Schizophrenia Is Costly, So Why Didn't Evolution Remove It?

"Given the depth of these symptoms on psychological processes, such as forming and maintaining relationships, a simple evolutionary view would likely have predicted that schizophrenia "should" have been de-selected from the human gene pool. Studies have suggested the (genetic) fitness loss that results from schizophrenia can range from 20 to 70 percent. The ongoing stable global prevalence of schizophrenia is an example of an evolutionary paradox."

"This model was first used by Mountford (1968) to understand litter sizes; the fitness advantage of having a large litter was outweighed by the severe cost of losing a large litter during a difficult year. Accordingly, the optimal strategy spread across the population, and over the years, would be to have a small litter size. It has since been used to explain other evolutionary paradoxes, such as the "obstetrical dilemma" and certain autoimmune disorders."

Schizophrenia affects roughly 23 million people worldwide, producing severe impairments in social, educational, and occupational functioning and symptoms such as delusions, hallucinations, restricted speech, and flattened affect. Genetic analyses estimate fitness loss among affected individuals of roughly 20–70 percent, creating an evolutionary paradox given stable prevalence. A polygenic explanation posits that schizophrenia represents an extreme expression of adaptive polygenic variation, where multiple genes collectively influence traits and some carrier expressions confer advantages. The cliff-edge model describes how incremental genetic effects can increase fitness up to a threshold beyond which fitness plunges, explaining persistence of risk alleles despite severe outcomes when the threshold is crossed.

Read at Psychology Today