ovarian cancer – could EGGS be the cause of disease? Vegan charity research REVEALED” 14th March 2017

The article goes on to explain that a Bristol based charity called Viva! Health has urged consumers not to eat eggs, claiming that one egg a week increases cancer risk by up to 70%. According to their own website, Viva! Health is a science-based health and nutrition charity, and being “science-based” you would expect them to have sufficient evidence to make a claim as eye-catching as the one above. So is this the case?food-eggs

Viva! Health claim that eggs are linked to ovarian and prostate cancer in two ways. First, the high cholesterol levels promote these cancers; and second, choline in eggs is linked to prostate cancer. They give references to scientific publications as evidence, but these publications show nothing of the sort. The journal article they point to regarding cholesterol explicitly states that any association between egg consumption and ovarian cancer risk is not due to the cholesterol in eggs. A quick look at the literature also shows that if there is any link between egg consumption and breast or prostate cancer, it is tiny. A similar pattern holds true for the link between choline and prostate cancer. The research that Viva! Health use to support their claim actually shows the opposite, that choline from eggs is not associated with cancer. It’s pretty clear, there’s nothing to worry about.

It took me roughly six minutes to debunk both of these claims, using the identical publications that Viva! Health used to support their claims, so an obvious question is how a charity that clearly thinks of itself as “science-based” could come to the opposite conclusion to me. There is a well-known phenomenon in psychology called motivated reasoning. It describes a process by which someone who holds a particular belief seeks out information that confirms what they already believe, rather than rationally assessing the evidence.

It is a fascinating mental trick that we are all guilty of. We all cling to different beliefs with different strengths. If I was to tell you that plastic bags are more environmentally friendly than cloth bags (unless the cloth bag is used more than 130 times), you are likely to look at the evidence and relatively quickly change your view without a huge amount of argument. On the other hand, if I was to say that immigration is economically bad for a country (or good depending on your point of view), you are far more likely to argue with me and ultimately reject that argument. Although both the plastic v cloth and the immigration arguments are contentious and depend on the studies you look at, the likelihood is that you reacted differently to each.

A lot of recent research has started to dissect these distinct types of beliefs. We have normal beliefs that change with additional information, but we also have a set of beliefs that form the core of our identities. These often take the form of religious or political views, and when these beliefs are challenged we don’t take a rational approach. Instead we employ motivated reasoning, dismissing awkward facts and cherry picking the ones that agree with us. Indeed, if one of these core beliefs is challenged, it is likely that the belief will be ultimately strengthened rather than weakened by the challenge, something called the backfire effect.

Motivated reasoning is extraordinarily common in pseudoscience. Topics like climate change and vaccine safety have decades of reputable research behind them, but despite this, deniers ignore the body of evidence and scientific consensus, deciding to rely on small bits of circumstantial evidence or simple untruths to “prove” their points. There seems to be very little we can do to convince people who hold these beliefs so tightly. However, the majority of the population doesn’t have beliefs like this at the core of their identity. They may have heard the arguments and be unsure about the topic, but with clear evidence and explanation, most people will make the right decisions. This is exactly why it is so important to talk about science and to educate people in how to recognize false claims.

Motivated reasoning may be behind the Viva! Health claim that eggs cause cancer. They are a charity dedicated to promoting veganism, so it is entirely plausible that their beliefs regarding non-vegan foods are central to their identity. Alternatively however, they may just understand that if you link something to cancer (whether it is true or not), you are far more likely to make it into the papers, and have random bloggers talk about you!

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