Thinking about Schizophrenia: Evolutionary Explanations

When tasked with the study of human thought and behavior, psychologists tend to adopt one, or a combination of several, perspectives. Cognitive psychologists examine conscious and subconscious processes that influence how we perceive and interact with the world around us. Social psychologists focus on our relationships with others, how they influence us and how internal factors influence them, in turn. Biological psychologists study the interface between the mind and the body to understand how behaviors and cognitions arise. Evolutionary psychologists attempt to decipher how patterns of behaviors and thoughts served an adaptive role led to increased chances of survival for our ancestors. This becomes particularly difficult when considering mental health disorders like schizophrenia. How can diseases like these be advantageous for survival when they are so debilitating?

For a trait - like a specific behavior or a mental illness - to be subject to evolution, four conditions must be met: 1) Reproduction occurs in the population. 2) The trait is heritable, or can be passed down from parent to child. 3) There is variation in the trait in the population. 4) A selective pressure exists on the trait. This means that possessing (or not possessing) the trait improves chances of survival and participating in condition number 1: reproduction. Condition #1 is certainly met, as humans have always relied on reproduction to make new humans. A long history of research has supported condition #2, with many mental illnesses shown to have high rates of heritability - schizophrenia has been found to be nearly 50% heritable. Put another way, if an identical twin is diagnosed with schizophrenia, the chances that their twin (who shares 100% of their DNA) also has schizophrenia is about 50%, even if the twins are raised separately. Further support for this idea is that, with the fairly recent advent of genome sequencing, researchers have identified genes associated with schizophrenia and other psychiatric illnesses. Condition 3 is also met, as 1% of the population suffers from schizophrenia.

Condition 4 has been the focus of most studies into the the evolutionary nature of mental illnesses like schizophrenia. Given the severe and debilitating nature of a disorder like schizophrenia, one would assume that there would be strong selection pressure against occurance of the disorder in the population. However, schizophrenia still exists. Why? It may be that schizophrenia is an extreme version of behaviors or cognitive processes that offer an adaptive advantage, similarly to how the allele that causes sickle cell anemia when two copies are present also provides resistance to malaria when only one copy is present. Recent work has provided support for this idea: that lower levels of psychotic-like traits or genetic risk may provide advantages that could explain the continued existence of psychotic disorders like schizophrenia.

Nichola Raihani and Vaughn Bell from University College London published a review on an evolutionary perspective of paranoia (2018). Although paranoia is not unique to schizophrenia, it is one of the most endorsed symptoms among schizophrenia patients. Paranoia is marked by beliefs that other people are secretly intending to cause one harm or that a conspiracy exists against you. The authors draw on theoretical and experimental literature to argue that paranoid thinking may have evolved as a sensitivity and vigilance toward social threat. From early human history through modern times, social group cohesion has served a crucial role in survival via resource gathering and distribution, as well as protection. Threats to this group cohesion potentially impacted the survival of every individual in the group. Since groups often contain(ed) related members, any threat to genetic survival was compounded. Therefore, it would be advantageous for groups or members of groups to be wary, even aggressively so, of new members or outside groups. As social groups have become more flexible and society has seen increased organization of external (including institutional) groups, extreme levels of paranoid thoughts can be targeted at a variety of people and groups, from family members to law enforcement agencies. While these extreme versions of paranoia likely impede optimal functioning, if paranoid thoughts continue to protect individuals from social threats, they will likely persist in the population.

Other hallmark symptoms of psychotic disorders are hallucinations. A recent study by Alderson-Day, Lima and colleagues examined the perceptual abilities of non-clinical voice-hearers (2017). Non-clinical voice-hearers (or NCVHs) are individuals who experience full auditory hallucinations, but may not display the other symptoms or functional difficulty associated with a psychotic disorder diagnosis. Many NCVHs appreciate their voice-hearing and do not wish to receive treatment for it. In this study, Alderson-Day, Lima, et al. recruited NCVHs and non-voice hearing controls to do a noise detection task during an MRI scan. The noise detection task involved listening to a series of “white-noise” clips, and pressing a button if they heard a distinct noise in those clips (they were trained on detecting the distinct noise prior to the scan). What the participants did not know was that half of the “white-noise” clips were actually recorded speech that had been acoustically degraded to a point that the speech is typically unintelligible at first. At the end of the task, the participants were asked if they noticed any speech sounds in the clips and if they remembered what they heard. Now that the subjects were aware of potential speech in the clips, the task was repeated.

