in many words in a semantically consistent manner (e.g., the clean in unclean, cleanly, cleaner, and cleanliness), then the learned distributed representations will come to treat that pattern independently of the rest of the word (Plaut & Gonnerman, 2000). These different characterizations of morphology are discussed later in this chapter. My use of language regarding morphological decomposition should not imply any specific theoretical perspective at this stage.
Morpheme frequency effects
Several studies have reported that visual word recognition is influenced by the frequency of the stem within a morphologically complex word (e.g., Taft, 1979). Typically, participants in these experiments are asked to make lexical decisions for sets of words matched on whole‐word frequency but varied on the frequency of stems (e.g., unreal versus refuel; Taft & Ardasinski, 2006). Results show faster lexical decision times for words with high‐frequency stems. This result has been demonstrated for both inflectional and derivational morphology, and across a number of different languages, including Italian (Burani, Salmaso, & Carramazza, 1984), Dutch (Baayen, Dijkstra, & Schreuder, 1997), and French (Colé, Beauvillain, & Segui, 1989). The fact that recognition time of morphologically complex words is based at least in part on the frequency of the stem (e.g., the real in unreal) indicates that the word recognition system is sensitive to morphological structure.
More recent research has begun to indicate that these morpheme frequency effects might be modulated by other factors. Baayen, Wurm, and Aycock (2007) used lexical decision data from the English Lexicon Project (Balota et al., 2004) to suggest that the effect of morpheme frequency might be restricted to words with low whole‐word frequency. This finding is unsurprising given that the recognition of low frequency words is slower and less accurate than for high frequency words; thus, there is less pressure in the system to represent morphemic constituents when words are high in frequency. Likewise, there is some evidence to suggest that stem frequency effects are larger when derived target words contain highly productive affixes (Bertram, Schreuder, & Baayen, 2000; Ford, Davis, & Marslen‐Wilson, 2010). Productive affixes tend to occur frequently, in combination with many different stems and in semantically consistent ways (e.g., ‐ness as in kindness is productive; ‐th as in warmth is not).
The morpheme frequency effects that I have described seem straightforward, but there is an important methodological point to consider. Typically, these experiments compare the recognition of words matched on whole‐word frequency while varying stem frequency. However, the way that we count stem frequency entails theoretical commitments. Is stem frequency simply the number of times that a particular stem occurs in a corpus of a certain size? Or, should stem frequency also include the frequency of the morphologically complex words in which it occurs? If readers analyse morphologically complex words in terms of their constituents, then the latter approach would seem to be the more valid, since each encounter with a morphologically complex word reinforces knowledge of its stem. Taking this argument further, if decomposition arises in some morphologically complex words more than others (e.g., those with highly productive affixes), then that may also influence how we should quantify stem frequency. Researchers do not always think about how their decisions to match stimuli on psycholinguistic properties reflect underlying theoretical commitments. In this instance, theoretical uncertainties make it difficult to design and interpret experiments of this nature.
Morpheme interference effects
If readers analyse the morphemic constituents of printed words, then they might have trouble deciding that a stimulus like quickify (i.e., comprising an existing stem and affix) is not a word. In a seminal study, Taft and Forster (1975) observed that nonwords built of morphemic units (e.g., dejuvenate) are rejected more slowly in lexical decision than nonwords that do not comprise morphemic units (e.g., depertoire). In this example, juvenate is a bound stem meaning “young” (as in juvenile), while pertoire is not a bound stem. Bound stems are stems that cannot occur on their own. These findings were important because they suggested that morphemic analysis is a phenomenon that arises prior to lexical identification.
The morpheme interference effect has been replicated and extended across many studies (e.g., Burani, Dovetto, Thornton, & Laudanna, 1997; Caramazza, Laudanna, & Romani, 1988). Replicating the effect using English derived nonwords, Crepaldi et al. (2010) reported that morphemically structured nonwords such as gasful are rejected more slowly in lexical decision than nonwords without morphological structure such as gasfil. In an extension to these findings, they reported that the interference effect vanishes when the constituent morphemes are transposed (e.g., comparing fulgas to filgas), suggesting that sensitivity to affixes may be position specific (Crepaldi et al., 2010). Dawson, Rastle and Ricketts (2018) used stimuli from Crepaldi et al. (2010) to test whether the morpheme effect arises in participants of different ages: children (aged 7–9), young adolescents (aged 12–13), older adolescents (aged 16–17), and adults. They found a morpheme interference effect on accuracy in all groups, but reported an effect on response time only in adults and older adolescents. These findings provide evidence that relatively young readers are sensitive to morphological structure, but that knowledge of these important indicators of meaning builds through the process of reading acquisition (see also Castles & Nation, this volume).
Morphological priming effects
If morphologically complex words are analysed in terms of their constituents (e.g., darkness ‐> [dark] + [‐ness]), then a morphologically complex prime (e.g., darkness) should facilitate subsequent recognition of its constituent stem (e.g., DARK) relative to an unrelated control prime (e.g., darkness – DARK vs. fullness – DARK). Priming effects of this nature have been observed extensively, across different priming paradigms and across different languages (see, e.g., Amenta & Crepaldi, 2012; Rastle & Davis, 2008, for reviews).
Morphologically complex words are morphologically related to their stems (e.g., cleanly, clean), but they are also semantically and orthographically related to their stems. Early studies thus considered the possibility that morphological priming effects might simply reflect a combination of orthographic and semantic priming (Devlin, Jamison, Matthews, & Gonnerman, 2004). However, Rastle and colleagues (2000) provided evidence against this possibility in a series of masked priming experiments. Masked priming is an experimental paradigm in which primes are presented so briefly that they cannot be perceived consciously (Forster & Davis, 1984). These researchers showed that masked priming effects for morphologically related items (e.g., darkness – DARK) were significantly greater than those for semantic relatedness (e.g., cello – VIOLIN), orthographic relatedness (e.g., tapestry – APE), or combined semantic and orthographic relatedness (e.g., ghost – GHOUL). These findings indicate that there is a special relationship between morphologically related words that goes beyond their combined semantic and orthographic relatedness.
The question that has provoked most interest over the past 20 years of morphological priming research concerns the role of semantic transparency in morphological decomposition. Marslen‐Wilson and colleagues (1994) reported a series of cross‐modal priming experiments (spoken primes, visual targets) showing that morphologically complex words prime recognition of their stems, but only when there is a semantic relationship between them (e.g., departure – DEPART, but not department – DEPART). These findings led them to conclude that morphologically complex words are represented in terms of their constituents only when the meaning of the whole word can be computed from the meanings of the parts. Visual priming studies using fully visible primes also observed this pattern (Rastle et al., 2000). However, visual masked priming studies in French (Longtin, Segui, & Hallé, 2003) and English (Rastle, Davis, & New, 2004) yielded different results. These studies showed that briefly presented, morphologically structured primes facilitate recognition of their stems, irrespective of whether there is a meaningful relationship between the words. This latter class of item includes morphologically related