"Who and what are the we in we?"

Humans are, to our knowledge, the only species who can inquire into the nature of nature. So it is not just narcissism that drives our efforts to understand what makes humans different from other animals. Often we are drawn to the great achievements of Homo sapiens in the arts, science, mathematics, and technology, because we view these achievements and the minds that created them as the paragon of what makes us special. The assumption is that these minds got an extra dose of the best of what makes humans human. But several lines of evidence are now coming together to suggest something a bit different and, for many people, more than a bit disturbing.

It is now well known that great achievers are disproportionately likely to suffer from mental illnesses. Severe mental illnesses, particularly bipolar disorder, are much more common among the greatest novelists, poets, painters, and musicians, than among your everyday H. sapiens, especially in recent centuries as the great accomplishments have become more abstract, that is, less normal. A Freudian might explain this association by suppressed social environment that generated both the creativity and their illness. To geneticists, consideration of familial associations suggests a genetic causes. What flows from these perspectives is the dogma that has dominated most of the past century: mental illness and mental creativity result primarily from an interaction between stressful environments and unusual human alleles.

A careful consideration of the evidence and application of natural selection, however, implicate another cause: infectious agents. People with schizophrenia and bipolar disorder, for example, are more likely to be born in late winter or spring, when born in temperate latitudes. This pattern is a smoking gun for prenatal or perinatal infectious causation, which can also explain the known familial associations as well as or better than human genetics. And human genetics does not offer sensible explanations of other aspects of these disease, such as the season-of-birth associations, the urban/rural associations or the high fitness costs associated with the diseases. People with severe mental illnesses commit suicide at a rate that is far too high to allow the maintenance of causal alleles simply by the generation of those alleles through mutation.

Noninfectious environmental influences may help explain some of these associations, but so far as primary causation of severe mental illnesses is concerned, none of the noninfectious environmental or allelic candidates have stood up to the evidence to date as well as infectious candidates. The arguments will eventually move toward resolution through the discovery of the causal agents whether they be alleles, pathogens or some noninfectious environmental influence. Alleles have been claimed as major causes of these diseases but retractions have followed claims as soon as adequate follow-up studies have been conducted. In contrast, evidence for associations between infectious agents and severe mental illnesses has mounted over the past decade in spite of much less funding support.

The associations between mental illness and creativity make sense from an evolutionary perspective. If our minds evolved to solve the challenges associated with hunting/gathering societies, we can expect the normal mind to be poorly equipped to solve some of the accomplishments valued by modern society, whether they be a new style of painting or complex mathematical proofs. If neuronal networks could fire differently, then new mental processes could be generated. Most of the re-networking that accompanies severe mental illnesses makes a person less functional for the tasks valued by society. But every now and then the reorganized brain generates something different, something that we consider extremely valuable. To distinguish this abnormality that we esteem from the abnormality that we pity, we use the term genius. If the geniuses of today were mentally ill at a rate no greater than that of the general population, then we could reasonably assume that genius was simply one tail of the naturally selected distribution of intellectual capacities.

The high rates of mental illness highest achievers, particularly in the arts, however, demand a different explanation. If the illnesses associated with such creativity are caused by infection and the infection cannot be explained as a consequence of the creative lifestyle, as indicated by the season of birth associations, then the range of feasible explanations is narrowed. The least tortuous conclusion is that prenatal infections damage the development of the brain, generating a brain that functions differently from the naturally selected brain. Most of the time these pathogens just muck up the mind, causing mental illness without generating anything in return. But in a few lucky throws of the dice, a different mind that is brilliantly creative.

At this level of accomplishment it is looking more and more like the we in we do not just belong to Homo sapiens but also to a variety of parasitic species. It may include human herpes simplex virus, borna disease virus, Toxoplasma gondii, and many more yet to be discovered species that alter the functioning of our brains, usually for the worse, but occasionally generating minds of unusual insight. Richard Dawkins's concepts of the extended phenotype and meme return with extended license. In addition to viewing characteristics of an organism as an extension of a manipulator species for the benefit of manipulator genes, some characteristics that humans prize as the best of what makes humans human may be side effects that do not actually benefit the manipulator. They are in effect cultural mutations generated as side effects of biological parasitism. Like biological mutations the cultural mutations are often detrimental, but sometimes they may create something that humans value: A Starry Night, The Raven, Nash equilibria, or perhaps even calculus. The devastation associated with these characteristics, which often involves extreme fitness loss — suicide with damage rather than benefit to kin — cannot be explained by natural selection acting solely upon humans. The principles of natural selection emphasize that we have to consider other species that live intimately within us as part of us, affecting our neurons, shaping our minds.

Paul W. Ewald is a professor of biology at Amherst College and author of Plague Time.