From research over decades in Harvard graduates and (initially) young men from inner-city Boston neighborhoods:

“We used to think that if you had relatives who lived to a ripe old age, that was the best predictor” of a long life, said Robert Waldinger, director of theHarvard Study of Adult Development, a psychiatrist at Massachusetts General Hospital, and an associate professor of psychiatry at Harvard Medical School. “It turns out that the lifestyle choices people make in midlife are a more important predictor of how long you live.”

We recently talked about the benefits of keeping a gratitude list.  The researchers note:

In a recent paper, Waldinger, Elizabeth Kensinger, and Marc Schulz utilized neural imaging to find that older adults with positive outlooks process emotional information differently from those with more negative views.