A student asked me today about the answer to this study question:
If we find a drug that reduces the activity of a particular brain chemical (Variable A) and also leads to a decrease in the symptoms of a particular mental disorder (Variable B), have we shown that there is a causal relationship between Variable A and Variable B? Why or why not?
Many people reason that, if a drug (e.g., an antidepressant medication) causes a change in the activity of a particular brain chemical (e.g. serotonin or dopamine) and also causes an improvement in the symptoms of a mental disorder (e.g., major depression), the change in the brain chemical caused the improvement in the symptoms. But this conclusion doesn’t follow. All we can conclude is that Variable A is correlated with Variable B, not that A caused B to change (or that B caused A to change).
Why? I think this becomes obvious if we look at an everyday example in which we know the causes. We know that s sunrise causes the outside temperature to increase (Variable A) and also causes people to wake up (Variable B). The increase in outside temperature doesn’t (usually) cause people to wake up inside their homes; and people waking up definitely doesn’t cause the outside temperature to increase. Variables A & B are just correlated: they change together because the sunrise causes both to change.
Introductory Psychology Readings 