This theory states that people with good relationships have more support when things are difficult. There is always someone to take them to the doctor, to help them stop a bad habit, or to calm them down when things are stressful. Social support can backfire though, especially in the case of younger adults. Peer pressure is an example of social support working against health by pressuring an individual into a bad habit. Social support is an important part of life and can be measured through surveys and interviews.
When researchers measured and ruled out social support, there still remained a longevity benefit to having good relationships. Some researchers think that this benefit is due to biochemical changes in the body when we are socially healthy. Much like antioxidants from red wine can help prevent damage, our bodies produce chemicals when we are in relationships that help protect our health. Though the exact way this works is not understood, the evidence is good that relationships promote health and longevity.