Empirical tests of natural selection-based evolutionary accounts of ADHD: a systematic review

ADHD co-morbidities: A review of implication of gene × environment effects with dopamine-related genes

ADHD is a major burden in adulthood, where co-morbid conditions such as depression, substance use disorder and obesity often dominate the clinical picture. ADHD has substantial shared heritability with other mental disorders, contributing to comorbidity. However, environmental risk factors exist but their interaction with genetic makeup, especially in relation to comorbid disorders, remains elusive. This review for the first time summarizes present knowledge on gene x environment (GxE) interactions regarding the dopamine system. Hitherto, mainly candidate (GxE) studies were performed, focusing on the genes DRD4, DAT1 and MAOA. Some evidence suggest that the variable number tandem repeats in DRD4 and MAOA may mediate GxE interactions in ADHD generally, and comorbid conditions specifically. Nevertheless, even for these genes, common variants are bound to suggest risk only in the context of gender and specific environments. For other polymorphisms, evidence is contradictory and less convincing. Particularly lacking are longitudinal studies testing the interaction of well-defined environmental with polygenic risk scores reflecting the dopamine system in its entirety.

Genetics in the ADHD Clinic: How Can Genetic Testing Support the Current Clinical Practice?

Attention-deficit/hyperactivity disorder (ADHD) is a neurodevelopmental disorder with a childhood prevalence of 5%. In about two-thirds of the cases, ADHD symptoms persist into adulthood and often cause significant functional impairment. Based on the results of family and twin studies, the estimated heritability of ADHD approximates 80%, suggests a significant genetic component in the etiological background of the disorder; however, the potential genetic effects on disease risk, symptom severity, and persistence are unclear. This article provides a brief review of the genome-wide and candidate gene association studies with a focus on the clinical aspects, summarizing findings of ADHD disease risk, ADHD core symptoms as dimensional traits, and other traits frequently associated with ADHD, which may contribute to the susceptibility to other comorbid psychiatric disorders. Furthermore, neuropsychological impairment and measures from neuroimaging and electrophysiological paradigms, emerging as potential biomarkers, also provide a prominent target for molecular genetic studies, since they lie in the pathway from genes to behavior; therefore, they can contribute to the understanding of the underlying neurobiological mechanisms and the interindividual heterogeneity of clinical symptoms. Beyond the aforementioned aspects, throughout the review, we also give a brief summary of the genetic results, including polygenic risk scores that can potentially predict individual response to different treatment options and may offer a possibility for personalized treatment for the therapy of ADHD in the future.

Testing the evolutionary advantage theory of attention-deficit/hyperactivity disorder traits

To reconcile the strong secular persistence of attention-deficit/hyperactivity disorder (ADHD) despite its impairing effects, ADHD traits have been postulated to offer an evolutionary advantage. It has been proposed that such advantages should in particular be observable under time-critical, novel, and resource-depleted conditions requiring response-readiness and high levels of scanning and exploration/foraging. Our objective was to provide the first behavioral test of this hypothesis. Schoolchildren from the general population with no/few (n = 56), mild (n = 50), moderate (n = 48), and severe (n = 48) ADHD traits, defined according to their ADHD-Rating Scale IV (ADHD-RS-IV) total score, participated in an exploratory foraging and response-readiness laboratory test. Here, children searched for coins hidden in locations of varying obscurity in an unfamiliar room for 1 min. Test-performance (number of coins found) adjusted for age, sex, and estimated IQ was analyzed categorically using multiple linear regression analyses and dimensionally by fitting a regression model including the ADHD-RS-IV score as a continuous measure. There were no differences in the mean number of coins between the No/Few (Mean = 7.82), Mild (Mean = 7.76), Moderate (Mean = 7.58), and Severe (Mean = 7.88) groups [F(3,195) = 0.24, p = 0.871]. Furthermore, excluding children with functional impairment, adjusting for verbal working memory and response inhibition, and stratifying for sex did not change these findings. Finally, continuous ADHD traits were not found to be related to test-performance [F(3,195) = 0.73, p = 0.536]. While our results do generally not support the evolutionary advantage theory (i.e., ADHD traits neither conferred an advantage nor a disadvantage), this does not disprove that ADHD traits may have offered advantages via other mechanisms.

