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

Grammaticalization is when lexical items (e.g., nouns) change to be used as grammatical items (e.g., prepositions). This kind of change is widespread throughout the world's languages. Change in the opposite direction, from more to less grammatical, is far less common and systematic. We ran two artificial language experiments to test whether this unidirectional tendency originates with an individual preference for extending lexical meanings to grammatical ones rather than extending grammatical meanings to lexical ones. We focus on the extension of body parts to spatial relations. In our first experiment, participants were told the English meaning of an artificial word and asked to rate how likely it is that that word can also be used to refer to a second meaning -- one meaning was a body part and the other was a preposition. We expected lower ratings when the first meaning was a preposition compared to when it was a body part, but we found no difference. For the second experiment, participants were paired up to perform semantic extension in a communication task. We varied whether they were extending words for body parts to spatial prepositions or vice versa. Again, we found no evidence of an asymmetric bias. Finally, we used a computational model of the second experiment to show that participants would have behaved asymmetrically if they had more words to use than meanings to express, indicating that having a larger number of lexical items than grammatical concepts could be an alternative source of unidirectionality.