Stressful life events

I was part of a dissertation defense this afternoon. The student was looking at race differences in stress and depression. One of the common measures of stress used in research is the "life events scale." This list has evolved over time as new research develops. Some researchers included both positive and negative events (weddings, birth of a child, death of a parent, loss of a job). Others use only negative events and include things like "victim of assault" or "victim of robbery."

In our discussion of the findings we went off on a tangent about what makes an event stressful. For instance, for anyone in the room being asked to get up in front of a room of 30 people and talk about some subject for an hour would not be considered stressful, we do it every day. For many people, however, public speaking is one of the most common items of stress. What determines a stressor for any individual? Can we define any life event as uniformly stressful for the population? How common of a stress does it need to be to be considered "universal"?