Genetic Crystal Ball?

It has long puzzled even the sharpest minds why some people succumb to depression while others march on no matter what life hurls at them. An example of the latter, Peter Hamer is a retired school principal who, over the years, has endured a divorce, the deaths of both parents and a job that often frayed his nerves; he's now supporting his wife through a battle with breast cancer. To many, those hardships would sound like the normal rough and tumble of life. But they'd be enough to tip others into a state that would pass these days for clinical depression....