Hyeree Kim. 2017. Syntactic Variation of Wh-Clefts and the Complexity Principle: A Corpus Study. Studies in Modern Grammar 95, 37-54. This study examines variable usage between to-infinitives and bare-infinitives in wh-cleft sentences in English. There are a number of previous studies dealing with either formal and functional analyses or regional and stylistic variation of wh-clefts. This study, however, attempts to find underlying factors determining the distribution of the two alternatives and investigates whether the so-called ‘complexity principle’ proposed by Rohdenburg (1998, 2000) is valid. Mair and Winkle (2012) used ten ICE corpora as an attempt to verify two out of four hypotheses of Rohdenburg’s principle. Although their findings partially supported Rohdenburg’s claims, the paucity of data turned out to neither prove nor disprove them. This study uses a much larger corpus, the COCA (Corpus of Contemporary American English), and shows that all four hypotheses of the complexity principle are valid: that is, the to-infinitive is more likely to occur, (i) if do is in more complex forms (did, done, doing) rather than in the simple present forms (do, does), (ii) if some elements intervene between do and be, (iii) if be is in the past tense (was) rather than in the simple present tense (is), or (iv) if be occurs in complex forms (e.g. will be, would be) rather than in the simple present or past forms (is, was). Furthermore, this study proposes and justifies a new hypothesis for the complexity principle: that is, the to-infinitive is more likely to occur, (v) if the intervening material between what and do are more complex (or lengthy).