Monday, October 10, 2011

Certain mental disorders may have evolved as costs of attractive mental traits

I find costly traits fascinating, even though they pose a serious challenge to the notion that living as we evolved to live is a good thing. It is not that they always deny this notion; sometimes they do not, but add interesting and somewhat odd twists to it.

Costly traits have evolved in many species (e.g., the male peacock’s train) because they maximize reproductive success, even though they are survival handicaps. Many of these traits have evolved through nature’s great venture capitalist – sexual selection.


Certain harmful mental disorders in humans, such as schizophrenia and manic–depression, are often seen as puzzles from an evolutionary perspective. The heritability of those mental disorders and their frequency in the population at various levels of severity suggests that they may have been evolved through selection, yet they often significantly decrease the survival prospects of those afflicted by them (Keller & Miller, 2006; Nesse & Williams, 1994).

The question often asked is why have they evolved at all? Should not they have been eliminated, instead of maintained, by selective forces? It seems that the most straightforward explanation for the existence of certain mental disorders is that they have co-evolved as costs of attractive mental traits. Not all mental disorders, however, can be explained in this way.

The telltale signs of a mental disorder that is likely to be a cost associated with a trait used in mate choice are: (a) many of the individuals afflicted are also found to have an attractive mental trait; and (b) the mental trait in question is comparatively more attractive than other mental traits that have no apparent survival costs associated with them.

The broad category of mental disorders generally referred to as schizophrenia is a good candidate in this respect because:
    - Its incidence in human males is significantly correlated with creative intelligence, the type of intelligence generally displayed by successful artists, which is an attractive mental trait (Miller & Tal, 2007; Nettle, 2006b).
    - Creative intelligence is considered to be one of the most attractive mental traits in human males, to the point of females at the peak of their fertility cycles finding creative but poor males significantly more attractive than uncreative but wealthy ones (Haselton & Miller, 2006).

The same generally applies to manic–depression, and a few other related mental disorders.

By the way, creative intelligence is also strongly associated with openness, one of the "big five" personality traits. And, both creative intelligence and mental disorders are seen in men and women. This is so even though it is most likely that selection pressure for creative intelligence was primarily exerted by ancestral women on men, not ancestral men on women.

Crespi (2006), in a response to a thorough and provocative argument by Keller & Miller (2006) regarding the evolutionary bases of mental disorders, makes a point that is similar to the one made above (see, also, Nettle, 2006), and also notes that schizophrenia has a less debilitating effect on human females than males.

Ancestral human females, due to their preference for males showing high levels of creative intelligence, might have also selected a co-evolved cost that affects not only males but also the females themselves though gene correlation between the sexes (Gillespie, 2004; Maynard Smith, 1998).

There is another reason why ancestral women might have possessed certain traits that they selected for in ancestral men. Like anything that involves intelligence in humans, the sex applying selection pressure (i.e., female) must be just as intelligent as (if not more than) the sex to which selection pressure is applied (i.e., males). Peahens do not have to have big and brightly colored trains to select male peacocks that have them. That is not so with anything that involves intelligence (in any of its many forms, like creative and interpersonal intelligence), because intelligence must be recognized through communication and behavior, which itself requires intelligence.

Other traits that differentiate females from males may account for differences in the actual survival cost of schizophrenia in females and males. For example, males show a greater propensity toward risk-taking than females (Buss, 1999; Miller, 2000), and schizophrenia may positively moderate the negative relationship between risk-taking propensity and survival success.

Why were some of our ancestors in the Stone Age artists, creating elaborate cave paintings, sculptures, and other art forms? Maybe because a combination of genetic mutations and environmental factors made it a sexy thing to do from around 50,000 years ago or so, even though the underlying reason why the ancestral artists produced art may also have increased the chances that some of them suffered from mental disorders.

A heritable trait possessed by males and perceived as very sexy by females has a very good chance of evolving in any population. That is so even if the trait causes the males who possess it to die much earlier than other males. In the human species, a male can father literally hundreds of children in just a few years. Unlike men, women tend to be very selective of their sexual partners, which does not mean that they cannot all select the same partner (Buss, 1999).

So, if this is true, what is the practical value of knowing it?

It seems reasonable to believe that knowing the likely source of a strange and unpleasant view of the world is, in and of itself, therapeutic. A real danger, it seems, is in seeing the world in a strange and unpleasant way (e.g., as a schizophrenic may see it), and not knowing that the distorted view is caused by an underlying reason. The stress coming from this lack of knowledge may compound the problem; the symptoms of mental disorders are often enhanced by stress.

As one seeks professional help, it may also be comforting to know that something that is actually very good, like creative intelligence, may come together with the bad stuff.

Finally, is it possible that our modern diets and lifestyles significantly exacerbate the problem? The answer is "yes", and this is a theme that has been explored many times before by Emily Deans. (See also this post, by Emily, on the connection between mental disorders and creativity.)

(All cited references are listed in the article below. If you like mathematics, this article is for you.)

Kock, N. (2011). A mathematical analysis of the evolution of human mate choice traits: Implications for evolutionary psychologists. Journal of Evolutionary Psychology, 9(3), 219-247.


David Csonka said...

Fascinating idea Ned. Great blog, I love how I can always come here to be exposed to a contrarian way of looking at things.

Sam Knox said...

I wonder how the differences in diet and lifestyle would have changed the expression of mental illness in pre-Neolithic societies.

It seems possible that schizophrenia, for example, might not only have been less common, but less dramatic, as well, and so less debilitating than its modern form.

Also, it occurs to me that what we now call schizophrenics might have been more highly valued by both men and women as "shamans" or "seers" and serve in the same or similar way that organized religions do in modern societies.

Unknown said...

I blogged about a large and compelling study here:

And yes, the two issues are 1) the increased expression of mental illness in our inflammatory modern world and 2) the acceptance of psychosis, etc. in hunter-gatherer populations (there's a popular meme that it is more accepted, and some are shamans or seers - but truth be told my professors who had done some world traveling told me that much of the time, a psychotic individual is tied to a tree and thrown food every once in a while.)

Ned Kock said...

Thanks David.

Sam Knox said...


I remember reading "The Origin of Consciousness in the Breakdown of the Bicameral Mind" by Julian Jaynes many years ago. If memory serves, he argued that, up until both hemispheres of our brains started talking to each other, we were directed by voices in our heads in much the same way that schizophrenics are today.

Was he on to something or full of doo-doo?

Ned Kock said...

Hi Sam. Indeed, it is possible that group support would reinforce selection via shamanism. But intra-group competition would likely have a detrimental effect on selection - shamans don’t seem to like competition.

Ned Kock said...

Thanks for the link Emily. Nice post.

Ned Kock said...

I’ve just linked Emily’s post to this post – nice overview of some key studies.

David Isaak said...


I thought Jaynes' book was quite compelling. Even if it isn't correct, it ought to be.

Lerner said...

If we were to broaden things by supposing that creative intelligence can manifest as charisma, then I'd say that the young Gadhafi must surely have been able to attract devoted followers in order to, at age 27, topple a king.

Of course, his bizarre psychological traits eventually became well known - but I also thought that his facial appearance was also indicating some pathology, whether diet or drugs or perhaps some innate deranged biochemistry.

Ned Kock said...

Hi Lerner. Interestingly, Robert Trivers has been working on a theory of self-deception lately; he often argues that certain leaders (e.g., corrupt politicians and iron-fist dictators) need to be masters of self-deception, to be able to deceive others.

Maybe they need some other mental traits as well …

Indian Pharmacy said...

The question often asked is why have they evolved at all?