Grammatical unidirectionality is not reflected in individual preferences when performing artificial semantic extension

Abstract Grammaticalization is the process whereby lexical items change into grammatical items. This phenomenon is widely attested, while the change from grammatical to lexical is far less common. We ran two experiments to test whether this unidirectional tendency originates with a preference for extending lexical meanings to grammatical ones rather than vice versa. We focus on body parts and spatial relations. In Experiment 1, participants were told the meaning of an artificial word then rated how likely it is that that word can also be used to refer to a second meaning – one meaning was a body part and one a preposition. We expected higher ratings when extending from body parts to prepositions than vice versa but found no difference. In Experiment 2, participants performed semantic extension in communication. We varied whether they extended words for body parts to prepositions or vice versa. Again, we found no asymmetry. Finally, we used a model of Experiment 2 to show that asymmetrical extension follows straightforwardly if there is an asymmetry in the number of words available relative to the number of meanings to express, indicating that having a larger number of lexical items than grammatical concepts could be an alternative source of unidirectionality.