I think one can deduce that there is an evolutionary disadvantage to a heritable predilection toward pessimism, or at its extreme, depression, but I think it’s indirect. Based on the ideas in Dawkins’ The Selfish Gene and elsewhere, the idea that evolution selects traits that have net positive benefits to an organism and eliminates traits (or reduces their frequency) that have net detriment. “traits”, loosely defined can be connected to heritable hunks of DNA (not necessarily “genes” per se, but possibly sets of genes and/or other functional bits). Predicting benefit vs loss arising from modification of the DNA is challenging and often game theory and psychology are enlisted to help.
Imagine a mutation that induces a person to commit suicide. If this acts on the person before puberty the mutation will, obviously, quickly disappear in the population. OTOH, if the mutation has its effect well after breeding age it may be represented in a stable fraction of the population. In short, a tendency to suicide only has negative consequences, and thus would tend to disappear, if it affects breeding adversely, which is possibly why a later-in-life tendency to suicide would actually be advantageous to the population (get rid of the old useless old folks so the young can consume more, an attitude the Repugs were counting on as they try to get rid of Medicare and Social Security.)
I think the “wild type” of human behavior vis-a-vis pessimism vs optimism is obviously in favor of optimism. If pessimism were dominant our species would often just give up and thus perish (or the individuals with the pessimism trait would become relatively less numerous). But “optimism”, especially in its extreme, foolhardiness may be good for the group (the foolhardy fight predators and many die but the rest of troop is protected by their “sacrifice”) but bad for the individual. But lots of studies show this trait (fearlessness to the point of foolhardiness) does exist in wild populations. Optimism drives an exhausted organism to keep moving and maybe find the sustenance it needs. OTOH, pessimism, especially when expressed in its extreme as depression does reduce the incentive to press on and find whatever the individual or group needs.
Now as humans can communicate and thus the behavior of a group of humans is influenced by “debate” among those individuals that disagree, I think the traits of optimism (an evolutionary advantage) vs pessimism (an evolutionary disadvantage) will find their equilibrium behavior through social selection, which often, in primates, means sexual selection. So I think that’s why we generally shun pessimistic or depressed people; we simply don’t want them around (from that unconscious evolutionary notion of fitness) but we also don’t want them influencing us. In short nobody likes a gloomy Gus so a sure path to unpopularity is to be a pessimist (or even a realist).
Sexual selection is believed, by many, to have strongly driven human evolution. Sexual selection consequences are most easily discernible in birds and are a bit more complex in primates. But both birds and primates have the same issue – it’s tough to be a single mom (for birds, given the mechanism of their reproduction, this is almost a certainty). In many mammals and other species males are essentially irrelevant to the survival of offspring and so are reduced to nothing more than sperm donors. But in species where nurturing offspring requires substantial parental contribution the male must also do some care-giving and thus for these species it is generally believed that the females can select males based on traits that will aid the survival of the offspring. The bright colors of male birds is most often interpreted that the male must risk its own life in order to draw away predators so thus a male that is both showy and fit is the ideal daddy and female birds try to detect this. In primates, especially humans, it is more the role of food gathering than defense that represents desirable traits for the female.
So, assuming non-human animals can show “optimism” or “pessimism” wouldn’t female selection always go for the “optimist”. I once was stuck, by snow, for several days in Lassen National Park and enjoyed watching the male robins aggressively looking for building materials for nests (including stealing from each other) which they use to show off to females to prove what great dads they’ll be. Obviously the “optimistic” robin wins this one, believing both he can seduce the female as well as build a great nest. So I think this is built into humans as well.
So if “optimism” is a positive trait why does the negative “pessimism” trait persist? I think that’s where we get confused by gender roles. A male robin has to prove his worth to the female but is the other direction required? Regardless of what we believe about gender as a social construct, there is a big biological difference. In terms of reproduction itself the short answer is that sperm is cheap and thus males are cheap; OTOH, ova are expensive (far more biological resources required) and for mammals pregnancy is very expensive, to the female. So the optimal male strategy (the assumption being every creatures wants to maximize its offspring) is to spread its cheap sperm around, in short, males are not programmed to be very choosy. OTOH, females pay a much higher price to produce offspring so the female will carefully guard her reproductive assets by attempting to chose the best male around (and generally only one).
