Attention Deficit Hyperactivity Disorder

Critically evaluate the evidence suggesting ADHD is a highly heritable disorder conferring an evolutionary advantage in our ancestors

Critically evaluate the genetic basis of Attention Deficit Hyperactivity Disorder (ADHD) and the evolutionary accounts for its persistence in the human genome

To what extent can Attention Deficit Hyperactivity Disorder (ADHD) be considered as a genetic disorder that was once adaptive?

‘Attention Deficit Hyperactivity Disorder (ADHD) is a genetic disorder that once was of adaptive benefit.’ Discuss.

Attention Deficit Hyperactive Disorder (ADHD) is a neurodevelopmental disorder that impacts 5% of the paediatric population. ADHD is a cluster of behaviours that are more often than not seen together consisting of: hyperactivity, impulsivity and lack of attention or in-attention.   The definition of ADHD has evolved during the last 20th century and still evolving, from George Still’s lecture (1902) to the DSM-5 definition (USA) and the ICD-10 (definition) however there are accounts throughout literature of children displaying symptoms to what we commonly known as ADHD. The aetiology of ADHD is most likely to be genetic, however evidence cannot be regarded as conclusive. In this essay I will evaluate evidence that ADHD is genetic conferring to an evolutionary advantage in our ancestors.

There is genetic evidence for the basis of ADHD that comes from a number of researchers. Between 10 and 35 per-cent of the immediate family members of children with ADHD also display this disorder.

Risk for siblings of children with disorder is approximately 32%

If a parent has ADHD the risk to offspring is on the order of 50+% A number of researchers looked at ADHD in twin studies of monozygotic and dizygotic twins and found heritability to be 70-80% Heritability estimate 0.75-0.91 (adjusting for different types of ADHD(Burt 2009, Levy, 1997).  Stevenson (1992) looked at 91 pairs of identical same-sex twins and 105 pairs of fraternal same-sex twins to demonstrate a strong heritability component. Results indicate significant genetic contribution to individual differences in activity levels and attention abilities.  

However, a problem with this is families may be exposed to the same environmental risk, with genetic disposition studies have shown the importance of environmental agents in ADHD Milberger et al (1997) found an positive association with pregnancy, delivery and infancy complications such as maternal substance use and family problems during pregnancy. Suggesting that these individuals may be pre-disposed by genetic factors to ADHD but environmental factors play a part in the development on the disorder. However, taking away environmental factors ADHD is still prevalent Sprich et al (2000) This study examined the rates of ADHD and associated disorders in the first-degree adoptive relatives of 25 adopted probands with ADHD and compared them with those of the first-degree biological relatives of 101 nonadopted probands with ADHD and 50 nonadopted, non-ADHD control probands.   The findings show, that Six percent of the adoptive parents of adopted ADHD probands had ADHD compared with 18% of the biological parents of nonadopted ADHD probands and 3% of the biological parents of the control probands. Other family studies have shown, Biederman et al for example 75% of the parents of children of ADHD were treated for it.

than unrelated cases Suggesting but not proving  it could be influenced by a persons genes

Although family and twin studies demonstate the genrtic they do not indetify the specific genes. So, if it is genetic what genes are being passed on not one simple gene does not follow mendelian principles (Lander & Schork, 1994),, A number of genes have been identified: DAT1, DRD4 and SNAP25.

So, going on that ADHD is inheritable what evolutionary advantage would it have for our ancestors.

The ability to survive and reproduce is the whole purpose is the whole purpose of life. Orgasnismisms who are not suited to their environment have a limited chance to do so. People with ADHD are more likely to reproduce at a younger age and have different partners, thus having more chance of passing on the ADHD genes Barkley et al., 2006 from an evolutionary perspective this is job done. Initially it was suggested that ADHD served as an adaptive function in particular the Hunter rather than the farmer,

Hunter rather than farmer hypothesis first proposed by Hartmann (1997) noted that most or all humans were nomadic hunter-gatherers for hundreds of thousands of years, but that this standard gradually changed as agriculture developed in most societies, and more people worldwide became farmers. Over many years, most humans adapted to farming cultures, but Hartmann speculates that people with ADHD retained some of the older hunter characteristics, hunters needed hyper focus more. From an evolutionary viewpoint, “hyperfocus” was advantageous, conferring superb hunting skills and a prompt response to predators, however never intended to be a hard science. However, there has been support for this hypothesis Chen (1999) A genetic variant associated with ADHD has been found at higher frequency in more nomadic populations and those with more of a history of migration.[3] Consistent with this the health status of nomadic Ariaal men was higher if they had the ADHD associated genetic variant (7R alleles). However, in recently sedentary (non-nomadic) Ariaal those with 7R alleles seemed to have slightly worse health, study done on 39 populations with over 2,000 individuals. Jesen et al 1997, proposed a model