Depth in English grammar

Corpus data are used to investigate Yngve’s claim that English usage avoids grammatical structures in which the number of left branches between any word and the root node of a sentence exceeds some fixed limit. The data do display a marked bias against left-branching, but the pattern of word-depths does not conform to Yngve’s concept of a sharp limit. The bias could alternatively reflect a statistical invariance in the incidence of left-branching, but whether this is so depends on how left-branching is counted. Six nonequivalent measures are proposed; it turns out that one (and only one) of these yields strikingly constant figures for left-branching in real-life sentences over a wide range of lengths. This is not the measure suggested by Yngve’s formulation; it is the measure whose invariance is arguably the most favourable for computational tractability.