In 1967, psychiatrists Thomas Holmes and Richard Rahe decided to study whether or not stress contributes to illness. They surveyed more than 5,000 medical patients and asked them to say whether they had experienced any of a series of 43 life events in the previous two years.
Each event called a Life Change Unit (LCU), had a different “weight” for stress. The more events the patient added up, the higher the score. The higher the score and the larger the weight of each event, the more likely the patient was to become ill.