Epigenetics is the phenomenon by which changes in gene expression occur not by changes in the DNA sequence, but by DNA methylation, chemical modification of histone proteins, control by non-coding RNA or other means. Three large-scale epidemiological studies published in 2016 raised the possibility that adult-onset attention-deficit/hyperactivity disorder (ADHD) differs biologically from that developing in childhood. Based on the hypothesis that ADHD develops in adulthood, we examined the possible involvement of epigenetic mechanisms.
 Studies on the heritability of ADHD have demonstrated that both genetic and environmental factors are involved in the development of ADHD. In studies on causative genes, many genes, including monoaminergic and neurotrophic factor-related genes, were found to be involved. Regarding the epigenetic theory of the development of ADHD, there are a number of reports supporting the theory that smoking and drinking by the mother during the fetal stage, and psychosocial stress during infancy cause epigenetic changes, leading to an ADHD-like phenotype. In this study, we reviewed the studies on the epigenetic theory of ADHD development, and discuss the pathogenic mechanisms from both genetic and epigenetic viewpoints.
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