Attention-deficit/hyperactivity disorder (ADHD) is a clinically heterogeneous neurodevelopmental disorder characterized by inattention, impulsivity, and hyperactivity, sharing comorbidities with other psychiatric disorders, including heightened anxiety, depression, personality disorders, and antisocial behaviors. From a cognitive science perspective, there has been much controversy in academic literature on the role of nature and nurture, as well as the influence of genetics and environmental factors on the pathogenesis of ADHD—these studies often focus on family, twin, and adoption paradigms to analyze the genetic basis of psychiatric disorders. The fundamental problem researchers have sought to answer is whether and to what degree behavior in ADHD can be attributed to genetic inheritance or external factors, such as social and parental influences in an individual’s environment—and whether we can aptly distinguish the genetic and environmental etiologies of ADHD. Scientists who study psychiatric disorders often accept that there is a shared role between genetics and environmental factors in behavior exhibited. Yet in a controversial article published in 2000, “Not in Their Genes: A Critical View of the Genetics of Attention-Deficit Hyperactivity Disorder,” ADHD researcher Dr. Jay Joseph rejects a genetic basis for heritability in ADHD, instead attributing its causes to “psychosocial factors,” causing considerable backlash in the scientific community (Joseph, 2002). Renowned ADHD researchers, including Dr. Stephen Faraone and Dr. Joseph Lieberman (2002), profusely rebuke Joseph’s conclusions and the implications of rejecting a genetic basis of ADHD. The vast and controversial literature on nature and nurture in ADHD—various family, adoption, and twin studies, in conjunction with molecular genetic research—show that, despite potential shortcomings of genetic studies as put forth by Joseph, the key symptoms of ADHD are largely inheritable and influenced by genetic factors and specific genes in the dopaminergic system; the particular fusion of inheritability studies coupled with molecular genetics makes a strong case for the genetic basis and inheritability of ADHD. Moreover, while the onset of ADHD can likely be genetically predetermined, social and environmental factors from a young age, such as parental influence and familial relationships, as shown through additional adoption studies, can prolong the symptoms of ADHD and its incidence throughout adolescence and adulthood. As such, there seems to be a synergistic and reciprocal relationship between genetic inheritability and psychosocial factors in the onset, development, and continuity of ADHD in children and adolescents.

To analyze the genetic and environmental etiology of ADHD, researchers have employed a combination of family, twin, and adoptions studies, which respectively look at the incidence of ADHD in families and biological relatives, monozygotic (MZ) and dizygotic (DZ) twins, and in adopted children within and across different families. Twin studies are often used as a measure for gauging inheritability, where differences between MZ twin pairs (identical twins who share 100% of genes) and DZ twin pairs (who on average share 50% of genes) are compared, relative to an estimate of a given trait and the proportion of its population variance that can be explained by genetic differences. On a range from 0 to 1, heritability measures closer to 1 suggest a strong genetic determination of a given trait. Adoption studies in ADHD often examine genetically related individuals adopted into different environments with non-biological parents, and also compare the adopted child with attributes of the biological parents, if available—these types of studies are often used to gauge the effect of nurture, or external factors on the incidence or development of ADHD.

Family studies of ADHD, extensively documented by prolific ADHD researcher Dr. Faraone, have found that there is a higher prevalence rate of ADHD in the biological relatives of ADHD positive subjects. Faraone and Larsson (2018) note that one family study found that parents of children with ADHD have a two to eightfold increase in the risk of ADHD; a similar increase in the risk of ADHD across biological siblings shows that ADHD is highly familial, or runs in families. Thapar and Stergiakouli (2008) similarly note in a meta analysis that family studies highlight a larger prevalence of ADHD in biological relatives of ADHD subjects, as opposed to relatives of controls. Faraone and Larsson (2018) additionally note a family study consisting of 894 ADHD probands and 1135 siblings with ages ranging from 5–17 years old found that there was a ninefold increase in the risk of ADHD in siblings of ADHD probands, compared with siblings of controls. These higher rates, however, can potentially be attributed to either genetic or environmental factors, for which twin and adoption studies prove useful in distinguishing between both.

