Why Do Children Think Words Are Mutually Exclusive?

How do children learn what a word means when its uses are consistent with many possiblemeanings? One influential idea is that children rely on an inductive bias that ensures that novelwords get assigned distinct meanings from known words – Mutual Exclusivity. Here, we explorethe possibility that Mutual Exclusivity phenomena do not reflect a bias, but rather informationencoded in the message: learners might effectively be told when and when not to assume thatword meanings are mutually exclusive. In three experiments (N=106, ages 2;0−2;11 from acrossthe United States), we show that 2-year-olds only assume that novel words have distinctmeanings if the words were spoken with 'focus', an information-structural marker of contrast.Without focus, we found no Mutual Exclusivity; novel words were understood to label familiarobjects. These results provide a novel account of Mutual Exclusivity and demonstrate an early-emerging understanding of focus and information structure.