It is not unusual for developmental psychologists to become frustrated with the theory of universal grammar (UG), whose proponents have tended to dismiss most research on children's language production and comprehension as irrelevant to explaining how human languages are acquired. This is because children's actual linguistic behaviour is presumed to reflect factors besides their grammatical competence, rendering most methods of sampling linguistic behaviour unsuitable for evaluating UG theory. This means, in practice, that UG proponents do not view performance errors as evidence against their hypothesis that grammatical knowledge is largely innate. When children perform at ceiling on a given task, this is usually taken as proof of their adultlike grammatical competence, while poor performance is dismissed as due to research design flaws or limitations in information processing capacities (e.g. working memory). Crain & Thornton (1998) attempt to eliminate what they consider to be post hoc processing accounts of children's linguistic behaviour by arguing, counter to Chomsky (1965) and many others, that children and adults share identical language processing mechanisms, and that linguistic performance directly reflects grammatical competence. Therefore, if UG principles are available from an early age, child and adult performance should be the same when tasks are properly constructed to avoid extra-linguistic demand characteristics (excepting adult–child differences predicted by parameter-setting or maturational models). It should not be surprising then that some psycholinguists, such as Drozd (target article), would find C&T to be misguided with respect to these issues, because children's linguistic behaviour surely differs from adults' in seemingly unpredictable ways.