A meta-analysis of 16 studies revealed no differences in the rates of non right-handedness between community individuals who scored high and low in psychopathy, psychopathic and non-psychopathic offenders, and psychopathic and non-psychopathic mental health patients, partially supporting the adaptive strategy model of psychopathy. This research was published in Evolutionary Psychology.
Psychopathy is characterised by “antisocial, impulsive, manipulative, and callous behavior” and was long considered a mental disorder. Today, many of the defining features of psychopathy fall under the diagnostic criteria of Antisocial Personality Disorder in the DSM-5. Despite a widespread belief among the scientific community that psychopathy is a mental disorder, “an alternative, evolution-minded perspective has been proposed: that psychopathy is instead a life history strategy of social exploitation maintained by negative frequency-dependent selection,” write Lesleigh E. Pullman and colleagues.
The evolutionary view of psychopathy posits that “the risk taking, opportunistic, and callous behavior” characteristic of psychopaths would have increased reproductive success in ancestral environments. These traits would emerge when they are expected to promote fitness. For example, under conditions of a high ratio of cooperators to psychopaths, psychopathic individuals could exploit trusting and cooperative others to enhance their reproductive opportunities. Psychopathic traits in such conditions would have been favoured by selection.
A testable hypothesis regarding this debate is the extent to which psychopathic (vs. non psychopathic) individuals exhibit signs of neurodevelopmental perturbations. One indirect measure of such perturbations is handedness, as non right-handedness signals neurodevelopmental problems. Thus, in this meta-analysis, the researchers focused on handedness as a proxy of neurodevelopmental perturbations. Non right-handedness has been associated with low birth weight, birth complications, prenatal stress, and prenatal exposure to hormones, suggesting it may be associated with disruption to pre- and perinatal brain development of critical brain structures. For example, compared to healthy control groups, individuals with schizophrenia and depression are more likely to be left-handed.
In a recent meta-analysis of over 2 million people, the researchers found that approximately 11% of the population was left-handed. The bias for right-handedness is believed to stem from selection pressures throughout our evolutionary history. There are both structural and functional asymmetries in the human body, and brain lateralization (i.e., “neural functions being more dominant in different hemispheres of the brain”) is one such asymmetry. Given the brain’s left hemisphere controls the right side of the body, “right-hand dominance may be a byproduct of genetically determined functional asymmetry of brain lateralization for left hemisphere language processing and/or fine motor skills.”
This meta-analysis only included studies that: were conducted in English, included an identifiable sample of largely adult participants, had at least 10 participants in each group, used a validated measure of psychopathy (but not antisocial personality disorder), measured handedness, and contained enough statistical information for the calculation of effect size. “Furthermore, non right-handedness was defined as left-handedness, mixed-handedness, or ambidexterity, measured with self-reported hand preference, hand preference when writing, or validated handedness inventories.” The meta-analysis included 16 studies from 25 individual reports published between 1985 to 2017, with an overall total of 1818 participants.
Pullman and colleagues did not find support for the mental disorder model of psychopathy, with the meta-analysis providing partial support for the adaptive life history model of psychopathy. There were no differences in the rate of non right-handedness between community participants scoring high (vs. low) in psychopathy. And while there was no difference in rates of non right-handedness between psychopathic (vs. non psychopathic) offenders, there was a tendency for those with higher scores on the interpersonal/affective dimension of psychopathy to have lower rates of non right-handedness, while those scoring higher on the behavioural dimension had higher rates of non right-handedness. The behavioural dimension of psychopathy may be “conceptually more similar to [antisocial personality disorder] and life-course-persistent offending.” Lastly, there were no differences in rates of non right-handedness between psychopathic (vs. non psychopathic) mental health patients.
The authors note several limitations. There were a small number of primary studies that examined the relationship between psychopathy and handedness; as such, low statistical power could prevent the discovery of any group differences that exist. As well, the authors were unable to address the confounding variable of comorbidity with psychopathy; it is possible for psychopathic individuals to have other mental illnesses (undetermined in this work), which could confound the results. Interestingly, the rate of mental illness comorbidity with psychopathy tends to be lower than other mental disorders, which is consistent with the adaptation perspective. Lastly, the included samples were partially or completely composed of women; however, the adaptive model of psychopathy pertains to men and may not extend to women. Thus, future work should examine the relationship between psychopathy and handedness in men specifically.
The study, “Is Psychopathy a Mental Disorder or an Adaptation? Evidence From a Meta-Analysis of the Association Between Psychopathy and Handedness”, was authored by Lesleigh E. Pullman, Nabhan Refaie, Martin L. Lalumière, and DB Krupp.