In over a century of research into the English definite article "the", two main theoretical factors have been identified as relevant to its meaning: namely, (i) uniqueness and (ii) familiarity. The identification of these factors has led to an extensive debate in semantics about which of them is more fundamental to the meaning of "the". In this paper, we contribute to this debate by introducing novel data obtained through two controlled psycholinguistic experiments. We manipulated uniqueness and familiarity of potential referents, examining how these factors affect the comprehension and production of English definite descriptions. The behavioral results reveal an asymmetry between these two factors, with familiarity being a weaker cue than uniqueness – a pattern that is unexpected under any existing theory of definiteness. We close with a discussion of possible extensions to existing theories in light of this result, as well as avenues for future work