14.5: Criteria for a Causal Relationship

I would commit the post hoc fallacy if I said that the sun regularly comes up after the rooster crows, so he’s the cause of the sun coming up. The fallacy lies in supposing that A caused B when the only evidence is that A has been followed by B. For another example, suppose you get a lot of headaches and you are trying to figure out why. You note that you are unusual because you are the sort of person who often leaves the TV set on all day and night. You also note that whenever you sleep near a TV set that is on, you usually develop a headache. You suspect that being close to the TV is the cause of your headaches. If you were to immediately conclude that the closeness to the TV does cause the headaches, you'd be committing the post hoc fallacy. If you are going to avoid the post hoc fallacy, then how should you proceed?

First you should ask someone with scientific expertise whether there's any scientific evidence that sleeping close to a TV set that is on should cause headaches. If it should, then concluding that being close to the TV is the cause of your headaches does not commit the post hoc fallacy. Let's assume that there is no convincing evidence one way or the other, although a few statistical studies have looked for a correlation between the two. If the data in those studies show no association between sleeping near the TV and getting headaches, you can conclude that your suspicions were wrong. But let's suppose such an association has been found. If so, you are not yet justified in claiming a causal connection between the TV and the headaches. You─or the scientific community─need to do more. Before making your main assault on the causal claim, you need to check the temporal relation, the regularity, the strength, and the coherence of the association. What does that mean?

1. Temporal Relation: To be justified in saying that A causes B, A should occur before, not after, B. The future never causes anything in the present. This temporal relation is important because effects never precede their causes. Fear of sleeping near a TV in the future might cause a headache, but the future sleeping itself cannot cause it now. That is one of the major metaphysical presuppositions of all the sciences. Our claim, or hypothesis, that sleeping close to a TV causes headaches, does pass this first test.

2. Regularity: Suppose that three scientific studies have examined the relationship between sleeping near a TV and having headaches. In two of the studies an association has been found, but in one, none was found. Therefore, the association has not been so regular. Sometimes it appears; sometimes it doesn't. The greater the regularity, the more likely that the association is significant.

3. Strength: Even when an association is regular across several scientific studies, the strength of the association makes a difference. The weaker the association between sleeping near a TV and getting headaches, the less justified you can be in saying that sleeping near the TV causes headaches. If, after sleeping near the TV, you get a headache 98 percent of the time, that's a much stronger association than a 50 percent rate.

4. Coherence: The coherence of an association must also be taken into account when assessing whether a causal claim can be inferred from an association. Coherence is how well the causal claim fits with the standard scientific ideas of what is a possible cause of what. Suppose a researcher notices an association between color chosen by painters for the Chinese government's grain silos and the frequency of headaches among Canadian schoolchildren. On years when the percentage of blue silos in China goes up, so do the Canadian headaches. In green years the headaches go down. Suppose the researcher then speculates that the Chinese colors are causing the Canadian headaches and provides these data about the correlation to make the case. Other scientists would write this off as a crackpot suggestion. They would use their background knowledge about what could possibly cause what in order to deny the causal claim about the colors and the headaches. This odd causal claim does not cohere with the rest of science. It is too bizarre. It is inconsistent with more strongly held beliefs. The notion of coherence is quite fascinating. We will examine it in more detail in the next chapter when we discuss the paradigm of a science.

Exercise \(\PageIndex{1}\)

Have you ever noticed that night is associated with day? First there's the night, then there's the daytime. Again and again, the same pattern. Regular as clockwork. A consistent, strong association. This would seem to establish the causal claim that night causes day. Or would it?

Answer

The correlation does support the claim somewhat, but the claim is ridiculous, as we now know from our knowledge of scientific theories. The correlation is spurious. Unlike the people of 3,000 years ago, we know what causes daylight, and it's not night. We can suggest a much better explanation of why there's an association between night and day. No doubt you are thinking about the rotation of the Earth under the sun's spotlight. If you had lived 3,000 years ago when everyone believed the world to be flat, you'd probably never have imagined the alternative suggestion that the correlation between daytime and nighttime is caused by rotation of the Earth.