Depression's Evolutionary Roots: Scientific American
http://www.scientificamerican.com/article.cfm?id=depressions-evolutionary
Depressed people often think intensely about their problems. These thoughts are called ruminations; they are persistent and depressed people have difficulty thinking about anything else. Numerous studies have also shown that this thinking style is often highly analytical. They dwell on a complex problem, breaking it down into smaller components, which are considered one at a time.