Similarity between referents constrains the processing of contrastive focus during reading.

Contrastive focus implies a contrast between two elements. However, it is unclear whether and how any interplay between such a contrast and similarity between potentially contrasting elements might affect focus processing. Accordingly, we report an eye movement experiment investigating this issue. The experiment used a background story to introduce eight characters whose social identities were manipulated to be similar or dissimilar. Participants first read this background story, then a series of two-sentence discourses while their eye movements were recorded. Each discourse referred to two characters from the passage who had either similar or dissimilar identities, with one (the target character) either focused using the Chinese particle zhiyou (meaning only) or unfocused. The results showed a typical focus facilitation effect, such that target character names were processed more quickly when focused than unfocused. We also observed a main effect of the similarity/dissimilarly of characters and, crucially, an interaction between this variable and focus. This interaction was due to slower processing of a post-target region when the target character was focused and the two characters had similar rather than dissimilar identities, but no such effect when the target character was unfocused. The findings suggest that establishing a contrast between referents is effortful during reading when these have similar rather than dissimilar social identities and so are more difficult to differentiate. The distinctiveness of referents in a discourse context may therefore constrain the establishment of contrastive focus during reading. We discuss these findings in relation to current theories of focus interpretation.