PsychEducation.org (home)                                                                                                                                The Biologic Basis of Bipolar Disorder (start)

Chapter 5: The Big Picture -- 
There Must Be Some Evolutionary Advantage?

Summary:
Evolution did not select out bipolar disorder because the genes which lead to bipolar disorder have, in low "doses", significant value. The traits to which they lead were valuable in society when humans were evolving by natural selection, and may be acted upon by social selection pressures even now. one theory about depression, which can be extended to understanding mania, this suggests that depression is similar to being at the bottom of the latter in a social hierarchy.  The best thing to do is keep a low profile and save your energy.  Similarly, mania might be akin in some way to being at the top of a social hierarchy, where aggressive use of available resources can lead to significant accomplishments.


 Why didn't evolution "select out" bipolar disorder?  

(You don't believe in evolution? Here's some brief help with that)

Ten to twenty thousand years ago humans were still strongly affected by evolutionary selection. Only highly "fit" individuals could survive and prosper.  If they did prosper, they could help their children survive and reproduce.  So genes that created increased "fitness" would be preserved, and amplified: more children with these genes would survive to reproduce -- and prosper enough to help their children survive, and so forth.  

The opposite is also true: genes that decreased "fitness" -- the ability to survive and prosper -- would be reduced in every generation, as the humans with those genes struggled and failed to reproduce, or their children struggled without prosperous parents to help them.  

Severe bipolar disorder clearly reduces "fitness".  In bipolar I, an individual who has delusions that his wife is unfaithful and kills her, loses her support for their children and his opportunity to reproduce (to put the matter in blunt evolutionary terms).  Becoming "manic" and giving away all one's grain because of a belief that more can easily be harvested, when actually it's all been harvested already -- this too would decrease survival and reproduction of the individual and probably his children.  

So why wasn't the "gene", or genes, for bipolar disorder eliminated by evolution many thousands of years ago?  Surprisingly, there are numerous genes that appear to decrease an individual's reproductive success yet still have not disappeared.  The most common explanation for this puzzle is that the gene causes some change that in small doses provides increased fitness, and only with a "large dose" of this gene, and its effects, does the individual function less well than average. 

Fitness in bipolar disorder

One of the classic examples of this is "sickle cell anemia".  When an individual gets two copies of the gene that causes this condition, she has a crippling anemia and will die young.  However, if she gets only one copy of this gene, and a normal gene from her other parent, she can actually have increased survival success.  If she lives in an area with lots of malaria, she will have a lower risk contracting this lethal infection (her red blood cells contract into a sickle shape in which the malaria bug cannot survive -- but only some of her cells do this, because half of them are being governed by a "normal" gene, so she doesn't get the crippling effect of many cells doing this at once).  

The bipolar geneticists are thinking this same kind of thing has happened in bipolar disorder.  There must be some advantage that getting a "small dose" of bipolar genes provides.  And that's not too hard to imagine.  What is a person like in a manic phase?  What if you could have just a little of that?  For example: 

Gene Gene Effect Just a little Too much
A Connect unrelated ideas Creativity Tangential, disorganized
B Seek novelty Fascinated by change, curious Jumping from project to project
C Take risks Courageous Bad judgment about harm
D Be aware of others' opinions Socially polished Anxious, suspicious, paranoid
E High energy level Very productive Can't stop, slow down
Racing thoughts
Unable to focus
Scattered activity

If there are multiple genes that cause bipolar symptoms, then having a few was probably a good thing, in terms of one's reproductive success 10,000 years ago.  Later,  human evolution became dominated by social selection:  those who rose up the social ladder, or started there by being born of social leader, were more reproductively successful.  This pattern has been diminished in the last several hundred years as more and more humans are able to reproduce regardless of their position in the social hierarchy.  But until then, a genetic selection process probably still had major effects on bipolar gene "frequency" -- how many individuals, in the total population, carried one or more genes that in large doses cause illness.  

