Bzdok and Dunbar (open source) do a definitive review of evidence that psychosocial embedding in interpersonal relationship is crucial for survival. Dunbar is well known for his demonstration of the correlation between the size of social groups and the brain size of their members, which places the natural size of groups of humans at ~150 (roughly the size of human hunter gatherer groups whose isolation has permitted them to survive into the modern era. The number of friends and family relationships we can manage at any given time is limited by our cognitive constraints to ~150). Their summary:
From babies to the elderly, psychosocial embedding in interpersonal relationships is crucial for survival.
Insufficient social stimulation affects reasoning and memory performance, hormone homeostasis, brain grey/white matter connectivity and function, as well as resilience to physical and mental disease.
Feelings of loneliness can spread through a social network, causing negatively skewed social perception, escalating morbidity and mortality, and, in older people, precipitating the onset of dementia (e.g., Alzheimer’s disease).
