Andrew Shaner, Geoffrey Miller, Jim Mintz

Len Hamakado and Ngoc Tran

1% global prevalence of schizophrenia too high to be due solely to a single mutation (Wilson 1997).

Twin and family studies suggest schizophrenia may be due to many genes.

These genes may be fitness indicators involved in mutual mate choice and sexual selection.

Schizophrenia is the unattractive extreme of sexually selected fitness indicator (SSFI) genes.

SSFIs may be involved in courtship behaviorSymptoms of schizophrenia impairs courtship ability.

Results in reduced reproductive success.

Focus on sexual selection for human language and “verbal courtship behavior.”

Use verbal skills to attract a mate (humor, social and emotional sensitivity).

Humans may use behavioral and mental abilities like language, music, and humor as indicators of fitness (Miller 2000).

Behavior mediated by complex brain systems—affected by both mutations and environmental hazards.

genetic fitness refers to mutation load (high fitness = low mutation)Both prenatal and postnatal environmental conditions can affect development of schizophrenia.

A person with both high genetic fitness and environmental conditions develop a brain that is successful courtship behavior.A person with low genetic fitness and environmental conditions develop a brain that is unsuccessful in courtship behavior.

Successful verbal courtship may require ability to internally critique thoughts.Impaired internal critique may result in the disorganized speech and delusions.Human language used for both contest and courtship.

Model minds of rivals to produce more intimidating conversation.Model minds of potential mates to produce more attractive conversation.

Symptoms of schizophrenia should begin around age when courtship normally begins.Symptoms should become more severe in situations with high courtship and sexual competition.Schizophrenia shows up post-puberty (around ages 15-26).

Rare before puberty. Range of late onset ages due to variation in onset of courtship.

Predict average age of symptoms onset is correlated with average age of courtship onset (defined as age of first sexual intercourse).

Symptoms should peak in severity during age of peak mating effort.

Factors/situations that stimulate courtship and sexual competition should worsen schizophrenic symptoms.

Dopamine (DA) agonists stimulate courtship behavior in different animals and humans.

DA agonists are drugs that to bind to DA receptors and mimic effect of DADA agonists worsen schizophrenic symptomsbut schizophrenics usually have increased DA activity compared to normal

Schizophrenia should impair the ability to attract and retain mates.

Reduced rates of marriage (15-73% of normal) and reproduction (30-70% of normal)

Have almost normal number of children (not a fertility issue).

Sexual Selection Theory suggests that males will invest more time, energy, and risk in mating efforts than females.

This could explain the more frequent and severe symptoms in males

Sex differences that amplify mating effort (genetic, hormonal, neurophysiological) also amplify any abnormalities, such as Schizophrenia

The Young vs. The Old Shaner et al. suggest females prefer older males and males prefer younger females…

This means younger males and older females must try harder to attract and retain mates.Therefore the peak of mating effort is younger for males than for females; which could be an explanation of schizophrenia’s early age onset in males.

3.3 SSFIs show predictable sex differences

Fitness-sensitivityAllows SSFIs to perform main evolutionary function

Conversion of subtle genetic traits into blatant phenotypic traitsThis allows for the opposite sex to choose a high quality mate

If Schizophrenia is the unattractive extreme of an SSFI then…Neurodevelopmental abnormalities are increased to indicate poor fitness Schizophrenia is polygenic because SSFIs show high genetic variance to serve as fitness indicatorsEnvironmental hazards, pre- and post- natal, increase risk of schizophrenia because environmental hazards are one way SSFIs show “good genes”Increased mortality complicates schizophrenia because the unattractive extreme of an SSFI represents poor fitness and condition.

Balance between deleterious mutation and stabilizing selection produces an equilibrium of fitness-reducing mutations.

Thus…heritable variation may reflect individual differences in evolutionary transient fitness-reducing mutations.

Based on this reasoning Shaner et al. suggests most susceptibility alleles will be evolutionarily transient, lineage specific mutations that reduce fitness.

This reasoning explains why schizophrenia shows significant heritability in twin and adoption studies.Schizophrenia is not widely seen in the general population because the evolutionary half-life is only a few hundred generationsEvery population has a set number of fitness-reducing mutations that maintain heritability of SSFIs, and therefore schizophrenia.

Why does Schizophrenia run in families?2 reasons:

Some families have lower than average fitness (higher mutation loads)Some families have higher than average fitness (lower mutation loads)

Consider…An allele that increased the fitness-sensitivity of an SSFI

It increases the correlation between general fitness and the SSFI so that:

Low fitness individuals have much worse-than-average fitness indicatorsHigh fitness individuals have much higher-than-average fitness indicators

If the benefits of reproduction in the high-fitness individuals balance out the cost of reproduction in low-fitness individuals the allele for higher-fitness will persist in a population.

Families with high fitness sensitivity expect to see several unusual features:1. Higher variation in the SSFI and it’s neurological origin between

individuals2. Higher correlation between fitness and SSFI quality3. Higher variation in attractiveness and reproductive success

High fitness-sensitivity would be favored under conditions of more intense sexual competition; the reverse being true for low fitness-sensitivity

Schizophrenia rates should be highest in the populations that have the most intense sexual competition and where the SSFI associated with schizophrenia is most valued in mate choice.

Model for attractiveness of indicator trait and prevalence of schizophrenia

As traits evolve they become more informative fitness indicators, Therefore:

High quality SSFIs are highly sexually attractive; the opposite being true for low quality SSFIs

Shaner et al suggest:Schizophrenics receive so much stigmatization because if their low-fitness SSFI indicatorsAnti-schizophrenia biases should increase after puberty, when mating effort in peaking and should be more critical in females during ovulation (when “good genes” matter most)

Shaner et al. hypothesis explains many key features of schizophrenia across a wide range of topics.

Onset in adolescence and adulthood, greater severity in males, reduced reproductive rate , developmental abnormalities, increased reproductive success of unaffected relatives. Etc.

The hypothesis also resolves the issue: why does schizophrenia persist when it impairs survival and reproduction?

An attractive SSFI is attractive because its development is so easily disrupted by mutation or environment, therefore…

All SSFIs must contain, both, a high and low fitness extremeIf this is true then schizophrenia is that unattractive extreme which will persist as an inevitable side effect of sexual selection (most likely based in language or social cognition).