I heard about the Grant Study a few days ago, although I can't remember where, and then again this evening, I was reading The Happiness Project and the article below was mentioned there.
As a psychology major in college, the Grant Study fascinates me. It's a study conducted over 72 years with 268 men from Harvard and is one of the longest running longitudinal studies of mental and physical health in history.
Although the article is long, it's well worth the read.
What Makes Us Happy? by Joshua Wolf Shenk in Atlantic, June 2009