Complexity of Dependencies in Discourse: Are Dependencies in Discourse More Complex than in Syntax?

This paper investigates the complexity of dependencies at the discourse level, in particular the dependencies between discourse connectives and their arguments. Our study is based on data from the Penn Discourse Treebank (PDTB) and is therefore an exploration into the ways treebanks can inform linguistic issues. We observe that, unlike in syntax, there is more uncertainty and flexibility with regards to the location and extent of discourse arguments. This leads to a variety of possible patterns of dependencies between pairs of discourse relations, including nested, crossed and a range of other non-tree-like configurations. Nevertheless, our main conclusion is that the types of discourse dependencies are highly restricted since the more complex cases can be factored out by appealing to discourse notions like anaphora and attribution. We conjecture that the complexity of dependencies is far more restricted at the discourse level as compared to the syntactic level.