Note: this is just a theory, developed in reaction to recent conversations.
As we were discussing Friday, one form of female sociopathy (at least relevant to this conversation) likely involves manipulating or coercing others into providing resources for her children.
There are a couple of obvious tropes:
- The evil stepmother, who shunts resources away from a man’s first child, toward his later children.
- The cuckoldress, who tricks or convinces a man to care for another man’s children (this is not always seen as evil, since the male drive to provide for children is triggered at least partly by their proximity, since men cannot give birth, and thus men feel genuine affection for children who happen to be around them,)
- The crazy ex, who sues a man for all he is worth, doing her best to prevent him from being able to provide for any future children.
How crazy are women?
22%–slightly more than 1 in 5–women have been diagnosed with a mental illness, at least according to all of the data I’ve seen. Since mental illness peaks during the childbearing ages and falls off quickly after menopause, we can also assume that this rate is closer to 1 in 4 during these years.
(The dramatic problems our Native American communities are facing is a separate matter, deserving of its own post.)
The odd thing about this data is that mental illness rates are higher for women than men, despite the fact that mental retardation and mental disability rates are higher for men than women. Men are more likely than women to have serious conditions like non-verbal autism and schizophrenia, more likely to be homeless or commit suicide. When things go terribly wrong, the sufferers are disproportionately male (an unfortunate side effect of the Y chromosome causing greater male variability than female variability on a variety of traits.)
So why on earth do more women than men suffer from mental illness?
Perhaps some forms of mental illness confer some unexpected benefits on women.
Many (perhaps most) “mental illnesses” correlate with a single personality trait–neuroticism:
“Previously we thought that mental illnesses such as depression, schizophrenia, bipolar disorder, and substance abuse, were completely separate diseases,” Ystrøm says.
But research has now shown that these illnesses are often linked. If you suffer from one mental illness, you are more likely to develop another. And if someone in your immediate family has a psychiatric illness, your risk increases not only for this disorder, but for all other disorders.
These findings have led researchers to suspect that there could be a common underlying factor that increases an idividual’s risk of mental illness, overall. …
Ystrøm and colleagues have used new statistical methods to look for patterns in personality, mental disorders, genes, and environmental factors, among the twins in the Twin Register.
And the answer to the question the researchers asked is: yes, neuroticism seems to be the personality trait that best describes the risk of all mental disorders. …
“This one trait doesn’t explain everything. Anyone can develop a mental illness…”
And in women, neuroticism correlates with… more surviving offspring (in at least one study):
Taking an evolutionary approach, we use data from a contemporary polygynous high-fertility human population living in rural Senegal to investigate whether personality dimensions are associated with key life-history traits in humans, i.e., quantity and quality of offspring. We show that personality dimensions predict reproductive success differently in men and women in such societies and, in women, are associated with a trade-off between offspring quantity and quality. In women, neuroticism positively predicts the number of children, both between and within polygynous families. Furthermore, within the low social class, offspring quality (i.e., child nutritional status) decreases with a woman’s neuroticism, indicating a reproductive trade-off between offspring quantity and quality.
What is neuroticism, in the Big 5 Personality Traits* sense?
*Note: I am not endorsing or denying all five traits one way or another.
It’s worrying. Mothers who worry more about their offspring have more offspring–though it’s quite easy to imagine that the causality points in the opposite direction as the study’s authors conclude–poor women with lots of skinny babies have more reason to worry about their children than women with a few fat babies.
When are women most likely to experience mental illness?
Immediately after the birth of a child. It’s called post-partum depression, and it can be very bad–one woman in my moms’ group ended up in the mental hospital after developing post-partum psychosis. Andrea Yates famously drowned her five children during a bout of post-partum depression/psychosis.
Why on earth would women develop a debilitating mental illness at the most vulnerable time in their offspring’s life? Wouldn’t natural selection select rather quickly against anything that makes women worse at taking care of their offspring?
Let’s turn to everyone’s favorite genetic disease, sickle cell anemia. SCA is famous for being a relatively simple genetic mutation of the sort where if you have one copy of the sickle cell gene, you are less likely to get malaria, and if you have two copies, you tend to die. In areas where malaria is common, the cost of having a quarter of your children die from SCA is lower than the cost of loosing them to malaria.
Personality traits, including neuroticism, generally exist on a continuum. People may become more neurotic when life warrants it, and less neurotic when they don’t need to worry. A mother with a new baby is in a very vulnerable state–she has just lost a good deal of blood, may not be able to walk, and has an infant to care for every other hour, day and night. It is not a normal state by any measure. It is a time when being extra attentive and extra aware of threats and predators is in a woman’s interest.
It is also a time when women are most in need of help from their mates, relatives, or other friends. Increased neuroticism may also prompt others to attend more closely to the new mother, helping her out. . Increased neuroticism may be so helpful during this time period that a few women getting way too much neuroticism and becoming extremely depressed or even killing their children is a cost outweighed by the increased survival of babies whose mothers had moderate levels of neuroticism.
Let us note that nature doesn’t care about your feelings. Male praying mantises who allow themselves be eaten by their mates have more offspring than the ones who don’t, but that doesn’t mean male praying mantises enjoy getting eaten. Children who die of sickle
cell anemia don’t much appreciate that their siblings were protected from malaria, either.
