Social interaction as a unique form of reward - Insights from healthy ageing and frontotemporal dementia

The drive for positive social interactions, or "social rewards", is an important motivator of human behaviour, conferring several adaptive benefits. Social motivation fluctuates across the lifespan, reflecting changes in goals and priorities at different developmental stages. In older adulthood, for instance, priorities tend to shift toward maintaining emotional wellbeing and resources over seeking novel gains. Contemporary theories of social interaction must account for such motivational shifts, addressing the enhancement of social processing in ageing and its decline in dementia. Here, we propose a framework to track the evolution of social motivation across the lifespan, focusing on three mechanisms: (i) social interactions as rewards, (ii) learning from social interactions, and (iii) the effort required for social interactions. We posit that social rewards hold equivalent or increased value later in life, enhancing older adults' social connections. Conversely, social rewards become devalued in neurodegenerative disorders such as frontotemporal dementia (FTD), resulting in social withdrawal. This integrative framework serves as a foundation for understanding adaptive and maladaptive trajectories of social motivation throughout the adult lifespan.