Psychosis is the price we pay for being what we are. And how unfair, how bitterly
unfair it is that the price is not shared around but paid by one man in a hundred
for the other ninety-nine.
(Sebastian Faulks, Human Traces)1
Hallucinatory experiences are as old as humankind and until the
nineteenth century these experiences were generally attributed to
mystical or divine sources such as gods or demons, whereas in
modern times, hallucinations have generally been regarded as
pathological and as signs of illness.2Recently, however, our ideas
about hallucinations and delusions are being challenged by
findings from epidemiology. A new viewpoint is emerging. Simply
put, hallucinations and delusions are more common than we
think. Population-based studies using both self-report and
interview surveys show that the prevalence of psychotic symptoms
is far greater than had been previously considered, with a recent
meta-analysis suggesting a prevalence rate of 5–8% in the general
population, which is about ten times higher than the prevalence of
symptoms may be even higher among young people. Ten years
ago, Poulton et al4reported that 14% of 11-year-old children in
the Dunedin birth cohort reported psychotic symptoms on
interview and showed that these symptoms were associated with
a 5- to 16-fold increased rate of psychotic illness in early
adulthood, depending on the strength of the initial symptoms.
Since then, large, population-based studies surveying psychotic
symptoms among adolescents have found rates of 9–14% in
interview-based studies,5,6and greater than 25% in some studies
using self-report questionnaires.7–9Positive answers on self-report
questionnaires have been validated on clinical interview.9
Therefore it is becoming increasingly clear that a sizable minority
of young people experience psychotic symptoms.
This high prevalence of non-clinical psychotic symptoms in the
population prompts us to re-evaluate these symptoms in the light
of evolutionary theory.10In this editorial we discuss how this
non-clinical phenotype (i.e. psychotic symptoms) may hold the key
to understanding the persistence of psychosis in the population and
provide a new perspective on aetiology and treatment.
Evolutionary theories of psychosis
Psychosis is highly heritable and exerts strong negative fitness
maintains a relatively stable prevalence worldwide. Several theories
drawing on the Darwinian paradigm of selective advantage have
been formulated to explain the persistence of psychosis in the
human population. Crow’s ‘speciation’ hypothesis argues that
psychosis is the ‘price that Homo sapiens pays’ for development
of language.11Burns proposed that schizophrenia is a ‘costly
by-product’ in the evolution of complex social cognition12and
Nesse developed this idea further in terms of ‘cliff-edge’ fitness,13
whereby certain traits may increase fitness up to a critical
threshold, but beyond this point, fitness falls precipitously. For
instance, strong tendencies to use metarepresentation and theory
of mind can increase the ability to predict other people’s
behaviours and discern their intentions, but, as Nesse explains,
‘it is only one step further, over the cliff’s edge of psychotic
cognition . . . to finding secret meanings and evidence for
conspiracies in other people’s most casual gesture’.13Recently,
Dodgson & Gordon have proposed that certain types of
hallucinations could be viewed as evolutionary by-products of a
cognitive system designed to detect threat since, from a survival
perspective, it is much worse to fail to recognise a threat such as
the sound of an approaching predator than to mistakenly believe
that a predator is approaching when it is not.14Evolution may
therefore favour a selective skew towards propagation of genes
that promote false positives over false negatives, thus resulting
in ‘hypervigilance hallucinations’ in the population.14In social
or pack animals, such as humans, hypervigilance is not necessary
for every member, but the presence of this trait in at least some
members stands to benefit the entire group. Such a trade-off allows
the persistence of advantageous traits even in the presence of
increased risk of disorder.
Implications for genetic studies
The possibility that vulnerability to psychosis may be a by-product
of ‘normal’ human brain evolution, may offer an explanation for
Psychotic symptoms in the general
population – an evolutionary perspective
Ian Kelleher, Jack A. Jenner and Mary Cannon
Our ideas about the intrinsically pathological nature of
hallucinations and delusions are being challenged by findings
from epidemiology, neuroimaging and clinical research.
Population-based studies using both self-report and interview
surveys show that the prevalence of psychotic symptoms is
far greater than had been previously considered, prompting
us to re-evaluate these psychotic symptoms and their
meaning in an evolutionary context. This non-clinical
phenotype may hold the key to understanding the
persistence of psychosis in the population. From a
neuroscientific point of view, detailed investigation of the
non-clinical psychosis phenotype should provide novel leads
for research into the aetiology, nosology and treatment of
Declaration of interest
J.A.J. is director of Jenner Consultants, Haren, The
Netherlands, which provides advice and training on
psychological treatment of psychosis.
The British Journal of Psychiatry (2010)
197, 167–169. doi: 10.1192/bjp.bp.109.076018
Ian Kelleher (pictured) is a postgraduate student in the Department of
Psychiatry at the Royal College of Surgeons in Ireland, Dublin. Jack A. Jenner
is emeritus associate professor in the Department of Psychiatry at the
University Medical Center Groningen and the Mental Health Care Foundation
in Friesland, The Netherlands. Mary Cannon is an associate professor in the
Department of Psychiatry at the Royal College of Surgeons in Ireland and a
consultant psychiatrist in the Department of Psychiatry, Beaumont Hospital in
the difficulty in locating a genetic ‘point of rarity’ between
individuals with schizophrenia and controls.15,16We would also
suggest that the limited success in findings in schizophrenia to
date may be a result of shared genetic variation between the
clinical (disease) phenotype and the non-clinical (symptom)
phenotype. Genetic research thus far has used the simple
dichotomy of people with psychotic disorder compared with the
general population. However, the relatively high prevalence of
the non-clinical psychosis phenotype in the general population
may have masked genes of importance. Rather than searching
for genetic variation only between individuals with psychotic
disorders and the rest of the population (which contains, as we
have discussed, significant numbers of individuals with the non-
clinical phenotype), it may prove more fruitful to investigate
genetic variation among individuals with the non-clinical
phenotype in the general population (plus or minus individuals
with the disease phenotype) compared with the individuals
without such symptoms. Furthermore, an understanding of these
genes and their functions may hold the key to explaining the
persistence of psychosis in the population. It is possible that there
are evolutionary advantages associated with genes that contribute
to the non-clinical phenotype but which, in a fitness trade-off,
also increase the risk for psychotic disorder. If, for example, as
suggested by cliff-edge fitness theory,10psychosis results from
the development of certain useful traits beyond an optimal peak,
we might find that people with the non-clinical psychosis
phenotype demonstrate advantageous traits compared with the
rest of the population in, for example, certain language, cognitive
or metacognitive skills. Detailed investigation of the non-clinical
psychosis phenotype might, therefore, allow us to uncover a
Darwinian explanation for the persistence of psychosis genes in
the general population that has not been possible by looking only
at psychotic disorder.
