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Some peer-reviewed studies have found that happy people tend to live longer than their less-happy counterparts. But now, for the first time, researchers have found that happiness all by itself—regardless of marital status, income, physical health and other indicators—is a key factor in longevity.

“Nature teaches beasts to know their friends,” wrote Shakespeare. In humans, nature may be less than half of the story, a team led by ��ñ���� researchers has found.