Linguistic evolution minimizes the amount of syntactic hierarchy in grammar

A core characteristic of language is the capacity for hierarchical syntax, but it remains debated to what extent languages favor hierarchical structure in their grammars, that is, the extent to which they favor hierarchical structures like "When driving, look ahead!" over concatenations like "You drive, you look ahead!". Through phylogenetic modeling of the grammars of clause combining in three extensive language families (Tupi-Guarani, Sino-Tibetan, and Indo-European), we find an overall bias towards minimizing hierarchy, with significant cultural variation. Our findings suggest that the computational and representational benefits of hierarchical structure tend not to be universally exploited in clause combinations, likely because they come with additional costs for learning. In consequence, the amount of hierarchy in language might differ from what is found in animal communication in a more graded way than commonly assumed.