Friday Fallacy! This week, the Conjunction Fallacy

Hello everyone, it’s Friday Fallacy time!

Today we’ll discuss the Conjunction Fallacy. The Conjunction Fallacy is one of those “oops” fallacies that many people who are usually good at critical thinking fall victim to without realizing it. In committing the Conjunction Fallacy, one assumes that multiple, specific conditions are MORE likely to be true than one general condition.

For example,

Dr. Smith has a PhD in women’s studies.
Dr. Smith has a PhD in women’s studies and is a feminist.

One of these statements is more likely to be true than the other. While everyone who studies women’s studies I’ve ever met is a feminist, to be logical about it, you are making an extra assumption by saying that it’s more likely Dr. Smith is a feminist than not. Every time you “stack on” another condition, you are *decreasing* the likelihood that all of your conditions are true. As a further example:

Dr. Smith has a PhD in women’s studies. She’s also a feminist.

This statement is even less likely to be true than the first two. Now we have included the condition that Dr. Smith identifies as a woman. Although many women’s studies doctorates are earned by women, to be logical about it, it is less likely that Dr. Smith is a woman AND a feminist AND has a PhD in woman’s studies than it is that Dr. Smith is a feminist AND has a PhD in women’s studies.

To bring this to religion, here’s an example:

Which is more likely to be true: Judaism, or Christianity?

Many people, even atheists, might be tempted to answer that, as two competing religions, the likelihood that they are true is roughly equal (near 0%). But in fact, Judaism is MUCH more likely to be true than Christianity (of course, neither are even close to true, but that’s a separate post).

The reason for this is that in order for Judaism to be true, the following must be true:

1. There is a god
2. This god is the “God of Abraham” aka Yahweh aka a hundred other names
3. The stories in the Old Testament are true to whatever degree is necessary in order to consider themselves Jewish if you believe them
4. A Messiah will bring peace on Earth and blah blah blah.

In order for Christianity to be true, the following must be true:

1. There is a god
2. This god is the “God of Abraham” etc
3. The stories in the Old Testament are true to whatever degree is necessary in order to consider oneself Jewish if you believe them
4. A Messiah will bring peace on Earth and blah blah blah.

5. This Messiah has, in fact, already been here once, but we killed him.
6. This Messiah will come back at some point in the future.
7. Jesus of Nazareth was that messiah
8. You can go to heaven if and only if you accept this Jesus as your savior

It should be pretty clear by now which religion requires fewer necessary conditions conjointly. Another way to put this is that Christianity is basically Judaism plus a whole bunch of other unlikely stuff. And Mormonism is basically Christianity plus even MORE unlikely stuff.

With awareness of the conjunction fallacy (assuming equal evidence for each i.e. nothing substantial), we can say that Mormonism is less likely than Christianity, which is less likely than Judaism, which is less likely than theism, which is less likely than deism, which is less likely than the lack of belief in any gods.

This can be quite useful in discussions about religion, because every time a religious person stacks on more specific beliefs, you can justifiably require additional evidence for each claim individually.

Have a great weekend, heathens!

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  • Bob Rowland

    Very interesting.

  • ken

    very logical

  • SmartTheist

    very stupid

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