Dependency-length minimization in natural and artificial languages

A wide range of evidence points to a preference for syntactic structures in which dependencies are short. Here we examine the question: what kinds of dependency configurations minimize dependency length? We consider two well-established principles of dependency-length minimization; that dependencies should be consistently right-branching or left-branching, and that shorter dependent phrases should be closer to the head. We also add a third, novel, principle; that some ‘‘opposite-branching’’ of one-word phrases is desirable. In a series of computational experiments, using unordered dependency trees gathered from written English, we examine the effect of these three principles on dependency length, and show that all three contribute significantly to dependency-length reduction. Finally, we present what appears to be the optimal ‘‘grammar’’ for dependency-length minimization.