The authors found that NCVHs were more likely than their peers to spontaneously notice the presence of speech in the degraded sound clips. Further, if they noticed speech, they noticed it earlier than the controls and were slightly more likely to recall specific words that they heard. Group differences were reflected in activation of the anterior cingulate cortex, a brain region that has been associated with a variety of functions, but seems to facilitate attentional processes, including monitoring internal and external speech and sounds. Although the paper did not offer evolutionary implications, we can see how such processes in psychosis could have contributed to fitness for our evolutionary ancestors. The authors concluded that NCVHs may have a cognitive bias towards hearing speech. Such a bias for speech or other salient sounds, generally, may have provided an advantage in detecting threats from ambiguous sounds. This bias may have also been beneficial during the early development of spoken communication and language. Maybe future research in psychology or linguistics will test these possibilities.

A third possible adaptation associated with schizophrenia and psychotic disorders is creativity. The idea that madness often accompanies genius dates back centuries, but only starting in the 20th century do we have data to back this up. The link between creativity and psychosis or psychotic-like traits is fairly robust. Schizophrenia patients perform better than controls on logic puzzles that require practical limits to be relaxed (Fink et al. 2014). Well siblings and children of individuals with schizophrenia are more likely to have successfully creative careers than individuals without familial psychosis. Further, schizotypal traits (personality traits that have overlap with several schizophrenia symptoms and are associated with increased risk of developing psychosis) are also associated with various measures of creativity from laboratory tasks of divergent thinking (e.g. coming up with nonstandard usage for objects) to artistic success.

Power and colleagues (2015) took this link between psychosis and creativity further by studying the shared genetic basis of psychosis and creativity. In a large dataset in Iceland, consisting of genetic, medical, and occupational information. They calculated a polygenic risk score for two psychotic disorders: schizophrenia and bipolar disorder. A polygenic risk score is basically a single value that describes the number of alleles associated with development of a certain condition. The authors found that both schizophrenia and bipolar disorder polygenic risk scores predicted creativity, indexed by membership in national artistic societies (for dancers, musicians, visual artists, etc.). The authors also found that polygenic risk also predicted attainment of a university degree, but predicted creative professions even when controlling for degree attainment. They also determined that the genetic effects on creativity are not confounded by having relatives with psychotic disorders.

An evolutionary perspective on psychological disorders forces researchers to study not only how the illness negatively impacts those who suffer from it, but how it may have offered advantages to our evolutionary ancestors, like protecting the group from threats. We are also able to see how genes related to disorders or non-impairing levels of symptoms can provide certain advantages even today, like enhanced speech perception or creative ability. Further research may help us understand how potentially beneficial mechanisms become pathological, but for now, we can that even the most severe and impairing disorders, like schizophrenia, can have their roots in adaptive evolutionary mechanisms.

References:

Alderson-Day, B., Lima, C. F., Evans, S., Krishnan, S., Shanmugalingam, P., Fernyhough, C., & Scott, S. K. (2017). Distinct processing of ambiguous speech in people with non-clinical auditory verbal hallucinations. Brain, 140(9), 2475-2489.

Fink, A., Benedek, M., Unterrainer, H. F., Papousek, I., & Weiss, E. M. (2014). Creativity and psychopathology: are there similar mental processes involved in creativity and in psychosis-proneness?. Frontiers in psychology, 5, 1211.

Reginsson, G. W., Ingason, A., Euesden, J., Bjornsdottir, G., Olafsson, S., Sigurdsson, E., ... & Steinberg, S. (2018). Polygenic risk scores for schizophrenia and bipolar disorder associate with addiction. Addiction biology, 23(1), 485-492.

Raihani, N. J., & Bell, V. (2018). An evolutionary perspective on paranoia. Nature Human Behaviour, 1.