Are theory of mind and metaphor comprehension causally related? A training study in middle childhood

The main aim of this study was to investigate the existence and the direction of a causal relationship between theory of mind (ToM) and metaphor comprehension during middle childhood. To this purpose, we used a compelling training design, involving 53 typically developing children (25 M, pre-training Mage = 9 years; 3 months, SD = 0;4, age-range = 8;9-9;10) who were randomly assigned either to a ToM or a metaphor comprehension training (MetaCom) condition. Training programs were matched in structure and length and already proved effective in previous studies. Before and after the intervention, children were assessed for both advanced ToM (Strange Stories task) and metaphor comprehension (Physical and Mental Metaphors task). Groups were equivalent at baseline for the focus (ToM and metaphor comprehension) and the control variables (grammar, receptive vocabulary, reading comprehension, and working memory). Results showed that the two training programs significantly differ in their effect on ToM and metaphor comprehension. While both groups significantly improved in the respective focus variables (children in the ToM training improved in ToM skills and children in the MetaCom training improved in metaphor comprehension), the MetaCom training had a significant cross-domain effect on ToM. Conversely, the ToM training did not enhance children's metaphor comprehension. Overall, this study suggests a driving role of metaphor comprehension in the development of advanced ToM, rather than vice versa.