The Grant Study of 268 gifted Harvard students extensively over the course of their whole life from the classes of 1942, 1943 and 1944. Many of these men fought in World War II and endured life's traumas. Several were successful, including JFK who became president. By 1948, 20 of the men displayed signs of psychiatric illness. At 50 a third of them were mentally ill. This study made a strong link between depression and physical health, with 70% dying or being chronically ill by age 63 if they were diagnosed with depression by 50.
Martin Seligman and Chris Peterson,studying the Grant Study data, linked reduced health and longevity to pessimism.
Laura Kubzansky, a Harvard psychologist, who studied 1300 men for 10 years and found that heart disease in pessimists was at twice the rate as of optimists.
Madelon Visintainer who studied rat groups to discover that learned helplessness in rats who could not escape electric shocks weakened their immune system to the point that only 27% could reject the implanted cancer tumor cells. Those rats who were given escapable shocks rejected the cancer 70% of the time.
David Snowdon who studied a convent of nuns with fairly controlled and similar living habits - same quarters, bland diet, etc. In studying the life stories of these nuns, researchers found that 90% of the most cheerful nuns were still alive at age 84 compared to 34% of the least cheerful.