Moreover, researchers note that the increased prevalence of ADHD in families is more likely due to genetic factors as opposed to shared environmental factors, as shown through additional adoption studies—in one adoption study, the prevalence of ADHD was shown to be greater among biological relatives of non-adopted ADHD children than adoptive relatives of adopted ADHD children (Faraone & Larsson, 2018). In another adoption study, it was found that adoptive relatives of hyperactive children were less likely to exhibit hyperactivity than that of the biological relatives of hyperactive children, further supporting a genetic basis for ADHD between biological relatives (Faraone, 2005). In other words, the genetic prevalence of ADHD as shown in the adoption study makes a strong case for the genetic inheritability associated with ADHD, in tandem with the family study.

Moreover, Faraone and Larsson (2018) note that across 37 twin studies of ADHD, the mean heritability found was 74%, along with approximately 80% in a study conducted on MZ and DZ twins, full siblings, and maternal and paternal half-siblings. Of a few select studies that used categorical measures of ADHD—inattention or hyperactivity—the range of heritability found was from 77% to 88%, suggesting strong genetic inheritability (Faraone & Larsson, 2018). As inattention and hyperactivity are the two most common and prevalent subtypes of ADHD, the strong inheritability values associated with these traits, which act as categorical measures of ADHD, further highlight a strong genetic basis of ADHD.

While these studies on their own suggest a strong genetic basis for ADHD, research into the molecular genetics of ADHD taken in tandem further highlight the strong genetic basis of ADHD, augmenting the familial, twin, and adoption studies and extending their genetic implications—particularly, studies have highlighted that specific dopamine receptor and transporter genes that are involved in the neurobehavioral mechanisms of ADHD. In a meta analysis of the D4 dopamine receptor gene, Faraone et al. (2005) find a statistically significant association between ADHD and the DRD4 7-repeat allele—the study used the odds ratio to measure the relative risk of ADHD subjects having the 7-repeat allele as opposed to non-ADHD subjects. This was found for both case-control and family based studies, supporting the genetic basis of ADHD in pinpointing specific genes and their transmissions across individuals. The molecular genetic perspective serves as a corroborator for the genetic basis put forth by the family, twin, and adoption studies, supporting the idea of genetic inheritance in ADHD. However, while Faraone et al. posit a strong genetic basis for ADHD, often times over shared environmental factors, there are potential social factors that also affect the development of ADHD, and particularly prolong the incidence of ADHD and its symptoms—an intriguing study employs statistical analysis for examining hostile relationships between children and parents and its impacts on early ADHD development. Sellers et al. (2019) describe an adoptive design study employing reports filled out by parents—for measuring adoptive parent-to-child hostility, the Iowa Family Interaction Ratings Scale was employed and children behavior questionnaires were administered to parents to gauge the severity of ADHD among children. A correlation analysis highlighted that early child impulsivity and later levels of ADHD symptoms were correlated with maternal and paternal hostility, and that adoptive maternal and paternal hostility similarly predicted prolonged ADHD symptomatology in children (Sellers et al., 2019). The findings of this study uniquely shed light on how familial dynamics are both impacted by members with a genetic predisposition to ADHD, which causes maternal and paternal hostility, and in turn prolongs these symptoms of ADHD; in other words, there seems to be a reciprocal relationship environmental factors and ADHD severity, where the initial presence of ADHD can often originate genetically, and psychosocial familial dynamics are impacted and prolonged by members who have ADHD and parental hostility. On the topic of genetics, perhaps the most intriguing opposition to the influence of genetic factors in ADHD comes from Dr. Jay Joseph whose repudiation of genetic studies precipitated back-and-forth commentaries with Faraone and Lieberman on the validity of genetic influences in ADHD. Joseph (2002) posited that genetic studies that employed mainly family, twin, and adoption studies were flawed in proving genetic inheritance for a few key reasons: (1) family studies on their own are unable to determine genetic predispositions, as environmental factors can potentially affect ADHD incidence in families and (2) twin studies make an “equal environment” assumption that posits that the trait-relevant environments of MZ and DZ twins are the same.

