There's a fairly interesting article
here about investigation into the links between depression and use of language. Perhaps surprisingly, the most noticeable change to language use of people with depression was not in emotional words, though there was an increase in use of "sad" words, but in pronouns and "absolutism". People with depression use more first-person pronouns than other people. This might be because depression causes people to focus on themselves. Or it might be that people who are inward-looking are more likely to get depression. Even more marked than the change in pronoun use was the increase in "absolutist" words, like always, never, all, nothing. These were found to be the best linguistic marker for mental health.
This has potential use in diagnosis of mental health issues as well as in understanding of the processes. Although it occurs to me that as the analyses were done on mental health forums, they were of people who were already aware of their depression, so it's not yet clear how useful this would be as diagnosis.
