Hard Words Despite Early Concepts: The Acquisition of “Same” and “Different”

A longstanding assumption in language acquisition is that comprehension precedes production. However, for some words, children produce them long before they master their adult meanings, suggesting that such words are “hard to learn.” We investigate this phenomenon for the relational terms “same” and “different.” Although pre-lexical infants represent sameness and difference early in life, it is unknown when children acquire the corresponding word meanings. Using corpus analyses of English-learning children aged 2–4 years, we show that “same” and “different” are frequent in caregiver speech and are produced by all middle class English learning children by age 2. Nevertheless, three comprehension experiments with over 200 high SES English learning children aged 2–5 years reveal that only a minority of 2- and 3-year-olds understand the relational meanings of these words, with reliable comprehension emerging at age 4. Moreover, children almost never comprehend one word without the other, suggesting that “same” and “different” are learned in close temporal proximity. We discuss possible underlying reasons for this particular extended acquisition trajectory, including the different formats of non-linguistic and linguistic representations of sameness and difference. These findings clarify what makes certain relational words hard to learn and illuminate how language acquisition can impact relational thinking.