 When the interaction between two variables is labyrinthine it is meaningless to ascribe percentage values to the contribution made by either. And if this is true for just one gene interacting with one environmental variable, the argument must hold with even greater force for something as complex and multifactorial as human intelligence. Ironically, the IQ evangelists (such as Arthur Jensen, William Shockley, Richard Hernstein, and Charles Murray) use the heritability of IQ itself (sometimes called "general intelligence" or "little g") to argue that intelligence is a single measurable trait. This would be roughly analogous to saying that general health is one thing just because life span has a strong heritable component that can be expressed as a single number--age! No medical student who believed in "general health" as a monolithic entity would get very far in medical school or be allowed to become a physician--and rightly so--and yet whole careers in psychology and political movements have been built on the equally absurd belief in single measurable general intelligence.
V. S. Ramachandran
Book: The Tell-Tale Brain: A neuroscientist's quest for what makes us human. (2011). Norton; p 170-1
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