An Introduction to Critical Thinking and Creativity - always yours

134 REASONING ABOUT CAUSATION
Similarly, most people are fine after taking aspirin, and there is only a low positive
correlation between aspirin and allergic reactions. But aspirin does cause allergic
reactions in about 1% of the population.
If a low correlation does not preclude causation, then is a high correlation sufficient
for causation? Not at all! Confusing positive correlation with causation is
a common mistake in causal reasoning. Even if C does not cause E, there can be
many reasons why C is positive correlated with E. Here are the main possibilities:
• The correlation between C and E is purely an accident.
• E causes C and not the other way round.
• C does not cause E but they are the effects of a common cause.
• The main cause of E is some side effect of C rather than C itself directly.
This is not to say that data about correlation are irrelevant to causation. As a
matter of fact correlation is often a guide to causation. But we need to rule out the
alternative possibilities listed above if we want to infer causation on the basis of
positive correlation. Let us discuss these cases further.
15.1 WHY CORRELATION IS NOT CAUSATION
15.1.1 Accidental correlation
Sometimes high correlation is the result of not having enough data. Suppose I
have been in only one car accident my whole life, and that was the only time I ever
wore red trousers. There is a perfect correlation between the color of my trousers
and my being involved in a car accident, but this is just a coincidence. Correlation
data are more useful when they involve a large range of cases.
But still we need to be careful. It has been suggested that the sea level in Venice
and the cost of bread in Britain have both been generally on the rise in the past two
centuries (Sober, 1988). But it is rather implausible to think that the correlation is
due to some underlying causal connection between the two cases. The correlation
is presumably an accident due to the fact that both have been steadily increasing
for a long time for very different reasons.
There is also a kind of accidental correlation known as spurious correlation (or
Simpson's paradox) that has to do with the aggregation of statistical data. It is
a rather interesting but somewhat technical topic. If you are interested you can
read more about it on our companion website.
15.1.2 The causal direction is reversed
Sometimes C is correlated with E not because C causes E, but because E causes
C. Drug users are more likely to suffer from psychiatric problems. This might
be because drug use is the cause, but perhaps preexisting psychiatric problems
cause people to turn to drugs. Correlation by itself does not tell us which of these