Why Depression? Why Bi-polar? A Short Discussion
The Journal of Theoretical Biology published very interesting study by Daniel Nettle - An evolutionary model of low mood states. The article explains the possible evolutionary fitness of depression and mania. In other words, it tries to answer, “why do these conditions exist?”
The model [of the optimal response to conditions] predicts that individuals in a good state will be prepared to take relatively large risks, but as their state deteriorates, the maximum riskiness of behaviour that they will choose declines until they become highly risk-averse. However, when their state becomes dire, there is a predicted abrupt shift towards being totally risk-prone.
In other words, depression [low-mood state] is evolutionarily fit. The mood inhibits risk taking when circumstances suggest it would be dangerous.
As painful as depression is, it is near certain to have a biological rational. The better we understand why, perhaps the easier it is to accept periods of low mood. In a way, this exercise is objectifying depression, providing perspective.
Moods are appropriate to conditions; context matters. When resources are limited and energetic-activity is unlikely to be rewarded, an optimal response would include:
fatigue, loss of motivation and interest, anhedonia (loss of pleasure in previously pleasurable activities), pessimism about future actions, locomotor retardation, and other symptoms such as crying.
In other words, depression. (The author is careful to distinguish ‘adaptive’ depression from its more clinical counterpart.)
Another significant contextual signal is death, an outcome suggesting high survival-risk to risk-prone activities. ‘Death’ could also include qualities such as disruption to family, presence of predators, the unexplained loss of food, and so on. Optimal responses could look like ‘mourning,’ a mood-state similar to depression.
Nettle also addresses the circumstances when manic responses (and here I read bi-polar in general) are evolutionarily fit:
Agitated depression is more common in patients who also have manic episodes, which leads to the further question of whether mania could be related to the predictions of the model [of the optimal response to conditions].
Mania is characterised by feelings of increased energy and locomotor activity, impulsivity and disinhibition; all the features, in short, which would promote the pursuit of high-risk activities.
During periods when the optimal response is to be risk prone, members of a population need to pursue such risk. If a minority of the population has excessive sensitivity and reactivity to the opportunity to take risk, then such behavior would both be selected for and also biologically fit for the population. This statement could well describe individuals who are episodically manic (bi-polar).
It is possible for behaviors to be fit for a population and unfit for an individual (and this possibility not that rare either). I don’t want to dig out a 25-year-old text book on sociobiology to support this point.
Bi-polar, moreso than depression, is a challenging condition for individuals, families and even society. Exploring "why bi-polar?" is not to diminish pain but rather to find understanding.
3 Comments
Reader Comments (3)
Depressive states can be temporarily adaptive, but I don't think chronic depression is adaptive at all. When someone can't go to work to pay bills, it's not the risks that come with going to work and paying the bills that's the problem, there's something else going on. Science still hasn't cracked that nut yet.
I really wouldn't say that bipolar disorder is selected for because of an excessive sensitivity and reactivity to risk. The theory will need some ecological validity -- when has bipolar disorder been linked to various measures of "successful risk-taking," where it's been absent or impaired in non-bipolars? What's the real-life context there?
Evolutionary theories are necessary to explore, but at some point they break down when we start entering the complexity of human cognition and affect. Our constructs are becoming increasingly sophisticated with the scaffolding of technology and culture compounding that. That can often trump other explanations that are based on a more simplified model of human behavior. I think the trick is working the various viewpoints at the right angle.
Thanks for the short discussion, and for a interesting study to chew on.
Thanks for the reply.
You point about this analysis 'breaking down' is very appropriate. Evolutionary biology focuses on populations. It looks past the extreme cost of certain evolutionary traits for individuals.
Also, clinic depression is a harsh condition and different from the use of 'depression' in note. I tried to mirror the terms used by Nettle. The best term might have been 'low-mood.' The study's notion of mood-state is how a population is affected on average, rather than how individuals are uniquely affected.
It is possible that bi-polar is an evolutionary accident, a condition inevitably weeded from the future gene-pool. More likely is that bi-polar provides an advantage for a population when a few members are bi-polar and are likely to forage with manic energy when conditions are good or dire.
If many members with bi-polar are unsuccessful but a few are heroically successful, then those few should have significant reproductive success. An interesting list to look at - famous people with bi-polar.
My statement also looks past the high cost of bi-polar for an individual.
Cole, thank you for your patient response. Your original statements are now much clearer to me. I think the concept of an evolutionary accident is an interesting one. Thanks again.