Implications for diagnosis and treatment
The current diagnostic nosological systems are undergoing
revision. Evolutionary theories lend support for the idea of a
continuum approach to the diagnosis of psychosis15and provide
some clues as to why the traditional Kraepelinian or categorical
approaches to psychosis are beginning to prove problematic.16
Breaking down the categorical barriers may also help reduce the
stigma associated with psychosis, which remains one of the
rehabilitation. Taking an evolutionary perspective may also lead
to changes in how we think about treatment – for instance the
idea of schizophrenia as a disorder of social brain development
points to the importance of family therapy and social skills
training. Some therapies such as hallucination-focused integrative
therapy already incorporate such elements alongside more
traditional cognitive–behavioural therapy approaches.17The idea
of hallucinations as a ‘hypervigilance’ reaction also suggests the
value of engaging directly with the content of the symptoms in
psychosis and acknowledging the fact that some individuals may
wish to preserve positive or ‘useful’ voices that they feel support
or help them.17
the way ofrecoveryand
Broadening the evolutionary perspective
An evolutionary approach may also prove fruitful in other areas of
psychiatry. Higher than expected prevalences have been reported
for depression and anxiety. Prospective studies are showing that
up to half (and perhaps even a majority) of the population can
expect one or more episodes of depression over their lifetime.18,19
Taking an evolutionary perspective, one could speculate that the
tendency for unhappiness and dissatisfaction is intrinsic to the
human species (man as the ‘unhappy ape’) and this is one of
the features that has been a contributory factor in the success of
our species, driving Homo sapiens onwards, even to the surface
of the moon, in search of new terrain and challenges. Applying
evolutionary theories to depression may also lead to new
approaches to treatment and novel aetiological theories.10
Charles Darwin’s anniversary year has ended but that does not
mean we should put the theory of evolution back on the shelf
and dust it off only when the next anniversary comes round. As
Theodosius Dobzhansky famously stated, ‘Nothing in biology
makes sense except in the light of evolution’.20From a neuro-
scientific point of view, detailed investigation of the non-clinical
psychosis phenotype should provide novel leads for research into
the aetiology, nosology and treatment of psychosis and other
Ian Kelleher, MSc, Department of Psychiatry, Royal College of Surgeons in Ireland,
Dublin, Ireland; Jack A. Jenner, MD, PhD, University Medical Center Groningen,
Groningen and the Mental Health Care Foundation, Friesland, The Netherlands;
Mary Cannon, MD, PhD, Department of Psychiatry, Royal College of Surgeons in
Ireland and Department of Psychiatry, Beaumont Hospital, Dublin
Correspondence: Mary Cannon, Department of Psychiatry, Royal College of
Surgeons in Ireland, Education and Research Centre, Beaumont Hospital, Dublin
9, Ireland. Email: firstname.lastname@example.org
First received 24 Nov 2009, final revision 8 Apr 2009, accepted 27 Apr 2010
M.C. is supported by a Health Research Board (Ireland) Clinician Scientist Award and a
NARSAD Independent Investigator Award and is a member of the European Network of
National Schizophrenia Networks Studying Gene–Environment Interactions (EU-GEI) funded
We thank Alison Yung for suggesting the quotation from Human Traces by Sebastian
Faulks, S. Human Traces. Vintage Books, 2006.
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Psychotic symptoms in the general population
Alice Hatter (b. 1979)
Alice Hatter decided to leave her medical studies after developing a bipolar illness. This decision was not made because she felt
that her illness prevented her from continuing or coping with her studies, but because her outlook on life changed afterwards.
Being able to express herself and her experiences through the medium of art has been a great help to her while trying to combat
her illness. She had always enjoyed the arts when growing up and hopes to develop this area of her life further. Alice’s interest
lies in the link between creativity and mental illness, in particular affective disorders, and she hopes in the future to have the
opportunity to look more closely at this area of mental health.
This picture represents Alice’s idea and experience of a bipolar illness. One side is blue and the other is pink, representing
depression and mania, respectively. The colours get deeper as you go to the centre where the core appears black – but it is
actually black on the depression side and very dark purple on the mania side. The centre is open to interpretation – the red
can either represent ‘rage’ or ‘anger’ which is at the centre of the illness but gets buried under the other emotions; or it could
be the rawness of the person when the ‘end point’ of mania or depression is reached; it can be like falling into the depths and
being exposed and naked. Around the top and bottom edges of the picture the colours merge into one another to give a turquoise
colour eventually which Alice feels represents the ‘normal’ mood state, but the fact that all the colours merge like a spectrum is still
preserved as, like the illness, it is a continuum rather than specific discrete entities.
Edited by Allan Beveridge.
The British Journal of Psychiatry (2010)
197, 169. doi: 10.1192/bjp.197.3.169