ABSTRACT Words with larger morphological families elicit shorter response times in visual lexical decision, but in auditory lexical decision, family size effects have been reported to be facilitative, absent, or inhibitory. We revisit the auditory family size effect and different family size definitions by re-analysing data from two large lexical decision experiments. The results suggest that words with larger families elicit smaller response latencies not only in visual but also in auditory lexical decision. However, in auditory lexical decision, this effect is insignificant when the word contains a prefix. In both modalities, it is the degree of semantic similarity and the degree of form overlap between a word and its family members, rather than the mere presence of overlap, that seem to drive the family size effect.