Why Do Children Think Words Are Mutually Exclusive?

How do children learn what a word means when its uses are consistent with many possible meanings? One influential idea is that children rely on an inductive bias that ensures that novel words get assigned distinct meanings from known words-<i>mutual exclusivity</i>. Here, we explore the possibility that mutual-exclusivity phenomena do not reflect a bias but rather information encoded in the message. Learners might effectively be told when (and when not) to assume that word meanings are mutually exclusive. In three experiments (<i>N</i> = 106 from across the United States; ages 2 years, 0 months-2 years, 11 months), we show that 2-year-olds only assumed that novel words have distinct meanings if the words were spoken with <i>focus</i>, an information-structural marker of contrast. Without focus, we found no mutual exclusivity; novel words were understood to label familiar objects. These results provide a novel account of mutual exclusivity and demonstrate an early emerging understanding of focus and information structure.