There is a gradual shift early in word learning such that at first children accept both linguistic (words) and non-linguistic (sounds and gestures) signals as labels for objects, but later on, tend to constrain word-forms to words. The associationist account suggests this shift is due to children's attention to co-occurrences in their environment. Previous evidence shows that children will treat linguistic and nonlinguistic signals that are systematically correlated with a category of objects as labels. This study asks whether this trend continues throughout development given a more demanding mutual exclusivity task. Two experiments suggest that 20-montholds treat both words and animal sounds as labels for animals provided they are produced by a human. By 26-months-old children only treat words, and not animal sounds, as labels for animals. These results provide support for the associationist framework in addition to evidence supporting the trend to increasingly narrow the definition of label form.