The Uncommon Sense Effect: experimental evidence for underspecification as the mental basis of logical inference

Logical reasoning is one of humanity's most powerful abilities. A widespread assumption across psychology, linguistics, and philosophy holds that reasoning operates over concepts that refer to objects and properties in the world, yet this has rarely been tested empirically. We introduce a novel paradigm that exploits lexical ambiguity to differentiate candidate representations for human inference: word-forms, reference-fixing concepts, or more abstract "underspecified representations" that constrain meaning without fully determining it. Across three experiments (N=158), participants evaluated deductive arguments equivocating over polysemous or homonymous terms. Although these arguments are logically equivalent under standard analyses, participants reliably accepted non-truth-preserving polysemous arguments while rejecting homonymous ones. This asymmetry -- the Uncommon Sense Effect -- points to underspecified representations as the default basis of logical inference. This challenges a foundational assumption about meaning, and opens new empirical avenues for studying the link between what we think and why we think it.