So our model looks like this: 

Having a few too many genes begins to decrease reproductive success, because the behaviors they cause are becoming too extreme -- in other words, a person with that many genes is becoming "symptomatic".  If you get a few more genes than that, you may have so many symptoms that you cannot function well.  This is what we regard as "mental illness".   

Possible evolutionary value of mania itself

Does anyone have even a hunch as to what mania is?  The most compelling guess I've heard also falls in well with a long-standing guess about what depression is.  The guess presumes that somehow these mood changes must have some evolutionary benefit, at least when they are not so extreme (otherwise we'd have to wonder why these potentially lethal mood changes wouldn't have led to the removal of the genes associated with them, whatever those genes are).  So what might be the benefit of depression?  When faced with overwhelming stress, perhaps it might have been "smart", at one point in our evolutionary history-- reproductively speaking, anyway -- to be able to "shut down" and save resources for better times.  Give up on climbing the social ladder; give up on trying to start new projects or even complete the current ones (if you're really at the bottom of the heap already, anything you manage to make or gather might very likely be stolen anyway). Just hunker down and wait. Turn off your motivational engines. Heck, turn off your engines themselves as much as possible and go into a sort of hibernation if you can manage to do so. Wait for better times. Sleep a lot. Hoard your calories, because you may not get much to eat during this time. If you can grab any easy calories, eat a bunch of them, who knows when more are coming. "Perceptions of defeat" are the key ingredient leading to this state, according to one researcher (Gilbert).

That's the depression side of the hunch, obviously.  Somehow mania must be "opposite" in some ways? (even though it probably has a different mechanism, because we know that manic and depressive symptoms can occur at the same time, as you've learned about bipolar "mixed states") So the mania-side hunch goes like this:  what if it is caused by the brain chemistry associated with the opposite social experience, namely being on top of the social ladder, somehow goes to an extreme in mania? 

In our primate ancestors, these social ladders are very distinct and very obvious, even to researchers chasing them around their natural environments. Much of what we know about stress hormones and social ladders comes from the work of Robert Sapolsky and colleagues, for example, who did indeed literally chase baboons around the African Savannah routinely for years, gathering information on the chemistry of animals at the top and the bottom of their social "hierarchies". Based on and reasoning forward from such research, several mood experts have speculated that mania might be "too much of a good thing", where the good thing is the confidence, the drive, the ability to motivate oneself and get things done, the decrease in need for sleep, and even the increased sexual activity, of the top-of-the-heap animals in a social hierarchy. Go ahead, take risks; you've already established that you're the top baboon, so who's going to beat you up for bragging? Just strut right up to the top female in the hierarchy; she'll recognize that you're the alpha guy, and something good will happen. Take on that pack of hyenas? Sure, the pack is behind you, they'll follow your lead and you'll get rid of these pests for a while. Doesn't everyone see what is possible? Let's get going. The pickings are there for the taking. Everything will work out (believing all this may be necessary to work up the kind of confidence it takes to be the leader in this pack; and if it works, you'll have some great privileges it really does make sense to take risks for).  

This line of thought has been around a long time (e.g. 1982). But it is still very active; a recent resurgence in this reasoning (e.g. Wilson, 2002) is associated with the rise of "evolutionary psychology", now a field unto itself and growing stronger. A classic explanation of depression as an adaptation was provided by a leader in this field, Randolph Nesse. Not all mood scientists agree with this model, however; e.g.  Dubrovsky, 2002.  I've cited this line of thought here because it helps me to have at least some working guess as to what bipolar disorder might be, some idea on where it came from -- just in case there might emerge some suggestion on how to treat it. So far, the latter is lacking. Keep watching. 

(If you keep stumbling over the apparent emphasis on evolution here, perhaps my little essay on evolution might help.)  

Recommended reading
Dr. Kay Jamison is a professor at Johns Hopkins University.  She has written extensively on the connection between creativity and bipolar disorder.  Her book Touched with Fire is a good starting place on this subject. So is her newer work, Exuberance.