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Harder reasoning · Lesson 11 of 14

Causal reasoning: correlation vs cause

After this lesson, you can name at least 3 alternative explanations for a correlation and use one to weaken a causal argument.

5 min video, plus notes and 1 practice question below

In this lesson

• Spot when a stimulus claims one thing causes another based only on correlation
• Identify coincidence as an explanation when 2 unrelated things move together
• Identify a third variable that could be driving both results
• Identify reversed causation, where the effect gets mistaken for the cause
• Weaken a causal argument just by naming a believable alternative

Why correlation doesn't prove causation

LSAT arguments often point out that 2 things move together: as one goes up, the other goes up, or one goes up while the other goes down. The argument then jumps to a causal claim, saying that because the 2 track each other, one must cause the other. That jump is where the flaw lives.

Weakening an argument like this doesn't take much. You just need to point out that another explanation fits the same evidence. Naming one believable alternative is often enough, because it shows the causal claim isn't the only story that explains the data. The 3 alternatives below, coincidence, a third variable, and reversed causation, cover most of the ways a correlation gets mistaken for a cause.

Alternative 1: it's just a coincidence

Sometimes 2 things shift together for no real reason at all. Newspaper ad sales and movie scores can rise and fall together with no relationship between them, and average temperature and the number of pirates worldwide have both increased over long stretches of time with no real connection either. Numbers can march in step without one causing the other.

This happens more than you'd expect. Compare enough different measurements and some pair of them will line up by chance alone, even though nothing connects them. That's random association, not causation, and it's worth naming whenever a stimulus tries to build a causal claim on nothing but 2 numbers that happen to sync up.

Part of the reason coincidences fool us is selection bias. You notice the pairs that happen to line up and skip past all the ones that don't. Temperature and the number of pirates can rise together over decades, but temperature and, say, the number of people taking the LSAT might not move at the same rate at all, because not everything in the world rises and falls together.

Alternative 2: a third factor causes both

Ice cream sales and murders both climb over the same period. It's tempting to think one causes the other, but a hidden third variable explains it better: temperature. As it gets hotter, people buy more ice cream, and people also get more short tempered, so conflict flares up more often. Temperature drives both numbers up without either one causing the other directly.

When you see 2 things correlated, ask whether some outside factor could be pushing both of them in the same direction at once. If the stimulus never rules that out, you've found your alternative explanation.

Alternative 3: the causation runs the other way

Sometimes an argument gets the direction of causation backwards. Cigarette consumption and cigarette prices track each other over time, and it's easy to assume rising prices are driving consumption down. But the causation could run the other direction: as consumption drops, companies raise the price per pack to keep the same profits.

Both directions are plausible, and the real picture gets more complicated once you factor in things like state taxes and how addictive the product is. The point for the LSAT isn't to settle which direction is real. It's to recognize that the argument assumed one direction without ruling out the other.

How to use this on test day

When a stimulus argues that A causes B because A and B are correlated, run through the 3 alternatives: coincidence, a third variable causing both, and reversed causation. If any of them fits the stimulus, an answer choice describing that possibility will weaken the argument.

You don't need outside expertise to spot this pattern. Once you notice a causal conclusion resting on nothing but correlated numbers, ask yourself what else could explain the same data.

You don't have to prove your alternative is what actually happened. On the LSAT, simply describing a believable alternative is usually enough to weaken a causal argument, because it shows the argument overlooked other explanations for its own evidence.

The short version

• Before you touch the answer choices on your next weaken question, prephrase your own alternative: coincidence, a third variable, or reversed causation. A choice that matches what you wrote is your answer, if none match, you likely picked the wrong conclusion to attack.
• Log which of the 3 alternatives, coincidence, third variable, or reversed causation, you miss most often on practice sets. That's the one to drill, not the one you already spot fast.

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