On the relationships between apathy, depression and anhedonia

Although apathy, depression, and anhedonia frequently co-occur, the differences between these syndromes remains poorly characterised. Here, we assessed 4,350 healthy individuals and 235 people with major depressive disorder or on antidepressants. The results demonstrated significant symptom overlap between the three syndromes. However, factor analysis revealed five distinct dimensions: depression, anhedonia and three apathy domains (behavioural, social, emotional). While depression was strongly correlated with behavioural apathy and moderately with social apathy, there was no such relationship with emotional apathy. Using a feature-selection algorithm, we identified ten key symptoms – phenotypic markers – that reliably distinguished each syndrome in isolation, with replication in independent datasets. Crucially, emotional apathy emerged as a key domain differentiating apathy from depression and associated with reduced affective empathy as well as diminished sensitivity to emotional intensity. These findings reveal key relationships between apathy, depression, and anhedonia, but also demonstrate their separability on the basis of distinct phenotypic patterns.