This research investigated the acquisition process of the color lexicon, specifically how color words are initially grounded and develop into the lexical system possessed by the adults in the ambient language. We conducted a longitudinal study in which Japanese-learning 2-year- olds were tested every month on their understanding of basic words denoting 8 chromatic colors, continuing until they were able to map these words onto their referents consistently. The results strongly endorse the view that acquisition of the color lexicon should be characterized as a process of system construction, through which children reorganize prelinguistic color categories onto the linguistic categories of the ambient language, thereby representations of individual words are continuously refined along with the refinement of the representation of the system as a whole.

The contingency symmetry inference, the inference to generalize a learned contingency to a reverse direction, is known to be extremely difficult for non-human animal species (Lionello-DeNolf, 2009). In contrast, humans are known to have the “affirming the consequent fallacy”, which reverses the antecedent and the consequence (if P then Q: Q therefore P). The contingency symmetry bias has been long discussed in relation to the ontogenesis of language learning, as word learning requires understanding of bidirectional relationship between symbols and objects. But how this bias emerges has not been known. This research tested whether 8-month-old human infants have this bias on a matching-to-sample task. The results demonstrated the possession of this bias in human infants before they start active word learning. This bias is likely a uniquely human cognitive bias, which may explain why only humans have language.