  Human languages use a wide range of grammatical categories to constrain which
words or phrases can fill certain slots in grammatical patterns and to express
additional meanings, such as tense or aspect, through morpho-syntactic means.
These grammatical categories, which are most often language-specific and
changing over time, are difficult to define and learn. This paper raises the
question how these categories can be acquired and where they have come from. We
explore a usage-based approach. This means that categories and grammatical
constructions are selected and aligned by their success in language
interactions. We report on a multi-agent experiment in which agents are endowed
with mechanisms for understanding and producing utterances as well as
mechanisms for expanding their inventories using a meta-level learning process
based on pro- and anti-unification. We show that a categorial type network
which has scores based on the success in a language interaction leads to the
spontaneous formation of grammatical categories in tandem with the formation of
grammatical patterns.
