Overlap Principle
LSAT glossary · Logical Reasoning
Definition
A quantifier inference rule: if "Most A are B" AND "Most A are C," then "Some A are both B and C." The principle holds because two majorities of the same set must share at least one member — if more than half of A are B and more than half of A are C, the two subsets cannot be disjoint. This is one of the most commonly tested quantifier inferences on LSAT Must Be True / Inference questions and is the only way to derive a "some" claim from two "most" claims.
Overlap Principle in plain English
Imagine 30 kids in a class. If more than half of them play soccer, and more than half of them own a dog, then at least one kid must do BOTH — there simply aren't enough kids to keep the two groups totally separate. That's the Overlap Principle: two majorities of the same group are each bigger than half, so they're forced to share at least one member. That's why 'Most A are B' plus 'Most A are C' guarantees 'Some A are both B and C' — it's the one move that lets you squeeze a solid 'some' conclusion out of two loose 'most' statements.
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