ADHD is traditionally viewed as a 'disorder' displaying a set of behaviours significantly different from normal, that negatively impact in many areas of the individuals life. These negative outcomes can include; social skills, organizational ability, academic attainment, career success and criminal involvement amongst others (Barkley, 1990).
Barkley is a leading writer on ADHD and he frames the condition as abnormalities causing a disorder, for which treatment is required to suppress the symptoms. Taking a neurodevelopmental approach to its origin, he argues that the critical deficit associated with ADHD is the failure to develop a capacity for "self-regulation".
ADHD's heritability of 0.8 is high, especially for a behavioural disorder. Evidence is provided by twin studies that have confirmed a strong genetic contribution to individual differences in activity levels and attention abilities (Baird, Stevenson & Williams, 2000). Couple this with the relatively high occurrence in 3-9% of the population, it has been suggested that ADHD traits were selectively advantageous in the past.
This evolutionary perspective formed an entirely new approach to the understanding and treatment of ADHD as not being pathological at all, but instead normal behaviour which was once and could still be in some contexts, adaptive.
There's a controversial debate, between two key authors in the field, Barkley with a traditional pathological view and a pop writer Hartmann, who takes an evolutionary view.
Barkley wishes to treat the individual, believing ADHD symptoms are abnormally exaggerated behaviours, causing pathological harm and requiring treatment.
Hartley on the other hand wants to change the environment to fit the individual, believing the symptoms are normal behavioural patterns, which only look exaggerated because they are now out of appropriate context.
In 1993 Hartmann proposed the metaphor of 'hunter Vs. farmer', describing those with ADHD as hunters. He suggested the behaviours associated with ADHD were misunderstood, and were in fact useful skills for hunters in the evolutionary past.
Hartmann explain that each ADHD trait can be seen positively, e.g distractibility as scanning and searching for novel stimuli and threats. Three symptom behaviours, distractibility, impulsivity and risk-taking were ideal for hunters, but detrimental to farmers. For example patience is essential for farmers to complete mundane tasks such as planting seeds. Conversely risk-taking behaviour would have been the day to day norms for hunters who risked their lives against predators and had to act quickly and on impulse in order to catch prey etc.
Hartmann's ideas quickly gained popular support from parents and media.
Children with any disorder are often stigmatized and can suffer damage to their self-esteem and confidence. Therefore any new perspective indicating ADHD to have potential benefits may lessen feelings of disability and instil pride .
Hartmann's hunter in a farmers world remained just a metaphor until 2002, when researchers found a significant positive selection for the genetic variation associated with novelty-seeking behaviour and ADHD. This gene was termed the Edison gene and was likely to have served a unique adaptive purpose for our ancestors.
A mutation was discovered by analysing the DRD4 genes of a worldwide sample, noting the degree of variation and working backwards to find the time when there was no variation at all. There is now evidence of a single mutation, that triggered a novelty seeking adaptation that is found in ADHD.
Evidence supports this, like Eisenberg et al, (2008) who looked at two tribal communities still around today, one that was still nomadic, and the other recently settled. Their findings showed that those with the DRD4/7R+ genes in both communities, were better adapted to the nomadic lifestyle than to the static lifestyle of those recently settled. They continue that the short attention span associated with this genotype may allow nomadic children to more readily learn effectively in a dynamic environment (without schools), while the same attention span interferes with classroom learning in the settled community.
Jensen (1997) is in agreement with the core assumptions of an evolutionary perspective, that a species typical design develops via natural selection, implying the behaviours evident in ADHD, are potentially adaptive responses of the organism to environmental demands.
This theory suggests ADHD tendencies are innate in everyone but are activated by differing environmental stimulus in the early days. ADHD behaviours are seen as a continuum, furthermore one that is malleable, with early development being a time for 'wiring the brain' to fit its environment.
Jensen states exploratory motor behaviour usually only occurs in the context of presumed safety. A characteristically hyperactive child will thus suppress this motor activity during times of danger, during separation from the care-giver and in situations with a high degree of novel stimuli.
Attention, often described as scanning and rapidly shifting attention in those with ADHD is necessary to monitor dangers and threats. Conversely over focused attention could indeed be maladaptive in high-threat, highly novel environments. Thus those living in environments with high risks i.e high predator to prey ratio, are likely to display increased scanning behaviours (or lack of attention).
Impulsivity, defined as a quick response without considering alternatives, can be viewed similarly. Jensen proposes early on an individual can learn to adjust the threshold and timing of responses based on the likelihood of a 'payoff' as a result of immediate, delayed or non-response and whether the response is correct.
Jensen believes as society changed, problem solving individuals would have become increasingly the most adaptive type for the environments of industrialised and organised cultures. The response ready characteristics of ADHD individuals have gradually become seen as less adaptive.
This brings entirely new research and treatment implications for ADHD. Jensen asks if a systematic and strategic approach over a period of time, could effectively modify or 'reset' the individuals early biological substrates and adapt them to the environment they current reside in.
To begin this process research must be done to determine the periods of greatest plasticity and ascertain which children are most susceptible given their biologically selected substrate and their environment, to 'up-regulate' to ADHD type traits.
One final thought is just how important ADHD individuals may have been to our survival- The personalities of those with ADHD may have predisposed them to migration. Active, restless and adventure-seeking individuals would have pioneered the migrations that populated the earth. (Bridgeman, 2003).
The hypothesis generates a testable prediction, that the frequency of the gene should be greatest in groups that moved farthest from the African cradle of humanity. This is proven by rarity of the gene in the African bushmen, the closest group left to ancestral populations and highest in south America, about as far from Africa as one can migrate! (Bridgeman, 2003).