Is native speaker variation in understanding complex sentences due to individual differences in working memory capacity or in syntactic competence? The answer to this question has important consequences for both theoretical and applied concerns in linguistics and education. This text gives a historical and interdisciplinary perspective on the rule-based and experience-based debate and in supporting an integrated account. In the study reported here, variation was found to be due to differences in syntactic competence and the author argues that sentence comprehension is a learned skill, displaying many of the general characteristics of cognitive skills.