When humans become animals: Development of the animal category in early childhood.

The current study examines 3- and 5-year-olds' representation of the concept we label 'animal' and its two nested concepts -animal(contrastive) (including only non-human animals) and animal(inclusive) (including both humans and non-human animals). Building upon evidence that naming promotes object categorization, we introduced a novel noun for two distinct objects, and analyzed children's patterns of extension. In Experiment 1, children heard a novel noun in conjunction with two non-human animals (dog, bird). Here, both 3- and 5-year-olds readily accessed animal(contrastive) and extended the noun systematically to other (previously un-named) non-human animals. In Experiment 2, children heard a novel noun in conjunction with a human and non-human animal. Here, 5-year-olds (but not 3-year-olds) accessed animal(inclusive) and extended the noun systematically to humans and non-human animals. These results underscore the developmental challenge facing young children as they identify the scope of the fundamental biological term 'animal' and its corresponding, nested concept(s).