Impaired theory of mind in Parkinson's disease: A systematic review of etiological debates.

Impairment of theory of mind (ToM)-the ability to attribute mental states to others-has been identified as a non-motor feature of Parkinson's disease (PD), potentially undermining social functioning and quality of life. A central controversy concerns whether ToM deficits arise directly from PD neuropathology or are secondary to general cognitive impairment, with growing evidence pointing to the latter. This systematic review examined 14 studies employing designs that allow stronger causal inference-including regression, between-group, and experimental paradigms-to assess the dependence or independence of ToM deficits in PD relative to cognitive impairment. Overall, ToM performance showed a robust dependence on cognitive abilities across studies: patients with cognitive impairment consistently performed worse on ToM tasks than cognitively preserved patients, while results comparing preserved patients to healthy controls were mixed. Moreover, ToM performance improved in PD patients following general cognitive training or when tasks minimized executive function demands, though the validity of such modified tasks is debated. These findings suggest that impaired ToM performance in PD largely reflects general cognitive dysfunction rather than a primary social-cognitive deficit. However, whether an intact intrinsic capacity for mental state inference persists in PD remains unresolved. Further high-quality studies are needed to clarify this relationship and to inform targeted interventions as well as theoretical insights into ToM itself.