Many syntactic phenomena have received competing accounts, either in terms of formal grammatical mechanisms, or in terms of independently motivated properties of language processing mechanisms (“reductionist” accounts). A variety of different types of argument have been put forward in efforts to distinguish these competing accounts. This article critically examines a number of arguments that have been offered as evidence in favour of formal or reductionist analyses, and concludes that some types of argument are more decisive than others. It argues that evidence from graded acceptability effects and from isomorphism between acceptability judgements and on-line comprehension profiles are less decisive. In contrast, clearer conclusions can be drawn from cases of overgeneration, where there is a discrepancy between acceptability judgements and the representations that are briefly constructed on-line, and from tests involving individual differences in cognitive capacity. Based on these arguments, the article concludes that a formal grammatical account is better supported in some domains, and that a reductionist account fares better in other domains. Phenomena discussed include island constraints, agreement attraction, constraints on anaphora, and comparatives.