Genomic analysis of the natural history of attention-deficit/hyperactivity disorder using Neanderthal and ancient Homo sapiens samples

Attention-deficit/hyperactivity disorder (ADHD) is an impairing neurodevelopmental condition highly prevalent in current populations. Several hypotheses have been proposed to explain this paradox, mainly in the context of the Paleolithic versus Neolithic cultural shift but especially within the framework of the mismatch theory. This theory elaborates on how a particular trait once favoured in an ancient environment might become maladaptive upon environmental changes. However, given the lack of genomic data available for ADHD, these theories have not been empirically tested. We took advantage of the largest GWAS meta-analysis available for this disorder consisting of over 20,000 individuals diagnosed with ADHD and 35,000 controls, to assess the evolution of ADHD-associated alleles in European populations using archaic, ancient and modern human samples. We also included Approximate Bayesian computation coupled with deep learning analyses and singleton density scores to detect human adaptation. Our analyses indicate that ADHD-associated alleles are enriched in loss of function intolerant genes, supporting the role of selective pressures in this early-onset phenotype. Furthermore, we observed that the frequency of variants associated with ADHD has steadily decreased since Paleolithic times, particularly in Paleolithic European populations compared to samples from the Neolithic Fertile Crescent. We demonstrate this trend cannot be explained by African admixture nor Neanderthal introgression, since introgressed Neanderthal alleles are enriched in ADHD risk variants. All analyses performed support the presence of long-standing selective pressures acting against ADHD-associated alleles until recent times. Overall, our results are compatible with the mismatch theory for ADHD but suggest a much older time frame for the evolution of ADHD-associated alleles compared to previous hypotheses.

Attention-deficit hyperactivity disorder (ADHD): progress and controversy in diagnosis and treatment

Knowledge about attention-deficit hyperactivity disorder (ADHD) is rapidly accumulating. Recent advances in diagnosis, genetics, neuroimaging, drug and non-drug treatments are considered, and the results are related to the critical attack on the ADHD diagnosis, which argues it a medicalising social construct, unhelpfully sustaining power relationships. The advances reviewed suggest that, while this attack can be conclusively dismissed as wrong and misleading, the phenomenological definition of ADHD is no longer sufficient for construct validity, though continues to be valuable as a guide for clinicians. The humanising and individualising concerns underlying the attack on the diagnosis could usefully be redirected to improving effective measurement of patient outcomes.

Teenage Parenthood and Birth Rates for Individuals With and Without Attention-Deficit/Hyperactivity Disorder: A Nationwide Cohort Study

OBJECTIVE

Prior studies have established that attention-deficit/hyperactivity disorder (ADHD) is associated with risky sexual behavior, but it remains unknown whether individuals with ADHD also are more likely to become parents while being teenagers. This aspect is clinically relevant because teenage parenthood is associated with adverse outcomes for parents and children. Therefore, the main aim of this study was to investigate whether individuals with ADHD would be more likely to become teenage parents compared with individuals without ADHD.

METHOD

This is a historical prospective cohort study based on nationwide data from Danish registers. The cohort consisted of all individuals (N = 2,698,052) born in Denmark from January 1, 1960 through December 31, 2001. The association between ADHD (n = 27,479 cases) and parenthood (first child) in age intervals of 12 to 16, 17 to 19, 20 to 24, 25 to 29, 30 to 34, 35 to 39, and 40 years and above was investigated by Poisson regression and expressed as incidence rate ratios (IRRs) with accompanying 95% CIs. IRRs can be interpreted as relative risks.

RESULTS

Compared with individuals without ADHD, those with ADHD were significantly more likely to become parents at 12 to 16 years of age (IRR for females 3.62, 95% CI 2.14-6.13; IRR for males 2.30, 95% CI 1.27-4.17) and at 17 to 19 years of age (IRR for females 1.94, 95% CI 1.62-2.33; IRR for males 2.27, 95% CI 1.90-2.70).

CONCLUSION

Individuals with ADHD are significantly more likely to become teenage parents compared with individuals without ADHD. Therefore, it might be appropriate to target this group with an intervention program that includes sexual education and contraceptive counseling.