<p>Previous studies have revealed conflicting results as regards whether second language (L2) learners decompose morphologically complex words during online reading or process them in a whole-word manner. To resolve such controversies, first language (L1) morphological characteristics have been proposed as one factor leading to variation in L2 performance. Evidence for the argument of L1 morphological effects includes (1) that L2 learners from an L1 with a similar/congruent morphological feature outperform those from an L1 without a similar corresponding morphological feature and (2) that there is an advantage for L2 learners from an L1 with a more complex morphological system. In this paper, we examine the design of these studies and identify two potentially confounding factors that most studies have failed to consider when comparing L2 learners from different L1 morphological backgrounds: cognateness and L1 writing system. We suggest that studies examining L1 morphological effects recruit L2 groups whose L1s differ in their morphological system characteristics but share the same type of writing system, while also stringently excluding cognates in the design of experimental materials.</p>