Attention-deficit/hyperactivity disorder (ADHD) as a product of evolutionary mismatch: Identification of possible benefits using mixed methods

Abstract

Characterized by persistent inattention, hyperactivity, and/or impulsivity that interferes with functioning across settings, attention-deficit/hyperactivity disorder (ADHD) is among the most commonly diagnosed psychiatric disorders, affecting around 11% of children and 6% of adults in the US and around 8% of children and 7% of adults, globally. Due to its high prevalence and heritability (~80%), ADHD has been hypothesized to represent a case of evolutionary mismatch, wherein previously adaptive or neutral phenotypes become maladaptive in new environments. ADHD appears to be context-dependent, with symptom severity fluctuating during the life course in response to environmental demands. Thus, ADHD phenotypes may be less impairing—and potentially even beneficial—in some contexts. This dissertation seeks to investigate the contexts in which ADHD is beneficial using a mixed methods approach, integrating methodologies from biological and sociocultural anthropology, demography, and genetics to identify whether and when ADHD confers meaningful benefits.The first study is a preregistered (https://osf.io/m7x69/) replication study examining the 7-repeat (7R) allele of the gene DRD4, an ADHD-associated allele that has been previously linked to better nutrition in nomadic adult men and worsened nutrition in settled adult men of the northern Kenyan Ariaal group. In a previous replication, 7R was associated with increased economic status in families of children belonging to a closely related group, the Rendille, but was not associated with nutrition in the children themselves. Here, I utilize a sample of 161 adolescents from a neighboring semi-nomadic, traditionally pastoralist group, the Samburu, to understand the effects of 7R on measures of nutrition and health, economic status, and metrics of school attendance. I hypothesized that 7R may influence nutrition and health directly by modifying adolescents’ behaviors or indirectly through the behaviors of a 7R-carrying parent. I found no associations between 7R and most measures of nutrition and health, or between 7R and school metrics. Contrary to expectations based on community-based differences in mobility and sex-based differences in hunting, I observed no relationships between 7R-by-community or 7R-by-sex interaction terms and any outcomes. 7R was significantly positively associated with hemoglobin concentration (0.99 g/dL increase; p = 0.01) and was non-significantly associated with increased household economic status, proxied by weighted livestock units (6.78 unit increase; p = 0.08), with this effect in the direction expected based on the results found in the Rendille. Overall, the results of this first study suggest that the ADHD-associated 7R allele may confer modest health and economic benefits, but interpretation is limited by the complex socioeconomic conditions of the region and the small sample size. The second study of this dissertation builds on existing qualitative research centered on identifying the positives of ADHD. In this study, I utilized semi-structured interviews and surveys of ADHD symptom profiles (Adult ADHD Self-Report scale; ASRS-v1.1) in a sample of adults with (n = 14) and without (n = 4) ADHD to identify the strengths associated with ADHD and the contexts in which they occurred. This chapter explicitly sought to center participant voices and experiences in the research and ground future hypotheses in rich qualitative data. Data gathered from the interviews and surveys of individuals with ADHD highlighted the variable nature of ADHD, as well as the complex feelings surrounding the disorder. Following coding and analysis of interview data, including selective coding of major groups, and comparisons with the non-ADHD sample, the results of this qualitative work identified five areas or contexts in which ADHD symptoms are experienced as beneficial: (1) problem solving and creativity, (2) physical movement, (3) fine, manipulative tasks, (4) dynamic, active environments, and (5) varied tasks. These areas were then used to generate testable hypotheses for the final chapter. The final, preregistered (https://osf.io/p4zag/) study tests the hypotheses generated in the previous qualitative work using nationally representative longitudinal data from the National Longitudinal Study of Adolescent to Adult Health (Add Health), linked with occupational characteristics from the Occupational Information Network (O*NET), to test whether individuals diagnosed with ADHD disproportionately occupy job roles involving greater amounts of those five strength domains. 21 O*NET job characteristics were isolated based on the five areas identified in the preceding study, and five O*NET characteristics were expected to be most associated with ADHD diagnosis: Originality, Adaptability, Innovation, Complex Problem Solving, and Thinking Creatively. Multiple models were used to understand the effects of individual-level and family-level covariates on outcomes, and a Bonferroni correction was used to adjust for multiple testing issues (Bonferroni-adjusted p = 0.0024). In the models including only ADHD diagnosis and age and no other individual- or family-level covariates, ADHD diagnosis was positively associated with increased values of two O*NET job characteristics: Innovation (0.100 increase; p = 0.047) and Visualization (0.090 increase; p = 0.013), although these associations were not Bonferroni-significant. Thus, these findings point to moderate advantages conferred by ADHD in occupations requiring innovation, or the use of creativity and alternative thinking in the development of new ideas, and visualization, or the ability to imagine the process needed to achieve a desired product. Importantly, however, ADHD diagnosis showed negative or null associations with many of the characteristics, including Bonferroni-significant negative associations between ADHD diagnosis and Level/Impact values for eight of the 21 characteristics across one or more model types. Generally, the results of this study did not support the areas identified in the previous study or in the broader qualitative literature regarding the strengths of ADHD, and critical re-evaluation of this hypothesis is warranted. This dissertation aims to reframe ADHD as a product of mismatch of naturally-occurring neurobiological and behavioral variation with modern environmental demands. Based on the results of the three studies presented here, it may be that ADHD is more broadly mismatched to environments than initially expected and only rarely confers any benefits in modern contexts or that ADHD was not as beneficial in past environments as an evolutionary mismatch hypothesis would suggest. Nonetheless, further research that seeks to identify the benefits or positives of ADHD would be broadly beneficial in that it may help to alleviate stigma around the disorder, improve wellbeing in those diagnosed, and provide potential avenues for therapeutic interventions.