Evolutionary psychology (EP) uses evolution to help understand human behaviour, personality traits, cognition, etc. I think it’s an excellent theoretical framework to understand psychology, as I see psychology as being a subset of biology, and as has been quoted to death (but needs to be repeated):
“Nothing in biology makes sense except in the light of evolution”
- Theodosius Dobzhansky
The point of this post is that EP has its own explanations for several mental illnesses that suggest mental illness could be a good thing, or at least not necessarily a bad thing and/or are evolutionary adaptive for a variety of reasons. For example:
Major Depressive Disorder
EP has proposed many different explanations for depression, such as:
- Avoiding sources of stress because current stressors are exceeding the capacity to cope (psychic pain or distress as a motivator)
- Rumination to facilitate problem solving towards long term goals, purpose and meaning
- Signalling suffering/unmet needs to others (cry for help)
- Avoiding others to prevent spread of infection
- To lower the demeanour of animals who have lost
rank to minimise harm
Feeling ‘depressed’ is not the same as major depressive disorder (MDD). Feeling depressed may be adaptive, but MDD is not. While MDD can be caused by infection and stress, EP can’t explain the following:
- How things like LCO3s, zinc and exercise are therapeutic
- Low serum BDNF in people with MDD
- Evidence supporting the role of MDD as an inflammatory disease
- How LPS can trigger MDD symptoms in those susceptible
- Immune dysfunction being associated with MDD (high CD4+:CD8+ ratio and simultaneous inflammation and immune suppression)
- Glucocorticoid resistance
Some of the explanations don’t add up:
- People with MDD are not in an ideal position to problem solve as some of the symptoms of MDD include logical fallacies (see part 1, 2 and 3)*
- Having MDD for months/years is unnecessarily
long when yielding rank quickly is sufficient to minimise harm
On top of that, the two strongest bits of evidence against EP’s explanation for MDD are hippocampal atrophy (suggestive of pathology and why would that be necessary, adaptive or good?) and suicide.
* Many of the logical fallacies could perpetuate MDD. I suspect that for many people a significant portion of time in cognitive behavioural therapy is spent dismantling them.
Postpartum depression affects 15-20% of women during the first year after giving birth . The EP has two main explanations for postpartum depression (PPD):
- PPD informs the mother that she has undergone a fitness loss in having a child and so it’s a cry for help and/or a driver to focus on the mother’s wellbeing
- To reduce investment in offspring who would have
been unlikely to survive to adulthood (mainly due to a lack of support from
others, not because the child was deformed).
Social support predicts PPD and is very therapeutic for it, which supports the second point. Though just like MDD there are biological factors that haven’t been accounted for:
- LCO3s increase BDNF , are anti-inflammatory and are therapeutic for MDD . Low plasma levels of DHA are associated with more depressive symptoms . PPD may be due to low LCO3s as they in high demand during early development (potentially depleting the mother) and PPD is associated with low fish intake and low DHA levels 
- During pregnancy pro-inflammatory cytokines are lower and the mother is immune suppressed to inhibit potential immune responses against the foetus*. Very shortly after delivery the mother switches to a pro-inflammatory state to deal with the aftermath of childbirth. While the PP period is pro-inflammatory, women with PPD have 3 times more IL-6, which may be caused by glucocorticoid dysregulation (not producing enough or resistance) 
- Women who have anti-thyroid antibodies are more likely to get PPD. Among women with anti-thyroid antibodies administration of T4 reduced PPD from 50.6% to 17.2% 
- A low selenium diet can cause mood disturbance  and selenium decreases with pregnancy . Women with low intakes of selenium (<8.9µg) were 2.95 times more likely to experience post-partum depression  and each 10µg of supplemental selenium reduces the risk of PPD by 24% 
- Lower serum zinc (but not magnesium) is
associated with PDD  and combined
treatment of zinc, magnesium and vitamin B1 improved depressive symptoms in
postpartum period in an animal model of PPD . Other minerals may also have a role 
The biological factors would somewhat agree with EP in that the mother has undergone a fitness loss and perhaps the child might be less likely to survive due to malnourishment, but the degree of fitness loss of the mother and malnourishment of the infant or the mother isn’t supposed to happen.
EP has explanations for other mental illness as well, I’m aware of EP’s explanations for bipolar disorder (BD) and schizophrenia*, which suggests the two mental illness could be/have been beneficial because they enhance creativity  and also schizophrenia because it promotes hypervigilance of danger.
However, BD and schizophrenia are associated with lower brain volumes in several brain structures , people with BD  and schizophrenia  have a high suicide rate, even by mental illness standards** and people with schizophrenia have fewer children  .
As for the underlying pathologies I’ll leave that to Emily Deans, a psychiatrist who has done a lot of research on the biological causes of mental illness.
* Most people think only of the psychotic symptoms (hallucinations and delusions) of schizophrenia, which are called ‘positive symptoms’, but more common and more debilitating are the ‘negative symptoms’, which are similar to MDD and don’t respond well to the standard anti-psychotic medications.
** At least 25-50% of people with BD have attempted suicide at least once  and people with BD have a suicide rate of about 1% per annum, compared with 0.015% in the general population. (People with BD are also more likely to have successful suicide attempts (1/3 vs 1/30) . In other words BD causes a lot of suffering that no one should have to experience.
Not all of EP’s proposed explanations for mental illness are evolutionary adaptive, some are psychologically adaptive, but all share the view that ‘mental illness could be a good thing, or at least not necessarily a bad thing’, which I think is a wrong and dangerous approach. One slippery slope that an evolutionary psychologist could take is to block the treatment or not make it as effective in order for the ‘adaptive’ elements of MDD (for example) to manifest.
“Finally, I disagree that psychotherapists should "seek to alleviate their [patients'] suffering, adaptive or not." By that logic, doctors should always alleviate physical pain, adaptive or not. Yet physical pain often helps prevent further injury and promotes healing—if you have an injured limb, refrain from using it! Similarly, sadness is almost certainly a healthy response to adversity that may limit losses and promote healing.” 
Evolutionary psychologists often seem to confuse normal human emotions such as depression and anxiety with mental illness such as MDD and anxiety disorders. Depression and anxiety have a function, MDD and anxiety disorders are dysfunctional and are often caused by an underlying pathology (hippocampal atrophy for example).
You’ve got to wonder whether the evolutionary psychologists have had first hand or second hand experience with mental illness. Because in addition to impairing function and causing suffering, mental illness is inherently irrational and maladaptive. I think the only benefit of mental illness is just improving your understanding of them, allowing you to have more compassion for those who are afflicted by them.