Genetic and Environmental Factors of Intelligence
One of the most interesting and controversial areas in behavioral genetics, human intelligence is currently assumed to be subject to both genetic and environmental influences.
While this assumption is accepted by a majority of geneticists and behavioral scientists, there is great disagreement on the degree of influence each contributes. Arguments for environmental influences are compelling; at the same time there is growing evidence that genetic influence on intelligence is significant and substantial (Eyesenck, 1998; Mackintosh, 1998; Plomin, 1994; Steen, 1996). The purpose of this paper is to explore the question: "How is intelligence influenced by heredity and environment?"
What is Intelligence?
It is often difficult to remember that intelligence is purely a social construct, and as such is limited to operational definitions. Binet & Simon (1905, as cited in Mackintosh) defined it purely in terms of mental ability: "the ability to judge well, to comprehend well, to reason well." Wechsler (1944, as cited in Mackintosh) added behavioral factors: "the aggregate or global capacity of the individual to act purposefully, to think rationally, and to deal effectively with the environment." Sternberg (1985) synthesizes the previous definitions, defining intelligence as "the mental capacity of emitting contextually appropriate behavior at those regions in the experiential continuum that involve response to novelty or automatization of information processing as a function of metacomponents, performance components, and knowledge acquisition components." Gardner (1993) took the definition to a societal level, as "the ability or skill to solve problems or to fashion products which are valued within a cultural setting."
Measurement of Intelligence: IQ Tests
Alfred Binet developed the first IQ tests to identify children who would not benefit from public school instruction. His concept involved the idea that certain mental tasks are appropriate to certain ages, such as the ability to recite the names of the months: while expected of a ten year old, such ability would be rare in a three year old. Binet quantified intelligence as the Intelligence Quotient (IQ): the ratio of mental age to chronological age, multiplied by 100. Reasoning that low intelligence stemmed from improper development, Binet envisioned the test as a first step in treatment: a diagnostic instrument used to detect children with inadequate intelligence in order to treat them using "mental orthopedics."
Binet argued forcefully against the idea that intelligence is fixed or innate: "We must protest and react against this brutal pessimism (Lewontin, Rose, & Kamen, 1984)." However, those who translated his test into English tended to disagree, arguing that the test measured an innate and immutable, genetically inherited characteristic. After Binet's death in 1911, the Galtonian eugenicists assumed control, shifting the focus...