After almost exactly six years since the initial idea for 'The Specialised Mind' dawned on me, after writing a full book draft, working alone for years, beginning my PhD, doing essentially nothing other than this (aside from some odd jobs and the occasional holiday), I'm extremely happy to announce that my first paper derived from the book manuscript has been published, open access! The title is Specialised minds: extending adaptive explanations of personality to the evolution of psychopathology. Find it here. A lay summary is below in this post.
For video content, a 1hr15min webinar of me presenting the paper to the Irish College of Psychiatrists is here: https://youtu.be/hx_pnEUpNmY and a 30min highlights video is on EPSIGUK: https://youtu.be/RAT9w6njQiU
Thanks go especially to Adrian Jaeggi, my supervisor and the senior author on the paper, Lou Barrett, who was a wonderfully supportive editor, and to the two peer reviewers, whose comments were excellent and insightful; and of course to the human ecology lab and many others who’ve supported my work.
Evolutionary theory has long been used to explain species and sex differences, but individual differences in cognition and behaviour have mostly been left out of the picture. Evolutionary psychologists have even claimed that personality differences are neutral or maladaptive variation away from an optimal personality type – nothing more than ‘noise’. In our paper “Specialised minds: extending adaptive explanations of personality to the evolution of psychopathology” we propose that various individual differences in cognitive traits stem from an adaptive process of specialization, and that dimensions of personality and psychopathology can be partially explained by this evolutionary process.
Traits of personality and psychopathology show many characteristics expected in adaptations: early onset, moderate heritability, plastic response to environment, no clear pathological cause and significant effects on life outcomes, including reproductive success. The paper begins by reviewing various areas of key evidence needed to build an evolutionary account. We start with theoretical models and non-human research indicating when cognitive differences can be adaptive. Key evolutionary forces are social selection (the effects of social partners on a trait’s fitness), social niche specialisation (populations incorporate individual roles) and negative frequency dependency (traits benefit from being rare).
We go on to consider human ancestral social structure and certain aspects of human psychology that are fertile ground for these forces: humans notice individual differences and can reward and punish group members. Status is allocated to helpful members of the group. Niches in hunter-gatherer societies could be explicit (e.g. in healers) or implicit (e.g. personality types which ‘fit in’). Specialist niches exist for social roles, specific crafts or performance or art. Co-operators in warfare, punishment and generosity are noted. Hunter-gatherer life is spent in bands of approximately 28 individuals, and those bands regularly interact with larger groups of about 165. This becomes important later, as these group sizes are the framing within which human cognition – and potential specialization – evolved.
We then consider the ‘Big Five’ personality traits. Past authors have speculated that the Big Five could be maintained as specialisations by the various evolutionary forces mentioned. There are various plausible benefits and costs within these personality dimensions. If no single personality type is optimal, balancing selection and adaptive developmental plasticity could maintain the individual variation and explain why our personalities differ so widely.
Table 1. Plausible benefits and costs of the Big Five, derived from Nettle (2006, 2011)
In contrast to personality, traits of ‘psychopathology’ are medicalized and analysed by different disciplines with very different methodology – despite sharing essentially identical empirical characteristics. We suggest this differentiation is empirically unjustified, but is a social product of the need for delineation of certain traits which require medical attention.
Figure 1. Traits of personality and psychopathology traits share basic characteristics but are studied differently.
Various evolutionary hypotheses have tried to explain psychopathological traits. Often these accounts are less grounded in evolutionary theory than the evolution of personality literature, and concentrate more on the associated cost and benefit profiles. Considering alternative accounts for five traits, there is less consensus between authors around ADHD, bipolar and schizophrenia, but more for autism and psychopathy. Autism is broadly depicted as a specialized trait enhancing systemizing at a cost of socializing, psychopathy as a strategy of callousness and unemotionality. Both psychopathy and autism show costs and benefits which are easily imaginable (and sometimes visible) in modern society, and they are almost paradigmatic cheating/cooperating specializing strategies.
Table 2. Benefits, costs and models in a selection of evolutionary accounts for psychopathological traits
We suggest that a general process of specialization has led to individual differences in traits of both personality and psychopathology – these are meaningful differences, not noise. The tendency to concentrate on extremes of dimensions in psychopathology has distracted us from recognizing subclinical spectrums where adaptation is more obvious. It’s also important to note that heterogeneity within trait labels needs to be parsed out – adaptive, neutral and maladaptive processes can lead to differences we label ‘autistic’ and so on – but our general point is that the adaptive component is likely specialized via the processes mentioned.
We finish by suggesting an avenue of solution for a longstanding problem in the philosophy of medicine: defining disorder as statistical outliers (here’s where hunter-gatherer group size becomes important). It should be possible to certify non-adaptive extremes of a dimensional spectrum. Specialisation can only occur at a minimum of once per group – selection usually couldn’t maintain adaptations below this frequency. Basically, if your ancestors would have regularly socialized with someone with a particular trait, like ADHD – if one ADHD person would have been present in every group – it’s possible it’s an evolved specialization.
Figure 2. Prevalence per group size in relation to MAP. (A) is approximately band sized, 26 individuals. (B) is approximately band-aggregation sized, 130 individuals. (C) is more than twice band-aggregation size, 416 individuals. Phenotypes appearing once in (A) and (B) could plausibly be adaptive and maintained by negative frequency-dependency; phenotypes appearing once in (C) could not.
This (with lots of assumptions) gives us the simple formula 1/G to calculate the ‘minimum adaptive prevalence’ of a trait; below that prevalence, we should assume non-adaptation. In humans, the relevant group size is likely about 165: traits rarer than 0.6% are highly unlikely to be specialized adaptations – so traits of psychopathology (and personality!) beyond those extremes are likely non-adaptive. However, certain mental disorders and many spectrum traits are more common than this! We suggest adaptive specialization explains these dimensions, with extremes sometimes being diagnosed as disorder.
Figure 3. The expected prevalence of five psychopathological traits (Table 2) and their subclinical spectra amongst a band-aggregation sized population of 165 adults and children.