Joseph adamantly rejects the basis of genetic transmission on the basis of these points; given the vast literature on these methodologies in genetic studies, however, it does not seem reasonable to conclude that ADHD cannot be genetically inherited, especially from the point of view of a cognitive scientist. As shown in evidence presented prior, twin and adoption studies can augment family studies in proving inheritance—family studies are useful in establishing a baseline, potentially genetic basis for ADHD, where the twin and adoption studies can differentiate between environmental and genetic factors. While the validity of those twin and adoption studies are questioned by Joseph, Faraone notes that Joseph’s position is an inference without ample evidence, and that these assumptions, particularly for ADHD, have not been shown to be violated for trait-specific characteristics of ADHD in MZ and DZ twins. However, it is important to note the equal environment assumption, and aptly adjust estimates in the case where this assumption may be violated. Cross-demographic corroboration may help strengthen the genetic basis of ADHD.

Moreover, the molecular genetic basis presented earlier, in tandem with these family, twin, and adoption studies, suggests that there is a strong genetic link present in ADHD, and is an aspect Joseph does not take into account in his arguments. Overall, Joseph takes an overly aggressive stance in wholly rejecting genetics in ADHD—from a cognitive science perspective, we choose to accept the potential shortcomings of this study, but acknowledge that given the extensive research done, there does seem to be a strong genetic basis for ADHD, amplified by certain environmental factors.

ADHD today is still often misdiagnosed; its pathogenesis is largely misunderstood, despite the genetic and environmental basis that has been established through research. More objective mechanisms for characterizing and understanding its causes are needed. Based on literature outlining the genetic and environmental factors that figure into ADHD onset and development, there is evidently both a nature and nurture component present, given its high heritability measures and evidence on correlations between parent relationships and ADHD continuity. However, these social factors seem too variable to objectify and use as metrics to examine ADHD development. A neurobiological perspective, particularly with the discovery of novel biomarkers and potentially building off of research done on genetic inheritance, can provide more objective metrics for understanding behavior in ADHD. Based on the preliminary analysis presented here, though, the genetic aspect of ADHD is an area worth exploring further, particularly with its relationships with psychosocial factors and how they influence the development of ADHD from childhood to adulthood.

Bibliography:

Faraone, S. V. (2005, March 1). Genetics of adult attention-deficit/hyperactivity disorder. Psychiatric Clinics of North America. Retrieved October 10, 2022, from https://www.sciencedirect.com/science/article/pii/S0193953X0300090X?via%3Dihub

Faraone, S. V., & Biederman, J. (2002, May 25). Nature, nurture, and attention deficit hyperactivity disorder. Developmental Review. Retrieved October 10, 2022, from https://www.sciencedirect.com/science/article/pii/S027322970090515X?via%3Dihub

Faraone, S. V., & Larsson, H. (2018, June 11). Genetics of attention deficit hyperactivity disorder. Nature News. Retrieved October 10, 2022, from https://www.nature.com/articles/s41380-018-0070-0

Joseph, J. (2002, May 25). Not in their genes: A critical view of the genetics of attention-deficit hyperactivity disorder. Developmental Review. Retrieved October 10, 2022, from https://www.sciencedirect.com/science/article/pii/S0273229700905112

Nigg, J. T., Sibley, M. H., Thapar, A., & Karalunas, S. L. (2020, December). Development of ADHD: Etiology, heterogeneity, and early life course. Annual review of developmental psychology. Retrieved October 10, 2022, from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8336725/

Sellers, R., Harold, G. T., Smith, A. F., Neiderhiser, J. M., Reiss, D., Shaw, D., Natsuaki, M. N., Thapar, A., & Leve, L. D. (2019, December 16). Disentangling nature from nurture in examining the interplay between parent–child relationships, ADHD, and early academic attainment: Psychological medicine. Cambridge Core. Retrieved October 10, 2022, from https://www.cambridge.org/core/journals/psychological-medicine/article/disentangling-nature-from-nurture-in-examining-the-interplay-between-parentchild-relationships-adhd-and-early-academic-attainment/DA05EE9878474DA02CF817543A382587

Thapar, A., & Stergiakouli, E. (2008, August). An overview on the genetics of ADHD. Europe Pubmed Central. Retrieved October 10, 2022, from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2854824/