Julianne Holt-Lunstad's 2015 meta-analysis of 70 prospective studies including 3.4 million participants found that objective social isolation increased mortality risk by 29%, loneliness by 26%, and living alone by 32%. These effect sizes exceeded those for obesity and physical inactivity in the same analysis. Strong social relationships increased survival odds by 50%, an effect size comparable to quitting 15 cigarettes per day.
The Molecular Mechanism
Steve Cole at UCLA has mapped the genomic pathway by which loneliness translates into biological harm. Lonely individuals show the CTRA pattern: upregulation of pro-inflammatory genes and downregulation of antiviral and antibody-producing genes. This pattern tracks with current subjective loneliness — the biology responds to the felt experience of social disconnection, not to any objective measure of time spent alone.