So while the female will choose an optimistic male, if she can, the male probably won’t care. Thus like any recessive trait, the female can be homozygous while the male probably has to be heterozygous. Thus the pessimism trait can continue to co-exist in equilibrium with the pessimism trait.
I think social situations among humans always discriminate against “pessimists” and especially against “depressed” people. We place a high premium on “upbeat”, “cheerful”, “optimistic”, etc. We do it because if the more negative attitudes become more widespread in the population our breeding success would be reduced. Therefore as with every other trait selected by natural selection our species will discriminate against a trait, and thus its frequency will be reduced in the population.
I always wondered about the typical prohibition against suicide by religion. And also about the human invention of the idea of hell (which works in conjunction with the prohibition against suicide, where imaginary hell is the penalty for suicide). For much of human existence life has really sucked. Yet people continued to struggle to survive and somehow also to breed (some modern humans may not want to end their own life but are willing to suppress their desire for offspring to avoid burdening the offspring with a sucky life, i.e. my parents made the choice for me, but I can make the choice for my potential offspring). So with such sucky conditions why didn’t more humans either just give up and let death come or act to end their life. Was it just the social prohibition against suicide? Or is it the preference toward heritable optimism?
Now I have no idea about the suicide rates in ancestral populations of the Japanese vs other peoples, but at least at the popular level it appears that Japanese are less afraid of death than westerners (undoubtedly the effect of religion) and even more willing to commit suicide. And for much of their history life has sucked there too, but did that translate into higher suicide rates (one can reasonably deduce no, since there is a thriving population in Japan). So western society really didn’t need the religious prohibition against suicide in order to keep people fighting to survive in a sucky world. Why? Well, that’s the natural selection of the “optimism” trait again.
I ponder all this because I think the U.S., especially the religious right, will face an interesting challenge in a few decades. And that is to reverse the traditional hostility to assisted suicide. I’ve never really understood it anyway (it’s not like religious types don’t like killing people, they love wars against infidels, love frying “criminals”, only specks of flesh in a woman’s body get their protection). But the religious right is supporting the plutocrats who are driving the price of medical care beyond the reach of most people and when the Repugs finally get their chance to kill Medicare that’s going to leave massive numbers of elderly without adequate health care. And exactly what then do the Repugs expect to do with the elderly 99%? Just hope they quietly die from lack of care. Instead I believe they will find they can endorse assisted suicide. The simple economic fact is that most of the medical costs a person will ever incur will happen in the last few years of their life. So the plutocrats can get their big grab at yet more tax cuts simply by killing off the elderly. Now genocide of the elderly isn’t too likely to be (directly) proposed by the Repugs, so they will want it to happen implicitly, via assisted suicide. Faced with a miserable last few months or years, without adequate health care, and the certainty of death anyway, why not get it over with?
The ex-wife of a friend went this route. She was inevitably dying and the treatments she was receiving were miserable. Her life was awful and her prospects were certain. So at some point she refused any further treatment, instead wanting to spend her last days in relative comfort. It worked. After refusing the treatment she improved considerably. The docs were compassionate and allowed painkillers. So she had lucid time to spend with her children. She didn’t last long, but her last days were good ones. She had that choice. Others might stretch out painful, expensive and hopeless treatments forever; that’s their choice. But when it takes a bit more push than simply refusing treatment will we permit that?
So I predict a great fight over assisted suicide, between some (primarily Catholic) of the religious right and the plutocrats in the Repug party, just one of many fights they’ll have in the future since fundamentally these two groups actually don’t share any common values. The religinuts have already shown this; they hate abortion but simultaneously hate (or certainly won’t help support) the unwanted children they force the mother to carry to term.
And thus when assisted suicide is available, how will social life then adjust? Will the dominant hostility toward pessimism/depression decline a bit? Time will tell.