A happy retirement depends on being rich, not in money but in friends, researchers say.

They said as people age they should invest as wisely in their friends as they do in money.Toni Antonucci and Alicia Tarnowski at the University of Michigan analyzed data on 100 people interviewed before and after retirement.

A quarter, 25 percent, said they were more satisfied with life after retirement, while 34 percent said they were less satisfied. The rest reported about the same levels of satisfaction before and after.

The researchers looked at a variety of factors in these peoples' lives, including physical health, income, divorce, death of a spouse, age and gender.

What most strongly predicted happiness after retirement was having a strong social support network, they told a meeting of the Society for the Psychological Study of Social Issues.

"Our findings suggest that new retirees may need more emotional support than they did when they were working," Tarnowski said.