Studies of L2 writing development usually measure T-units and clausal subordination to assess grammatical complexity, assuming that increased subordination is typical of advanced writing. In this article we challenge this practice by showing that these measures are much more characteristic of conversation than academic writing. The article begins with a critical evaluation of T-units and clausal subordination as measures of writing development, arguing that they have not proven to be effective discriminators of language proficiency differences. These shortcomings lead to the question of whether these measures actually capture the complexities of professional academic writing, and if not, what alternative measures are better suited? Corpus-based analyses are undertaken to answer these questions, investigating 28 grammatical features in research articles contrasted with conversation. The results are surprising, showing that most clausal subordination measures are actually more common in conversation than academic writing. In contrast, fundamentally different kinds of grammatical complexity are common in academic writing: complex noun phrase constituents (rather than clause constituents) and complex phrases (rather than clauses). Based on these findings, we hypothesize a sequence of developmental stages for student writing, proposing a radically new approach for the study of complexity in student writing development. (Contains 6 footnotes, 7 tables, and 3 figures.)