evolution of adhd
...ndicating that they must have a recent common ancestor. The fact that the 7R allele has been in the population for a long time at a low frequency suggests that a balancing selection has been preserving 7R, but inhibiting it from becoming common until recently (Ding et al, 2002). They claim that a mechanism such as frequency-dependent selection is most likely responsible for the selective advantage of the 7R allele, by which the evolutionary advantage for a personality trait depends on the existing distribution of personality types. In this situation, the frequency of the new 7R allele increases up to a certain point and then stabilizes. When the advantage of an allele is frequency dependent, different alleles can exist indefinitely at polymorphic frequencies (Ding et al, 2000). However, the selective forces for ADHD must not be the same in all populations because the 7R allele is very common in some, like the South American Indians, found at intermediate levels in Europeans and Africans, and very rare in others, like East Asians and the !Kung Bushman (Chen et al, 1999). Harpending and Cochran (2002) argue against the idea that ADHD could never be advantageous by maintaining that there must be a force of positive selection for 7R because an adaptive genetic variant cannot cause a reduction in fitness. The only way it could cause fitness reducing mental illness is if the illness is an indication of recent environmental change, one in which the environment is too different from the one that favored the 7R allele for the variant to continue being advantageous. Proponents of this side of the argument insist that natural selection would have eliminated debilitating alleles otherwise, contrary to the other language theory of ADHD which states that ADHD was always fitness reducing because it interferes with the processing of language (Baird, 2000). Instead, Harpending and Cochran (2002) and others hypothesize that those individuals with the 7R allele for ADHD benefited in the past, and do so presently in some societies, from a reproductive advantage. In these male competitive societies, this trait would aid in the competition for food as children, or in face-to-face and group male competition. In more detail, Jensen et al (1997) explain how ADHD traits (hyperactivity, inattention, and impulsivity) could be adaptive in both ancient and modern day environments. Increased motor activity (hyperactivity) would be useful in helping one explore his environment for threats and opportunities, and aid in effective foraging and anticipating dangers. Additionally, increased activity could help to develop muscle and motor skills in children. This exploratory behavior would function to create a reservoir of experiences upon which one could draw on throughout life. Yet this hyperactivity only occurs when the ADHD afflicted individual is in close proximity to a caretaker where he feels safe, and is suppressed when separated from the caretaker or in times of danger (Bowlby, 1973). Bowlby’s data demonstrates the idea of adaptational theory, since increased motor behavior is expressed when its advantageous, and suppressed when it is not. Inattention could be beneficial to an individual because it results in increased scanning behaviors and rapid shifts in attention (Jensen et al, 1997). In dangerous or new environments of ancestral times, over focused attention would be harmful; when threats and novel stimuli are minimal however, over attention could be a useful ability in planning out future events. While these behavioral potentials have evolved to exist within the genome, the environment plays a part in shaping the expressed responses (Jensen et al, 1997). This has implications in modern day educational systems, since definitions of over or under attention only exist when defined in relation to an established norm, such as the levels required for active versus passive learning. Impulsivity, Jensen explains (1997), encompasses making a quick response to environmental conditions without considering alternatives. The likelihood of a payoff off as a result of an immediate response was higher in pre-modern societies, where it was necessary to avoid predators and capture prey. The negative impact of a false positive response, or mistaking a neutral threat for a harmful one, was clearly outweighed by the negative possibilities of missing a harmful threat. Thus, it was highly advantageous to be impulsive in the environment in which this trait evolved. Baird et al (2000) believe that there is no situation in which ADHD could ever be advantageous. It is not a normal response taken out of its adaptive environmental context, but instead an expression of maladaptive deficiencies at the anatomical, behavioral, and biochemical levels that tend to cause problems in a variety of social contexts. Poor behavioral inhibition, leading to a lack of self-control, is the central impairment that prevents ADHD sufferers from exhibiting the normal levels of sophisticated self-regulation. According to Baird et al (2000), the selective pressures that enabled human communication also demanded the highest degree of refined inhibition skills, attention, and memory. Communication is essentially a two way feed back system between the brain and the association of symbols. Human cognitive features developed as a side effect of the development of communication, and resultantly, the two are linked by many neural subsystems (Berk, 1992). People with ADHD are not lacking any normal cognitive features, but are just less effective in implementing, controlling, and monitoring them. Baird et al (2000) term this theory the “side effect of human language evolution model of ADHD.” ADHD creates problems with the ability to internalize speech (Baird, 2000). Bronowski (1977) observes that communication necessitates mental reflection when alternate options are being considered and eventually tried, and that these reflections require a temporary suspension of action. “Executive functions” are defined as the abilities that help maintain the plans deemed necessary to accomplish future goals, and are part of the developmental process by which speech becomes internalized. Bronowski delineates four executive functions. 1) Prolongation- to mentally move around in time and plan for different possibilities; 2) Separation of affect- separating emotional response from deliberations on an idea; 3) Internalization- internal dialogue; 4) Reconstitution- original and novel speech constructions. These processes are integral to internalizing speech and constitute components of the human communication system. Barkley (1994) proposed a unifying theory of speech requirements and cognitive abilities. He suggested that diminished control of inhibition interferes with memory, reflection and planning, and is manifested in a slower attainment of internal speech, so that thoughts and emotions...