An increase in neuroticism immediately after the birth of a baby may prompt a mother to take better care of it, but that doesn’t mean she enjoys the neuroticism. Neither does it mean that post-partum depression is healthy, any more than sickle cell anemia is healthy just because it’s a side effect of a trait that helps people avoid malaria.
But wait, I have more studies!
The persistence of common, heritable psychiatric disorders that reduce reproductive fitness is an evolutionary paradox. Here, we investigate the selection pressures on sequence variants that predispose to schizophrenia, autism, bipolar disorder, major depression and attention deficit hyperactivity disorder (ADHD) using genomic data from 150,656 Icelanders, excluding those diagnosed with these psychiatric diseases. … Higher polygenic risk of autism is associated with fewer children and older age at first child whereas higher polygenic risk of ADHD is associated with having more children. We find no evidence for a selective advantage of a high polygenic risk of schizophrenia or bipolar disorder. Rare copy-number variants conferring moderate to high risk of psychiatric illness are associated with having fewer children and are under stronger negative selection pressure than common sequence variants. …
In summary, our results show that common sequence variants conferring risk of autism and ADHD are currently under weak selection in the general population of Iceland. However, rare CNVs that also impact cognition are under stronger selection pressure, consistent with mutation-selection balance. The hypothesis that a selective advantage accounts for the prevalence of sequence variants conferring risk of schizophrenia and bipolar disorder is unproven, but rather this empirical evidence suggests that common sequence variants largely escape selection as their individual effect sizes are weak.
Unfortunately, this study mostly looks at the data in aggregate, instead of breaking it down by males and females. (And I don’t know why they would bother excluding people who actually have the conditions they are trying to study, but perhaps it doesn’t make much difference.)
Thankfully, they did break down the data by male/female in the tables–Table 1 and Table 2. These tables are confusing, but the takeaway is that mental illness has a bigger effect on male fertility than female fertility.
Results Except for women with depression, affected patients had significantly fewer children (FR range for those with psychiatric disorder, 0.23-0.93; P < 10−10). This reduction was consistently greater among men than women, suggesting that male fitness was particularly sensitive. Although sisters of patients with schizophrenia and bipolar disorder had increased fecundity (FR range, 1.02-1.03; P < .01), this was too small on its own to counterbalance the reduced fitness of affected patients. Brothers of patients with schizophrenia and autism showed reduced fecundity (FR range, 0.94-0.97; P < .001). Siblings of patients with depression and substance abuse had significantly increased fecundity (FR range, 1.01-1.05; P < 10−10). In the case of depression, this more than compensated for the lower fecundity of affected individuals.
Conclusions Our results suggest that strong selection exists against schizophrenia, autism, and anorexia nervosa and that these variants may be maintained by new mutations or an as-yet unknown mechanism. Bipolar disorder did not seem to be under strong negative selection. Vulnerability to depression, and perhaps substance abuse, may be preserved by balancing selection, suggesting the involvement of common genetic variants in ways that depend on other genes and on environment.
Now, this study gets interesting in its graphs:
In every case, mental illness has a bigger effect on male fertility than female–and in the case of depression, it has no effect on female fertility.
This graph is confusingly labeled, but it is breaking down the correlation on the brothers and sisters of people with mental disorders. So the first dot represents the brothers of people with schizophrenia; the second dot represents the sisters of people with schizophrenia.
None of these effects are huge, and some of them changed when “comorbidities were included in the analysis,” though it’s not clear exactly what that means–the word comorbidity in this context refers to people with more than one diagnosis.
For the objectives of this study, we first analyzed each disorder separately without accounting for comorbidities. A secondary analysis was then performed that corrected for comorbidities by analyzing all disorders simultaneously.
So when you analyze all of the disorders together, sisters of schizophrenics had no increased fertility, and neither did the siblings of people with bipolar. Depressed men had average fertility, while depressed women actually had slightly above average fertility. The results for anorexia, substance abuse, and autism didn’t change.
Personality variation is increasingly thought to have an adaptive function. This is less clear for personality disorders (PDs)—extreme variants of personality that cause harm in most aspects of life. However, the possibility that PDs may be maintained in the population because of their advantages for fitness has been not convincingly tested. In a sample of 959 outpatients, we examined whether, and how, sexual selection acts on the seven main dimensions of personality pathology, taking into account mating success, reproductive success, and the mediating role of status. We find that, to varying extents, all personality dimensions are under sexual selection. Far from being predominantly purifying, selective forces push traits in diverging, often pathological, directions. These pressures differ moderately between the sexes. Sexual selection largely acts in males through the acquisition of wealth, and through the duration (rather than the number) of mates. This gives a reproductive advantage to males high in persistence–compulsivity. Conversely, because of the decoupling between the number of mates and offspring, the promiscuous strategy of psychopaths is not so successful. Negative emotionality, the most clinically detrimental trait, is slightly deleterious in males but is positively selected in females, which can help to preserve variation.
It’s interesting that the invention of birth control may have inadvertently selected against promiscuous psychopaths–rather similar to the theory that abortion is responsible for the decrease in crime since the early 90s.
“Negative emotionality” is likely equivalent to “neuroticism.”
There are two obvious reasons why mental illness might have more of an effect on males than females–one is that mental illness might simply be mores severe for males than females, on average. The second is that mental illness interferes more with holding down a job than with being a housewife, so women with mental illnesses have more options than men.
Less obvious, though, is that some of these traits might actually be beneficial–in small quantities–for women.
That’s enough for now; let’s continue this discussion on Friday. (Wednesday is book club.)