Virginia psychiatrists Kendler and Halberstadt describe a neat "natural experiment" into what causes depression - The road not taken: life experiences in monozygotic twin pairs discordant for major depression
They interviewed 14 pairs of identical twins. One of each pair had reported a history of depression while the other hadn't. The twins were interviewed together, and asked to describe their lives, in particular any differences between their experiences.
It's well worth reading, for the human interest stories if nothing else. Here's perhaps the most striking one:
Lisa (never depressed) and Leslie (depressed), interviewed at age 53, were identical twins... they were together constantly as children, but described their personalities as somewhat different from the start... Lisa described herself as getting quickly upset over adversity, but then rapidly changing gears and focusing on problem-solving. Leslie indicated that she had more of a temper and was more assertive, more likely to ‘mouth off’ and get into trouble than her twin.
Lisa knew she wanted to be a school teacher, attended a teacher’s college and has taught for her entire career. Leslie was less certain of her career goals and held a number of different jobs... Lisa, at 24, met her husband of nearly 30 years and had a big, traditional wedding. Leslie married the guy she dated in high school and college, invited only their parents to the wedding and divorced after 5 years, commenting "I picked the wrong man."
Late in the interview, Leslie reported (only after being asked about the most difficult time in her life) that just over 30 years ago, after drinking a modest amount of alcohol, she became pathologically intoxicated and drove onto a major highway off ramp going the wrong way. She got into the highway going against traffic and had a head-on collision, which killed the other driver - a woman with young children. Leslie was not seriously injured.
Recounting this event was clearly difficult for her as she openly wept in telling us this story even after all these years. In recounting her psychological reaction to the accident, she said, "Am I really to blame for this, and then I’ll have to live with this for the rest of my life? knowing there was somebody else whose family had been destroyed there."
The depression following this episode was her most severe and she talked about her deep sense of guilt. Although manslaughter charges against her were dropped, there was evidence that Leslie was at fault.
The other stories were less extreme, but no less dramatic in their own ways. The authors conclude that, of the 14 pairs, the depressed twin got depressed because of: romantic difficulties (7); single traumatic events (2, including Lisa & Leslie); employment difficulties (1), a mixture of factors (2), and for no clear reason at all (2).
However, what we can really conclude from all this? The study specifically took identical twins, who grew up together but only one of whom reported depression, so it ruled out genetic influences and also psychological and social factors shared by both twins - things like family background, growing up in poverty, etc. That's the whole point of the study but it's important to remember that these are unusual cases.
Beyond that, it's hard to know if the differences in the twins' lives caused the depression. All we know is that they're correlated. "Leslie" for instance became seriously depressed after causing the death of a woman in a drunk driving accident. A straightforward case of cause and effect, perhaps, but then why did she drink and drive in the first place? Was depression, or something associated with it, part of the reason?
Ideally, I would want to repeat this study in identical twins both (or neither) of whom had depression to see if their lives - before the depression at any rate - were more alike than these discordant twins. However, it's still a fascinating study.
Kendler, K., and Halberstadt, L. (2012). The road not taken: life experiences in monozygotic twin pairs discordant for major depression Molecular Psychiatry DOI: 10.1038/mp.2012.55