Samuel Johnson said 'Such is the delight of mental superiority that none on whom nature or study have conferred it would purchase the gifts of fortune by its loss'; for the modern reader this captures the importance of both nature and nurture, and also touches on, perhaps with slight discomfort, a contemporaneous social sensitivity to inferred elitism. An expert review by Plomin & Deary1 notes how intelligence is one of the most heritable behavioural traits as well as being among the best predictors of occupation, mental and physical health, and mortality outcomes. Among what they label the special findings of the genetics of intelligence, they describe how the heritability of intelligence increases dramatically from approximately 20% in infancy to 60-80% in later adulthood: the authors posit that although individuals clearly have genetic stability, they select, modify and create environments that are correlated with their abilities, resulting in a so-called genetic amplification with time. Assortative mating - having a partner with similar traits to oneself - is greater for intelligence than for other behavioural traits such as personality and psychopathology, or physical traits such as weight and height. Intelligence has a normal population distribution: the authors' statement that the exceptional end of performance is a model for studying 'positive genetics' is reasoned and reasonable, but sharply cuts back to the image problem this field historically has had, echoing Johnson's 'mental superiority'. Clear heads, as well as intelligence, are needed.
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