Morphological knowledge refers to the ability to recognize and use morphemes correctly in syntactic contexts and word formation. This is crucial for learning a morphologically rich language like Finnish, which features both agglutinative and fusional morphology. In Finnish, agglutination occurs in forms like aamu: aamu+lla (‘morning: in the morning’), where a suffix is transparently added. Fusional features, as seen in ilta: illa+lla (‘evening: in the evening’), involve allomorphic stem changes that reduce transparency. We investigated the challenges posed by stem allomorphy for word recognition in isolation and in context for L2 learners and L1 speakers of Finnish. In a lexical decision task, L2 speakers had longer response times and higher error rates for semitransparent inflections, while L1 speakers showed longer response times for both transparent and semitransparent inflection types. In sentence reading, L2 speakers exhibited longer fixation times for semitransparent forms, whereas L1 speakers showed no significant effects. The results suggest that the challenges in L2 inflectional processing are more related to fusional than agglutinative features of the Finnish language.

The aim of the present study was to investigate whether the recognition speed of Finnish nominal base forms varies as a function of their paradigmatic complexity (stem allomorphy) or productivity status. Nikolaev et al. (2014) showed that words with greater stem allomorphy from an unproductive inflectional class are recognized faster than words with lower stem allomorphy from a productive inflectional class. Productivity of an inflectional paradigm correlates with the number of stem allomorphs in languages like Finnish in that unproductive inflectional classes tend to have higher stem allomorphy. We wanted to distinguish which of these two characteristics provides the benefit to speed of recognition found by Nikolaev et al. (2014). The current study involved a lexical decision task comparing three categories of words: unproductive with three or more stem allomorphs, unproductive with two stem allomorphs, and productive with two stem allomorphs. We observed a facilitation effect for word recognition only for unproductive words with three or more stem allomorphs, but not for unproductive words with two allomorphs. This effect was observed particularly in words of low to moderate familiarity. The findings suggest that high stem allomorphy, rather than productivity of the inflectional class, is driving the facilitation effect in word recognition.