COMMENTARY
The article “Walking Linked to Less Brain Shrinkage, Study Says” describes research that found what scientists call a correlation between the amount people walk and the amount of gray matter in their brains. The study indicated that people who walk more are found to have more gray matter in their brains.
People without a scientific background may easily take this as meaning that walking can increase or prevent the loss of brain tissue, but in fact, we cannot infer any causal relationship from this study.
A correlational study is a study in which one records the numerical values of two characteristics (there are also studies that find correlations between more than two things, but that is highly complicated and not done often) describing a group of people (or animals, environmental phenomena, and so on), and then compares them to see if there is any similar pattern between them.
You can then draw a graph with one of the variables on the x-axis and the other on the y-axis. (In the case of the study in question, these values will be the amount of walking and amount of gray matter, respectively).
The more correlated these two variables are, the closer the resulting graph will be to a straight line. That is, if the people who walk more have more gray matter, and the people who walk less have less gray matter, there is a correlation between the amount people walk and the amount of gray matter they have. That is a positive correlation, as one increases with the other.
There can be another situation in which the people who walk more have less gray matter and those who walk less have more gray matter. That would be a negative correlation because they correlate inversely.
A third situation would be that there is no correlation whatsoever, and the data points just scatter everywhere without any pattern. These are the three basic results possible from a correlational study. (There are also more complicated relationships where, for example, one variable could increase with the other until a certain point, and then it would start to decrease with the other).
Now, if walking can prevent brain shrinkage, these two variables will definitely have a positive correlation. But their having a positive correlation doesn’t necessarily mean that walking can prevent brain shrinkage. It could also mean that brain shrinkage makes people walk less.
Or it could be because something else could lead to people’s both walking more and having more gray matter, say, for example, some kind of gene can make people like walking and at the same time have bigger brains. Or perhaps people who walk more may also be more likely to do other things that are different from what those who walk less do (such as eating healthier, spending less time watching TV, and so on) and that might contribute to having more gray matter.
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