The treatment of plural morphemes in English noun-noun compounds is significant because it provides a test case for competing theories of language acquisition and representation. Even when the first noun in a compound refers to plural items, native speakers frequently use the singular form (Murphy, 2000). Sometimes, they will use the irregular plural form ('mice chaser') but very rarely are regular plurals ('rats chaser') used as the first noun in a compound. This effect has been found with native English-speaking children (Gordon, 1985; Nicoladis, 2000; Oetting & Rice, 1993; van der Lely & Christian, 2000); native English-speaking teenagers (van der Lely & Christian, 2000); and native English-speaking adults (Lardiere & Schwartz, 1997; Murphy, 2000). The apparent dissociation between regular and irregular plurals (i.e. that irregular plurals are included before a second noun but regular plurals are almost never included before a second noun) is thought to be due to innate morphological constraints (Marcus, Brinkmann, Clahsen, Weise, & Pinker, 1995). Such constraints predict that all items of regular morphology should be treated differently from all items of irregular morphology by language users in all situations. However, if external factors such as input and response modality affect the number of plurals included in compounds, then this questions the internal constraint-based explanations of compounding and encourages investigation of how external factors might influence the number of plurals included in compounds.