Does language about similarity play a role in fostering similarity comparison in children?

Commenting on perceptual similarities between objects stands out as an important linguistic achievement, one that may pave the way towards noticing and commenting on more abstract relational commonalities between objects. To explore whether having a conventional linguistic system is necessary for children to comment on different types of similarity comparisons, we observed four children who had not been exposed to usable linguistic input - deaf children whose hearing losses prevented them from learning spoken language and whose hearing parents had not exposed them to sign language. These children developed gesture systems that have language-like structure at many different levels. Here we ask whether the deaf children used their gestures to comment on similarity relations and, if so, which types of relations they expressed. We found that all four deaf children were able to use their gestures to express similarity comparisons (point to cat + point to tiger) resembling those conveyed by 40 hearing children in early gesture + speech combinations (cat + point to tiger). However, the two groups diverged at later ages. Hearing children, after acquiring the word like, shifted from primarily expressing global similarity (as in cat/tiger) to primarily expressing single-property similarity (as in crayon is brown like my hair). In contrast, the deaf children, lacking an explicit term for similarity, continued to primarily express global similarity. The findings underscore the robustness of similarity comparisons in human communication, but also highlight the importance of conventional terms for comparison as likely contributors to routinely expressing more focused similarity relations.