Strengthening or Weakening a Causal Argument

The LSAT throws a lot of causal arguments at you, and asks you to either strengthen or weaken them. These questions can be tricky at times, but are fundamentally very simple and even predictable, so I thought I’d write a post about them that could hopefully save you a point or two on your next practice test.

A “causal argument” is an argument that X is the cause of Y.

It doesn’t argue that X is happening, or that Y is happening. These facts might be taken for granted, especially if they are both things that happened in the past. I’ll illustrate what I mean with an example.

Let’s say that Johnny ate celery for breakfast. And let’s say that later that day, he aced his chemistry test.

A causal argument might be that eating celery for breakfast was what caused Johnny to ace his chemistry test.

But what is not being argued is a) whether Johnny ate celery for breakfast or b) whether he aced his chemistry test. Those facts are both known and assumed to be true, whether or not you want to draw a causal connection between them.

So, let’s say you have an LSAT question that reads:

Yesterday, Johnny ate celery for breakfast. Later that day, he aced his chemistry test. Therefore, eating celery for breakfast must have been what caused him to ace his test.

Which of the following, if true, most weakens the argument?

As you read the paragraph in any Logical Reasoning question, you want to be thinking ahead and trying to predict what the answer could be. So what are some things that could weaken this argument?

Well, as we’ve said, what you’re not trying to do is prove that Johnny didn’t ace his test, or that he didn’t eat celery for breakfast. (Conversely, if the question was asking for something that would strengthen the argument, you wouldn’t be looking to prove that Johnny ate celery for breakfast, or that he aced his test.) As in any Logical Reasoning question, the facts brought as evidence in the paragraph must be taken for granted – what you are disputing is the conclusion drawn from them.

The argument is that X (Johnny eating celery) caused Y (Johnny acing his chemistry test). So, you know that the answer will be anything that makes it less likely that X caused Y.

There are essentially two ways to weaken or disprove that X was what caused Y: either prove that

1) X couldn’t have caused Y, or

2) it was really Z (some other thing) that caused Y!

In other words, if you want to argue that the reason Johnny aced his chemistry test is because he ate celery for breakfast, my two ways to prove you wrong would be either by showing that there’s no way eating celery has any effect on a person’s ability to take a chemistry test later that day, or by showing that he had studied for 30 hours and that THAT was probably why he aced it (or any other non-celery reason).

Remember that in trying to predict the answer, the goal is to be as general as possible. Instead of thinking of specific and disconnected possibilities of things that would weaken the argument, try to think in general terms of anything that could possibly weaken this argument, so that your thinking will have to encompass whatever the right answer is.

In these causal questions, the only possible way to weaken the argument is by showing one of these two things, so if you predict that either 1) or 2) above will be the answer to the question, you will be correct 100% of the time. This makes these questions much, much easier, since you always know what the answer will be.

In this case, possible answers in category #1 are:

    • Eating celery has not been shown to have any impact on a person’s chemistry abilities
    • Whatever effect celery has on a person’s brain usually takes longer than a day to appear
    • The particular celery that Johnny ate was spoiled, and celery loses its positive mental effects when it spoils
  • Many other people in the class ate celery that morning and didn’t ace the test

These are all proving the same thing: that eating celery was unlikely to have been the cause of Johnny acing his chemistry test.

Possible answers in category #2 are:

    • Johnny got 8 hours of sleep the night before, which has been shown to help people ace chemistry tests
    • Johnny studied for 30 hours for the test, which has been shown to help people ace chemistry tests
  • Johnny ate tomatoes for breakfast, which has been shown to help people ace chemistry tests

…and so on. All of these statements prove the same thing: that something else was just as likely to have been the cause for Johnny’s test performance, which means it wasn’t necessarily the celery.

What about if the question had asked this instead:

Yesterday, Johnny ate celery for breakfast. Later that day, he aced his chemistry test. Therefore, eating celery for breakfast must have been what caused him to ace his test.

Which of the following, if true, most strengthens the argument?

Strengthening an argument is the opposite of weakening it, so anything that, if true, would strengthen an argument, would weaken it if false, and vice versa.

In other words, the two ways to strengthen a causal argument that X caused Y are the respective converses of the above two categories. You must either prove that:

    1. X is likely to have caused Y, or
  1. No other thing (Z or otherwise) is likely to have caused Y.

Some examples in category #1:

    • Eating celery causes increased test-taking ability
    • Eating celery causes increased understanding of chemistry
  • Eating celery causes greater mental abilities in general

Some examples in category #2:

    • Other than eating celery, Johnny did almost nothing to prepare for this test
  • Other than eating celery, the way Johnny prepared for this test was the exact same way he prepared for his last three tests, all of which he failed

You get the point.

Let’s take a look at a question from the June 2007 LSAT (if you haven’t taken it yet, skip this part so you don’t spoil the question for yourself). (We also analyzed this same question here.)

The scientist is arguing that X (gas buildup) caused Y (warming), so the answer will be anything that makes it less likely that X caused Y. In other words, the answer will be anything that either proves that:

    1. This gas buildup could not have caused this warming, or
  1. There’s something else that’s just as likely to have caused the warming.

That’s what you want to be thinking as you read the paragraph. If you can do that, you’ve cast a wide net that will definitely include whatever the right answer is.

Now, maybe you can even think of specific things that would fall into these categories. For example, in category #1, maybe there’s evidence that even though these gases kept some heat in, the amount of heat kept in would have been way too minuscule to account for the reported increase in temperature. Maybe it could only have been responsible for 0.01 degrees Celsius, which would greatly undermine the scientist’s argument (the scientist says they are the primary cause). Or, in category #2, maybe there’s evidence the Earth’s orbit got a bit closer to the Sun over the past century, allowing more of its heat to reach the Earth. This provides an alternative explanation for the warming, which means maybe that was what caused the warming, and not the gas buildup.

Either of these things would weaken the argument, and either one would have to be the right answer if it was on the page. But you don’t have to carve out specific potential answers like we just did. Just note the two general ways this argument could be weakened, just like every other causal argument, so that you can encompass all of the possible right answers, and then you’ll be ready to spot it in the answer choices. Your mind may even automatically come up with a couple of specific possibilities, and that’s good – just don’t spend any time ruminating on it. (This general prediction process is something you want to be doing as you’re reading the paragraph, not something to spend extra time on.)

Now, go back and take a look at answers.

Lo and behold, if you were looking for an answer that fit into category #1, you would have found it in (B)! If the warming happened before the gas buildup, it’s impossible that the gas buildup could have been the cause of the warming – nothing can cause something to happen before itself.

If this was asking to strengthen the argument instead, a possible category #1 answer would be that the gas buildups in the atmosphere like the present one tend to raise global temperatures by about 0.5 degrees Celsius (maybe, for example, the last 3 times we had gas buildups like this, the temperature also rose 0.5 degrees Celsius – this would certainly strengthen the argument). A possible category #2 answer would be that no other known cause for global temperature increase was present on the Earth within the century.

So, to recap:

    • The two ways to strengthen an argument that X caused Y are either to prove that X was likely to cause Y, or to prove that nothing else could have caused Y
    • The two ways to weaken an argument that X caused Y are either to prove that X was unlikely to cause Y, or to prove that something else could have caused Y
  • By having these general predictions in mind as you read the answers to causal strengthen/weaken questions, you can predict and identify the correct answer with 100% accuracy!

Hope this tip helps! For full clarity on very tough logical reasoning questions, check out my comprehensive LSAT Course.

Oh, and if you haven’t already, download your Free MasterLSAT Study Guide, which will tell you exactly what you need to do to reach a 99th-percentile score on the LSAT.

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