432 British Journal of Developmental Psychology (2012), 30, 432–445 C  2011 The British Psychological Society The British Psychological Society www.wileyonlinelibrary.com Effects of morphological Family Size for young readers Kors Perdijk 1 ∗ , Robert Schreuder 1 , R. Harald Baayen 2 and Ludo Verhoeven 1 1 Radboud University Nijmegen, The Netherlands 2 University of Alberta, Edmonton, Canada Dutch children, from the second and fourth grade of primary school, were each given a visual lexical decision test on 210 Dutch monomorphemic words. After removing words not recognized by a majority of the younger group, (lexical) decisions were analysed by mixed-model regression methods to see whether morphological Family Size influenced decision times over and above several other covariates. The effect of morphological Family Size on decision time was mixed: larger families led to significantly faster decision times for the second graders but not for the fourth graders. Since facilitative effects on decision times had been found for adults, we offer a developmental account to explain the absence of an effect of Family Size on decision times for fourth graders. According to Miller (1991, p. 238), a child learns about 3,750 words each year, 10 new words every day. Miller emphasizes how amazing this achievement is: ‘Clearly, a learning process of great complexity goes on at an impressive rate in every normal child’. How does a child manage to learn such a great number of words in such a short period of time? Decomposing a novel unfamiliar word into familiar morphemes is probably important for this achievement. This paper focuses on the relation between the morphological structure of words, visual word recognition, and the mental lexicon of children. For most languages, the majority of the words in the language are morphologically complex. That is, these words consist of more than one morpheme, the smallest unit in a language that has a function or meaning. Morphologically complex words tend to occur with a lower frequency than monomorphemic words. When readers (both adult and young readers) encounter a novel word (which by definition has a very low frequency for that child), it is therefore likely to be morphologically complex (see Baayen, 2001). In this paper, we will study the growth and changing structure of the mental lexicon for children in primary school, using morphological Family Size. For this purpose, we use visual lexical decision instead of offline tasks (see, e.g., Anglin, 1993 for offline studies on Vocabulary Knowledge). The morphological family of a (monomorphemic) word is ∗ Correspondence should be addressed to Kors Perdijk, Radboud University Nijmegen, The Netherlands, PO Box 9104, Nijmegen 6500 HE, The Netherlands (e-mail: k.perdijk@gmail.com). DOI:10.1111/j.2044-835X.2011.02053.x