Idiom and proverb processing in autism: a systematic review and meta-analysis

Figurative language processing impairments in autism have been widely demonstrated, and have been considered a defining feature of autism. Studies in this area often consider different types of figurative language together, and less attention has been paid to identifying the factors that might contribute to difficulties in processing specific types of figurative language. Here we present a preregistered systematic review and meta-analysis of studies assessing the comprehension of idioms and proverbs in autism spectrum disorder (ASD) as compared to typically developing (TD) individuals. Idioms are widely used multi-word figurative expressions, which are understood by using contextual information. Proverbs are a related type of fixed, figurative, formulaic expression in a sentential form, typically linked with wisdom. Idioms and proverbs represent forms of figurative language which are more conventionalized and frequently opaquer than metaphors, pointing to a unique way that they are processed in conversational contexts. Our analysis encompassed a total of 11 studies from 10 papers (involving 235 autistic and 224 TD individuals), which met our inclusion criteria (the ASD and TD groups were matched on both chronological age and intelligence). The analysis of accuracy data revealed a group difference favouring the TD over the ASD group, with a medium effect size, and no indication of a publication bias. Participants’ age was unrelated to the magnitude of group differences, but there was a trend for smaller group differences in the case of participants with higher (verbal) intelligence. We discuss these findings with reference to theories related to the nature of figurative language impairments in autism.