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According to the DSM-5, ADHD is characterized by “a persistent pattern of inattention
and/or hyperactivity-impulsivity that interferes with functioning or development”. Common
inattentive symptoms include “wandering off task, lacking persistence, having difficulty
sustaining focus, and being disorganized…”, while common hyperactivity-impulsivity symptoms
include “excessive motor activity, fidgeting, tapping, or talkativeness when not appropriate” and
“hasty actions that occur in the moment without forethought and have a high potential for
harm…”. Diagnosis occurs if a certain number of characteristic symptoms are present by the age
of 12, are variably present in two or more settings, and “there is clear evidence that the
symptoms interfere with… social, academic, or occupational functioning” (APA 2013:97-99).
While prevalence rates vary greatly across many dimensions, as of 2011, about 11% of children
in the United States had been diagnosed with ADHD, and just over 6% had been prescribed
medication for ADHD treatment (Visser et al. 2014). Similarly, ADHD prevalence rates for
adults have been found to be around 5%. This makes ADHD one of the most commonly
diagnosed psychological disorders, and its prevalence rates are increasing (Franke et al. 2012).
Accordingly, much research into the etiology of ADHD has been conducted over the past few
decades, including heritability studies, molecular genetic analyses, maternal exposure studies,
and environmental exposure assessments. In this paper I will provide an overview of this
research and review the most up-to-date findings, beginning with genetics-related studies, and
then addressing studies related to maternal, prenatal, and perinatal factors, and finally discussing
environmental exposure studies. I will then conclude by summarizing my important points and
discussing the limitations of etiological inquiry into ADHD.
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GENETIC RESEARCH
Much of the early research into ADHD and its causes focused on the heritability of the
condition and its symptoms and consisted of numerous twin and adoption studies. Results of this
research were promising; heritability estimates tended to be high, hovering between 60% and
90% between various studies. Adoption studies showed a significant correlation between ADHD
diagnosis in adopted children and their biological parents, but no such correlation in diagnosis
between them and their adoptive parents (Sprich et al. 2000). Many twin studies, while not
measuring ADHD diagnosis directly as a variable, measured “attention problems” (Rietveld et al.
2003) and “hyperactivity” (Thapar et al. 1995), which are synonymous with ADHD symptoms.
These studies found such symptoms to have heritability scores of 68% to 76% in the case of
attention problems, and 88% in the case of hyperactivity. With the importance of genetic
contributions to ADHD seeming clear, later genetics research focused on the molecular genetics
of ADHD; consisting mostly of genome-wide association studies (GWAS) and candidate gene
analyses. The results of the GWAS studies were not promising. In the largest GWAS conducted
on ADHD to this date, a meta-analysis of four other such studies, still no significant genome-
wide associations were found (Neale et al. 2010). Two different conclusions have arisen out of
these results. On one hand, the sample size of even this large GWAS was smaller than most other
successful association studies, and ADHD diagnoses may vary between studies, introducing
more variability into the sample size. Increasing sample size and correcting for diagnostic
variability may produce more positive results in the future. On the other hand, the negative
results may indicate that the genetic component of ADHD is highly complex, perhaps involving
a wide variety of different genes with small effects. To test this explanation, candidate gene
studies have targeted genes thought to be related to the physiopathology of ADHD, and their
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results seem to support the idea of a wide variety of small-effect genes. Candidate genes
involved with the neurotransmitters dopamine and serotonin have been to most studied, and
shown the most positive results. Pooled odds ratios from 1.13 to 1.45 were observed for five
different genes related to the dopaminergic system. Similarly, two genes related to the
serotonergic system had observed odds ratios of 1.31 and 1.44 (Faraone et al. 2005). Despite the
positive results of candidate gene studies, no causal genetic or physiological mechanisms have
been found for ADHD. Rather, research has so far come to the conclusion that genes are only a
part, albeit an important part, of a larger etiology involving environmental exposures and gene-
environment interactions.
MATERNAL, PRENATAL, & PERINATAL FACTORS RESEARCH
A relatively large amount of research has focused on maternal behaviors during
pregnancy, as well as prenatal and perinatal conditions, and their effect on ADHD risk. Maternal
stress and smoking are two of the most commonly studied risk factors, and research has so far
shown positive results. An analysis combining numerous maternal smoking studies found an
odds ratio of 2.39 with a confidence interval of 1.61 to 3.52 for the risk of maternal smoking on
ADHD diagnosis (Langley et al. 2005). One study on the effects of maternal stress found that
moderate and severe stress during pregnancy increased the chance of a child being diagnosed
with ADHD (Grizenko et al. 2008), and another study of the same risk factor found an odds ratio
between 1.65-52.38, but only for boys (Rodriguez and Bohlin 2005). Low birth-weight and being
born pre-term are two other risk factors for ADHD that have been commonly studied with
promising results. A meta-analysis combining seven studies relating pre-term birth to ADHD
risk found a pooled relative risk of 2.64 (Bhutta et al. 2002). However, only three of the seven
studies included in the meta-analysis had a lower confidence interval of 1.00 or greater.
