“I have read that one aspirin taken every day reduces the risk of heart…

Concept

An informal fallacy is a defect in the content or the reasoning of an argument rather than in its logical form. The fallacy of composition is committed when a property or effect that has been established for a single part, member or unit is transferred without warrant to the whole, or to a larger aggregate of those units, on the assumption that the effect simply adds up in proportion to the quantity.

Application

The statement starts from an effect established for one unit: a single aspirin taken daily lowers the risk of a heart attack. It then transfers that effect to a larger quantity of the same thing, two tablets a day, and concludes that the protection must be twice as great. Nothing in the premise licenses that step, because a dose-response relationship need not be additive; in this case the protective benefit levels off while the risk of bleeding rises. The unwarranted move from what holds of one unit to what must hold of the larger aggregate is exactly the part-to-whole transfer described above.

Contrast

Contrast with the other fallacies named:

• Begging the question: the conclusion is already assumed by one of the premises, so the reasoning is circular. Here the claim about the double dose is not smuggled into the premise; it is inferred from it.

• Hasty generalisation: a sweeping general rule is drawn from a sample that is too small or unrepresentative. Here no generalisation from instances is made; one accepted claim is scaled up in quantity.

• Slippery slope: an initial step is said to set off a chain of further steps ending in an extreme outcome. Here no chain of consequences is asserted at all.

Conclusion

The statement therefore commits the fallacy of composition.