The Fallacy of Composition

In group discussions, debates about issues are informal and sometimes fast-moving. They can move so quickly that the group's members don't realize that they have reached incorrect conclusions. When this happens, one or more rhetorical fallacies probably played a role — accidentally or intentionally inserted by one of the group's members.

Because of its subtlety, the Fallacy of Composition is a favorite of those who intentionally use rhetorical fallacies. When people use the Fallacy of Composition, they make statements about some parts of a whole (or even every part of a whole), and then conclude something about the whole. To illustrate the use of the fallacy, here's a particularly transparent form, just to give you the idea of how it works:

Since the global economy is in recession, and our jobs are less secure than they once were, everyone should pay off their debts and increase the fraction of their income that they save.

If we were to follow this advice, the recession would quickly become a depression, because economic activity would contract severely. Although the statement is probably true for individuals, the conclusion about the entirety of all individuals is false.

The Fallacy of Composition also occurs in group discussions about managing projects:

We can make up some time and get closer to the original schedule if Tim and Ellen work through the weekend. So it's probably best if everyone works every weekend for the rest of the project.

If we take this approach, people will become fatigued, their work quality will degrade, some will seek reassignment, and the project might never complete.

Here's an example in which an approval board finds two issues in a proposal, and then rejects the entirety on that basis:

We've found serious problems in the proposal. The Localization budget is too low, and the schedule for customer extensions is too aggressive. You need to rethink the whole thing.

The above Because of its subtlety,
the Fallacy of Composition
is a favorite of those
who intentionally use
rhetorical fallaciescomment omits any proof that the two flaws cited are justification for the final recommendation.

And from Glen's performance review:

I'm sorry I couldn't recommend you for a promotion this year. You had some trouble working with Fran, and there were also problems with Leo. You need to learn how to work better with people.

Perhaps the conclusion above is correct, but two difficult relationships are probably not adequate proof. For instance, if both Fran and Leo have problems with everyone, perhaps the problem isn't Glen's. This application of the Fallacy of Composition offers a convenient alternative to justifying decisions made for other reasons.

One important distinction between a Fallacy of Composition and valid inductive reasoning is the nature of the generalization. An inductive argument contains a proof of its generalization; a fallacy of composition merely suggests its generalization. It is the subtlety of this distinction that makes the Fallacy of Composition so difficult to identify in the moment.  Top    Next Issue

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