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Is Evolution "Just a Theory"?

Post #3 in a series for Darwin Day 2012

Image of Charles Darwin statue

Image copyright Sebastian Ballard

Last Sunday, February 12th, was Darwin Day: a celebration of the life and work of Charles Darwin. Yet more than 150 years after the publication of his monumental On the Origin of Species, Darwin's theory of evolution by natural selection remains controversial outside the sciences, at least in the United States.

In this series of posts, I've been exploring the psychology behind our explanations for the biological world, and in particular behind the acceptance or rejection of evolutionary theory. In Post #1, I considered whether feeling powerless in an uncertain world makes explanations that offer a sense of order and control, such as Intelligent Design, more attractive than "random" natural selection. In Post #2, I considered whether people reject evolution because they think it has negative existential consequences. In today's post I turn to a topic that arose in the comments from my last post:

Is evolution "just a theory"? And is this dismissive attitude related to whether people accept or reject evolution?

It started with a comment by Alice, who shared the following view:

Who said evolution is true?!

As a believer (christian) I believe that God created the Universe and everything in it.

As a non-scientist, I think if "flies" in the face of science/reason to think that the Universe just happened.

That is why evolution is called a theory.

This comment sparked a flurry of replies, including the following highlights:

The theory of evolution is a theory, just like the theory of gravity. The word theory does not mean something is untrue. (from George)

In the scientific world, "theory" has a specific and more-or-less value-neutral meaning. It covers a wide spectrum, from the more speculative, such as at the wilder end of theoretical physics, to the firmly established and pretty stable, such as electromagnetism. (from Phillip)

There is a gross public misunderstanding of the semantics of the word "theory". The way scientists use the word is very different from how we use the word in our every day discussions. (from Derek)

In casual conversation, calling something a 'theory' can be a way to mark disagreement or distrust. Consider: "That's one theory," "That's just your theory," or "That's a nice theory, but is it true?"

In science, however, the word "theory" doesn't have these same connotations. Accepted scientific theories are typically supported by decades of research and have undergone intense scrutiny by the scientific community. So evolution is a theory, just like gravity and the germ theory of disease. But it isn't "just a theory," implying that it falls short of some more desirable scientific status.

Could it be that some people reject evolution because they think of it as "just a theory"?

Along with science educator Anna Thanukos and philosopher of science Michael Weisberg, I investigated this question in a paper published in Evolution: Education & Outreach in 2008. We had dozens of students complete questionnaires to assess their beliefs about science and evolution. Our aim was to find out whether there's a relationship between how well someone understand the nature of science – issues like what counts as a scientific theory and how science proceeds – and whether she or he accepts evolution as an explanation for humans and other species.

And in fact, we did find a moderate correlation between people's understanding of the nature of science and their acceptance of evolutionary theory. In other words, people who understood the nature of science were more likely to accept evolution. And while the correlation wasn't enormous, it also wasn't small. In fact, the correlation was almost as large as that between religiosity and accepting evolution – there we found a moderate negative correlation, meaning that students who were more religious were less likely to accept evolution. (Interestingly, there was no significant relationship between understanding the nature of science and religiosity, so it wasn't the case that religious students were less likely to understand the nature of science.)

While we examined many aspects of the nature of science, one of those that predicted whether people accepted evolution was precisely the nature of theories. For example, students who agreed with the following statements were more likely to accept evolution:

Scientific theories are subject to ongoing testing and revision.

Because they are supported by much evidence, accepted scientific theories and hypotheses are reliable.

In contrast, students who agreed with this next pair of statements (which mischaracterize the nature of scientific theories) were less likely to accept evolution:

Scientific theories based on accurate experimentation will not be changed.

Because they are inherently tentative, accepted scientific theories and hypotheses are unreliable.

So students who understood that theories are responsive to evidence – and therefore reliable but revisable – were more likely to accept evolution. But why might this be the case?

One possibility is that accepting evolution and understanding the nature of science have a common cause. For example, students with greater scientific literacy or who have a more positive attitude towards science could tend both to accept evolution and to understand the nature of science. However, we asked participants in our study to report their level of science education and their general attitude towards science, and we found that the relationship between evolution and understanding science still held up even when we took these factors into account.

If accepting evolution and understanding the nature of science don't have a common cause, then it could be that accepting evolution causes one to understand the nature of science, or (more plausibly) that understanding the nature of science has a causal impact on accepting evolution. But why would understanding – for example – that theories are revisable but reliable influence people's preferred explanations for the biological world?

Our findings can't answer this question directly, but here's my best guess about what's going on. A lot of anti-evolution propaganda aims to undermine evolution's scientific credentials by exploiting common misconceptions about science and scientific theories. For example, the highly publicized 2005 Kitzmiller v. Dover Area School District trial in Dover, Pennsylvania involved (among other things) a controversial statement that teachers were requested to read prior to teaching evolution, and which included the following text:

Because Darwin's Theory is a theory, it is still being tested as new evidence is discovered. The Theory is not a fact. Gaps in the Theory exist for which there is no evidence.

It could be that understanding the nature of science doesn't itself encourage acceptance of evolution, but rather that by understanding the nature of science, students are able to critically evaluate anti-evolution messages and understand that being "just a theory" isn't such a bad thing to be.

But that's just my theory.