What is the connection between brain structure and other things?
If you have a look at the papers that we have published using the MRI data you will often come across the word “association” used to describe the relationship between two things that were measured. In fact, 9 out of 16 of those papers have the words “associated” or “association” in their titles. For example, we might say a particular brain measurement is associated with walking speed. All that means is that we have found in statistics-land that the two things appear to be linked – when one measurement is higher the other measurement also tends to be higher, or it might be that when one is higher the other is lower and vice versa. You might have heard the expression “correlation does not mean causation” or something similar – this expression refers to the fact that a correlation or association between two things does not necessarily mean that one causes the other (although it could). In our case, even though it might seem reasonable that the structural differences in the brain could cause some of the other differences between people that we measured (walking speed, antisocial behaviour, mental health, etc.) this might not be the case at all. It could be that there are common factors that explain both brain structure and the other things. Or if one thing does in fact cause the other, it could be that the brain differences are caused by the other differences (for example, in one paper described above we showed that it is likely that cardiovascular fitness affects brain structure). Or it could be a combination of all of those things. It could even be completely coincidental although this is made much less likely by using the right statistical techniques. Unfortunately, because we only have MRI data from age 45 at the moment it is difficult to say with any confidence whether the brain differences or the other differences came first. Having MRI data from future interview phases will help answer these questions.
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