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:
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).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.
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
14 comments:
Not able to access the work, here, but am wondering if there were ever similar traumatic events/romatic failures/etc in the 'unaffected twin'?
Homozygot twins may look alike but often there is one "foreign affair minister" of the pair in my experience.
Often, there is one who suffer more if the twin sibling find strong personal interests and relationships outside of "the twin pair kingdom"
Seems to me from a very limited experience.
This to say that may be one twin is less resilient than the other according of the dynamics of the twin pair.
It is false to assume genetically identical twins = biologically identical.
Repeat: being genetically identical does not mean biologically identical.
Particularly in twins, there are inequities in the fetal / womb environment, such as superior blood flow/nutrient delivery to one twin over the other, hormone/endocrine differences in one twin vs the other... it is not normal for human fetuses to be sharing their living space with another fetus. Ironically twin studies may be a better example of the importance of a healthy prenatal environment rather than the heritability/preordained destiny of disease.
Lisa and Leslie sound quite biologically different even from young ages, the aggression/impulsivity demonstrated by leslie could *just as easily* be an endocrine difference or the result of a superior prenatal environment for lisa, even though their genes are 100% identical (and even then, they might NOT be 100% identical, if leslie or lisa contracted a virus early in life the other did not for example...)
Either way, one thing we will learn in time: there is NO SUCH THING as a "psychological" origin of disease. Psychology causes disease like scars cause disease: you are observing an effect of a biological phenomenon, which may go on to cause problems (e.g. scar tissue can lead to nerve pain) but never ever ever originates anything.
ItsTheWooo: Fair comment. They say they were different from a young age, and that's very important if true, but it could reflect retrospective memory bias (one ended up depressed so they projected that depression back on their childhood memories...)
Anonymous Neuroskeptic,
Its The Woo had written something absolutly unscientific ignoring chimerism and mosaic:
///even though their genes are 100% identical (and even then, they might NOT be 100% identical, if leslie or lisa contracted a virus early in life the other did not for example...)///
Chimerism and mosaic make homozygot twins'genes not necesseraly 100 % identical.
I cite wiki :
///In the more common mosaics, different genotypes arise from a single fertilized egg cell, due to mitotic errors at first or later cleavages.
Another form of somatic mosaicism is chimerism, where two or more genotypes arise from the fusion of more than one fertilized zygote in the early stages of embryonal development.///
http://en.wikipedia.org/wiki/Mosaic_(genetics)
I hope your ambition, anonymous neuroskeptic is not a blog full of people just writing scientific nonsenses in their comment with all the authority of a blogger.
Is it just me or is this discussion digressing a bit?
But you are touching on an ancient debate here about the brain-mind dualism. However, a materialist approach to neuroscience would indeed posit that all psychology is the result of biological phenomena inside the brain. Whether or not this is true is worth a whole discussion of its own and I have no desire to get into this right now.
I just want to point out that stating that there are essentially biological processes underlying mental illness isn't undermining psychiatry or even diminishing a non-biological approach to studying and treating these conditions. Think about it this way: memories are presumably encoded in a complex pattern of synaptic efficacy. That doesn't mean that with our present understanding of how the brain works the best way to learn vocabulary in a new language is to take a cocktail of drugs and targeted microstimulation. The best way to learn is still to memorize the words.
The brain being as complex as it is, and the myriad patterns it can form, the changes that even 110% identical brains develop exactly the same is infinitesimally small.
Identical twins or not, on such a interpretative level as the diagnosis of a vague mental disorder it can go separate ways for many reasons.
Most likely: bad diagnosis, in either one.
Else just random fluctuations in neural pathway development.
I've deleted a large number of comments as the discussion had become off-topic and unhelpful. Apologies to those whose comments were attempts to remedy the situation, your comments are gone too.
Any future comments of this nature will be deleted with no warning.
Neuroskeptic,
Thanks.
Oh good, we're all peaceful now :)
First, I haven't read the sudy and I'm not even a medical or biological professional, meaning I suspect I would have a hard time with it. I do know something about depression. I've been on Prozac for many years now because without it I have moods lasting for months and years that I once described to a therapist as "feeling like my soul is dying." When I say "months and years" I don't mean months and years of ups and downs, I mean months and years of down, unrelenting 24/7. Despite the thearapy and epic soul searching I could find no proximate or ultimate cause that was proportional to the way I felt.
So I want to ask a question that should be posed by any researcher in mood disorder. How are you defining depression? If the example in your article is indicative this would seem to be more of a study of the relationship between genetics and reaction to adversity.
I am a college student taking psych 101 and we are learning about the different research models used in psychology. In this post this experiment is described as being 'natural experiment' so does that mean that the research method used was naturalistic observation? It is a bit confusing because they do not actually observe the behavior, but instead rely on what the subjects tell them. I think this study is very interesting and could lead to other great experiments regarding how the environment influences disorders like depression in people.
http://www.psychologytoday.com/blog/twofold/200902/why-some-identical-female-twins-are-different
http://epiexperts.com/blog/identical-twins-are-not-identically-imprinted-epigenome-in-context-of-the-genome/
Note that women are all chimeric to some extent, due to X-chromosome inactivation. Which cells express which X-chromosome genes seems to be fairly random, and determined early in development. I don't know if anybody has looked at this in monozygotic twins, but it wouldn't be surprising if, like fingerprints, it is so critically dependent upon the fine details of the developmental trajectory that the twins are not identical in this respect.
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