Is intelligence genetically determined?
INTELLIGENCE IS IMPORTANT TO EXPLORE AS INDIVIDUAL DIFFERENCES CAN PARTLY DEFINE A PERSON IN SOCIETY (SHACKLETON & FLETCHER, 1984). INTELLIGENCE
HAS BEEN THEORISED IN DIMENSIONS OF GENERAL AND SPECIFIC INTELLIGENCE (SPEARMAN, 1927), MULTIPLE PRIMARY ABILITIES (THURSTONE, 1938), AND
HIERARCHICAL MODELS SUCH AS CARROLL (1993). MALTBY, DAY & MACASKILL (2010) SUGGEST A GOOD DEFINITION OF INTELLIGENCE (KNOWN AS IQ) INVOLVES
ENVIRONMENTAL ADAPTION, BASIC MENTAL PROCESSING, PROBLEM SOLVING, DECISION MAKING AND REASONING AS SOME OF THE CORE COMPONENTS. THE
DEFINITIONS AND DISCUSSIONS SURROUNDING THE CONCEPT AND MEASURABILITY OF INTELLIGENCE IS A TOPIC ON IT’S OWN THUS WILL NOT BE CONSIDERED IN
FURTHER DETAIL. NORMAL DISTRIBUTIONS OF IQ SCORES DERIVED FROM ACCUMULATED LITERATURE AND IQ TESTS E.G. WECHSLER’S (1994) MEASURE OF ADULT
INTELLIGENCE, HAVE SET THE BOUNDARIES BY WHICH WE NOW MEASURE IQ. USING THESE BENCHMARKS THE ESTIMATED ROLE OF GENETICS AND THE ENVIRONMENT
CAN BE RESEARCHED USING CORRELATIONAL EMPIRICAL RESEARCH, BETWEEN AND WITHIN POPULATIONS . THE MAIN PURPOSE HERE IS TO EVALUATE THE GENETIC
DETERMINATION OF INTELLIGENCE IN THE GENERAL POPULATION, AS THE NATURE VS. NURTURE DEBATE IS HIGHLY POLITICISED DUE TO THE POTENTIAL IMPACT OF
HERITABILITY ON ENVIRONMENTAL OR EDUCATIONAL POLICIES (COOPER, 1998). THE MAIN THEORIST’S TO CONSIDER ARE GALTON’S (1865; 1869; 1874) HEREDITARY
THEORY AND THE INTELLIGENCE CONTROVERSY BY EYSENCK & KAMIN (1981). THREE METHODS SUGGESTED AS APPROPRIATE BY PLOMIN (2004) INCLUDE FAMILY, TWIN
AND ADOPTION STUDIES WHICH WILL BE DISCUSSED IN TERMS OF THE RELIABILITY AND CONTRIBUTIONS TO INTELLIGENCE RESEARCH. RESULTS IN THE FIELD ARE
CONSISTENT ACCORDING TO RIDLEY’S (1999) REPRESENTATION (SEE FIGURE 2.) AND SHOW STRONG EVIDENCE THAT INTELLIGENCE IS TO SOME DEGREE GENETICALLY
DETERMINED (BETWEEN 40% – 80%), THE QUESTION REMAINING IS THE EXTENT TO WHICH GENETICS INFLUENCE IQ.
Gillie (1977) found many separated twins rarely apart. The separation criteria of twins and the scope of shared environments has been criticised.
Kamin (1977) identified two major flaws in adoption studies, children are carefully homed may be selectively placed.
Flynn (1987; 1999) favours environmental factors for
IQ increases suggesting genetics would produce lower IQ’s.
Mackintosh (1998) suggest assortative mating or
‘like attracts like’ is important to consider from an environmental and genetic perspective.
Galton (1865) theorised intelligence as an evolutionary and hereditary concept that is biologically passed from parent to child. Galton (1869) observed the greatness and ancestry in obituaries finding greatness appeared to run in families. He estimated 1st degree relatives (siblings, parents) share up to 50% of their genes and 2nd degree relatives share 25% of their genes (aunts). Results imply the stronger the genetic relation the less variance in greatness strongly suggesting the heritability of intelligence. Galton
(1874) began to recognise that intelligence as simply inherited is an oversimplified view and began to debate the role of nature and nurture.
•Kamin (1977) denies heritability viewing environmental influences contribute 100% to the variance in IQ. Plomin &
Bergeman, (1991), among others accept there is always a genetic aspect. In Eysenck versus Kamin (1981), Eysenck estimated heritability of IQ as high as 80% attributing 20% to environmental contributions. Rawles (1981) saw
Eysenck as an a interactionist which Shackleton & Fletcher
(1984) suggest most theorist’s now agree with. The debate now is the role of each and estimated influence.
•Environmental influences are extensive and much harder to explore as Bouchard & Segal (1985) found 21 factors alone and Neisser et al., (1996) identified 4 overall categories (see figure 2). The most comprehensive framework theorising genetic and environment interactions was produced by Bouchard