This is a follow-up to the popular post, “why we rarely use the word ‘prove’ in psychology.”

In their first months of studying psychology students love to use the word “prove” when explaining studies. This drives teachers and examiners nuts! But instead of making a blanket rule in class like “you’re never to use the word prove!” I like to get students to understand when and why it’s incorrect to use this term. But I think it’s equally important to know when it’s OK as well.


One study doesn’t prove anything!

The common phrase from students in the explanation of a study is something like “This study proves that…”

But here’s the problem: it’s just one study. In order to prove something you need really convincing evidence. It would be like a witness to a murder saying “there’s the man that did it” and the judge saying “well, that proves it then.” There isn’t enough evidence to jump to such a bold conclusion.

As I said in an earlier post, there are lots of things to consider:

  • Are the results generalizable?
  • Does the study demonstrate causation, or simply a correlation?
  • Have the results been replicated?

All these questions that can be raised over a single study mean that it’s difficult, if not impossible, to make such a bold claim about proof.


When can we say “prove” in psychology?

There are times when we can confidently use the word “prove” in psychology. Here are some examples:

  1. It’s been proven that our memories can be distorted and that eye-witness testimony may be unreliable. 
  2. Decades of neurological research have proven false the old claim that our brain growth is fixed from a young age; it’s now been proven that our brains actually change throughout our lives. 
  3. It’s a proven fact that many criminals have common brain abnormalities that may contribute towards their criminal behaviour. 

This raises interesting questions about what constitutes a “fact” in psychology. Great TOK discussion material.

So why are these acceptable claims? Because they’re not based on single studies. When writing about memory I was thinking about Loftus’ career in this field and the numerous studies by her and others that have shown this phenomenon. It’s the same with neuroplasticity: there’s so much evidence to show this phenomenon that I think we’d be crazy to say that it hasn’t been proven. And the same goes for my third point. I’m being really bold here and saying “fact” because I think the evidence is so strong.

The main point is that psychological studies can prove things, just not by themselves.


The importance of qualifiers: notice in the above examples that these are still not definitive claims. Let’s look at the difference in these two statements:

  • It’s a proven fact that many criminals have common brain abnormalities that may contribute towards their criminal behaviour. (Correct)
  • It’s a proven fact that criminals have brain abnormalities that cause their criminal behaviour. (Incorrect)

There are two key differences. The first statement uses the important qualifier “many.” It’s not saying all criminals and is leaving some room for exception, because there are always exceptions. Another difference is the first statement says “contribute towards” while the second says “cause.” There are so many variables that might influence criminal behaviour, not just neurology. So to say definitively that all criminals’ behaviour is caused by the brain is incorrect and shows a lack of understanding. So you can see that even when the evidence is compelling and it can be used to prove something, you still need to be careful with the conclusions you draw.


One of the qualities of a good psychologist is a healthy skepticism: that means you don’t believe everything you read. Instead, you ask questions of the research and the findings, including the methodology. This is a major part of critical thinking and one that’s essential to be an excellent psychologist.