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ENVIRONMENTAL EXPOSURE RESEARCH
Much environmental exposure research has also been conducted to find any potential
relationship between ADHD and environmental toxins and pollution. While many substances
have been studied in this manner, the ones I present here are the most commonly studied and
have shown the most promising results. Exposure to lead, PCBS, and certain pesticides, as well
as dietary choices, have all been shown to have some relationship to ADHD and ADHD
symptoms. Bouchard et al. (2010) found that higher urinary concentrations of organophosphate
pesticide metabolites in children were correlated with an increased likelihood of being diagnosed
with ADHD. Blood concentrations of lead in children have also been found to be associated with
ADHD diagnosis, though only children with hyperactive-impulsivity-type ADHD, not those with
inattentive-type ADHD (Nigg et al. 2010). Similarly, PCB (another common toxin that, like lead,
is found in much consumer electronics and appliances) exposure in early childhood has been
associated with ADHD-like symptoms (Sagiv et al. 2010). Finally, studies of dietary effects on
ADHD have recently been conducted, prompted by policies enacted in Europe to restrict certain
food addivites. Nigg et al. (2012) compiled numerous dietary studies and found that restriction
diets reduced ADHD symptoms in children, and that eliminating food colors in the diet had some
effect on children’s ADHD symptoms.
LIMITATIONS & CONCLUSION
Despite the successful findings of the large amount of ADHD-research, to this date the
largest limitation remains ADHD’s nebulous quality. ADHD’s diagnostic criteria cover a diffuse
cluster of symptoms ranging from over-activity (hyperactivity, impulsivity, etc.) to under-
activity (inattention, inability to complete tasks, etc.). Comorbidity with many other conditions
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like anxiety is also extremely common in those with ADHD (Shea et al. 2014), and symptoms of
comorbid conditions and ADHD frequently overlap. Thus, I would posit, it seems that ADHD
functions more as a diagnostic label for a cluster of symptoms that may or may not originate
from numerous somewhat similar conditions with equally numerous etiologies. The research
presented in this paper support this claim. There is good reason to believe that genetic and
environmental factors influence attention and hyperactivity-impulsivity, but that these factors are
myriad, sometimes interdependent, rarely independent, and lead to similar, but still diverse
psychological and behavioral outcomes. These psycho-behavioral outcomes, when they manifest
appropriately, are defined as ADHD.
It is here where we see socio-cultural factors becoming relevant and playing a role in the
proximate causes of ADHD. Diagnosis, as Jutel (2009:279) argues, “enables the social
incorporation of the afflicted individual, with the allowance for, or tools to palliate and explain”.
The processes of creating and using a diagnosis are not methods of discovery, either discovery of
a disease itself or its presence in a particular individual. Rather, diagnosis is a classification tool,
a cultural schema, which allows the medical community to carry out its role. Every culture has
ways of dealing with deviance and abnormality; sometimes schizophrenics become shamans,
sometime they become marginalized, and other times they are treated with medicine. The case of
ADHD is no different. One could argue that the proximate cause of ADHD is the culture that
demands behavioral and productive conformity to social institutions, namely school and work in
this particular case. The inattentive (unproductive) and hyperactive (abnormal) individual is
identified through diagnosis and allowed to enter the proper channels that mitigate the effects of
such deviance through medication, therapy, and other treatments. The importance of
sociocultural causes may serve as a powerful explanation for ADHD’s varying prevalence
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among different geographies, cultural groups, and genders, as well as numerous other variables
(Skounti et al. 2007).
However, ADHD cannot be so readily dismissed as a “cultural irreality” (ignoring
for now, the contradictions in such a term). People have ADHD, or perhaps more accurately,
people vary psychologically in their executive functioning, or their ability to sustain attention
and control impulsivity. Variety is nothing new, and some amount can be accommodated without
much effort in many cases. But when such variety crosses some threshold, is interpreted as being
too different, diagnosis occurs, and the variety is called “ADHD”. This is also not necessarily a
bad thing for those labeled with such diagnoses. Many instances of a disease being “created”
through such medicalization have led sufferers to receive help that has drastically improved their
lives, as is the case with PTSD (Conrad 1992). It is clear that ADHD-like symptoms are
detrimental to those subject to the demands of modern educational and occupational institutions,
and that inability to function in these institutions leads to suffering. The relevance of genetic,
environmental, developmental, and sociocultural factors to the etiology of ADHD-like symptoms
is demonstrated by the research. Variation in attention, hyperactivity, and impulsivity exist, and a
certain band of this variation has come to be defined as ADHD.
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