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Attention deficit hyperactivity disorder (ADHD) is a common condition affecting many thousands of children. It is a condition that can have adverse health, social and educational outcomes. It also exacts significant societal costs, economically and socially. This paper hypothesises that the population of children receiving a clinical diagnosis of ADHD is aetiologically heterogeneous: that within this population, there is a group for whom the development of ADHD is largely genetically driven, and another who have a ‘phenocopy’ of ADHD as a result of very adverse early childhood experiences, with the prevalence of this phenocopy being heavily skewed towards populations living with poverty and violence. A third group will have a high genetic risk and have been exposed to violence. These groups will overlap, with epigenetic phenomena and other environmental factors, for example, preterm birth, poor intrauterine growth, foetal exposure to teratogens, playing an important role for all affected children in determining the severity of their functional difficulties.
Phenocopies occur when people exhibit a phenotype that appears identical to that found in other individuals who have a linked genotype, but do not possess this genotype themselves. Environmental factors may cause this, as occurs in Rutter's autism phenocopy described in Romanian orphans adopted by western families in the early 1990s.1 These children had experienced extreme, profound neglect; after adoption, they began to present to services with abnormal or delayed development. Many presented a clinical and behavioural picture indistinguishable from autism in preschool children. However, follow-up revealed a different natural history to that of childhood autism, with these children no longer fulfilling criteria for autism, with mild cognitive impairment, low empathy and conduct problems prevailing in adolescence.2 Animal models suggest a possible epigenetic mechanism for this, with environments of profound neglect associated with alterations in gene expression as a result of …
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