Intelligence is defined as the ability to acquire knowledge, to think and reason effectively, and to deal adaptively with the environment (Passer and Smith, 2007, p. 313). Intelligence varies from each individual and there are multiple forms of intelligence. Harvard Psychologist Howard Gardner identified nine distinct intelligences. These consist of Verbal-linguistic, Logical-mathematical, Spatial-visual, Bodily-kinesthetic, Musical, Interpersonal, Intrapersonal, Naturalist and Existential inteligence. This essay will explore the influence of Nature (genetic heritage) and Nurture (environmental and experiences) on overall intelligence. This question has been a long source of controversy and debate and there are compelling arguments about how genes and our environment influence intelligence (“How Does Heredity and Environment Influence Intelligence?”, n.d.). The results of which has important social as well as scientific consequences (Passer and Smith, 2007, p.332).
Psychometric testing is widely used as a tool to measure an individual’s intelligence, IQ testing being one of the most popular forms of testing. Prior to psychometric testing, Intelligence or mental ability and the varying difference from individuals had often been discussed and written about by man. The great philosopher Plato (c. 428-347 BC) wrote in The Republic “those whose natures were fashioned of gold were made to rule; those of silver were made to be auxiliaries; and those formed from brass or iron were made to be husbandmen and craftsmen” (Schacter, D., Gilbert, D. and Wegner, D., 2012, p. 340). By this he suggested rulers should be thoroughly tested for their suitability to their roles (Schacter et al. 2012, p. 341). So for many years intelligence and its measurement has been an important topic for mankind and plays an important role in the development and direction of an individual’s life.
Francis Galton was a pioneer in the study of eugenics and intelligence and his book Hereditary Genius (1869) was the first statistical study of genius. He conceptualized general mental ability as a largely heritable trait with, like some physical traits, a normal distribution in the population (Schacter et al. 2012, p. 341). As no tests of intelligence were available to him he relied upon university and military examination results (Schacter et al. 2012, p. 341). He did later carry out genealogical testing of wealthy families and this data was still being analysed long after his death (Schacter et al. 2012, p. 341). Whilst Galton believed intelligence mainly to be hereditary he did only study the upper classes. Which due to their socioeconomic status allowed them access to education which must also be considered as a factor in the results.
The relevance of genes is important when comparing the intelligence scores of people who share genes and those who don’t. Siblings will share similar intelligence scores as they share similar genes. Identical twins for example share 100% of their genes whereas fraternal twins only share 50%. It has been shown that test scores of identical twins are higher when raised in the same household as opposed to when they are raised in separate households; r=0.86 compared to r=0.78 (Plomin et al., 2001a cited in Schacter et al. 2012, p. 357). People who share all their genes have very similar test scores regardless of whether they share the same experiences (Schacter et al. 2012, p. 358).
The Minnesota Twin Family Study (or MTFS) carried out by Bouchard and Lyyken was a longitudinal study of genetic and environmental influences on the development of psychological traits on monozygotic twins who were reared apart against monozygotic twins who were reared together. In this study the participants completed approximately 50 hours of testing which included two intelligence tests.
Hock (2009) stated in their findings that intelligence is for the most part determined by nature (70% of the variation in intelligence appears to be due to genetic influence).
However they did state that these findings did not imply that IQ cannot be increased. A survey covering 14 countries has shown that the average IQ test score has increased in recent years. The present findings, therefore, do not define or limit what might be conceivably achieved in an optimal environment. The authors are stating that 70% of the variation in IQ is due to naturally occurring genetic variation, 30% of the variation remains subject to increases or decreases due to environmental influences. These influences include many that are well known, such as education, family setting, toxic substances and socioeconomic status (Hock, 2009, p.24)
As seen above genetics are not the only contributing factor in the intelligence debate. Environmental influences also come into play. When comparing identical twins raised together to identical twins that were raised apart the correlation is higher for those raised together. As is the same for other types of siblings raised together and raised apart (Schacter et al. 2012, p. 333). This shows that experience is an influence on the individual’s intelligence. Home environment has a big impact on mental ability; findings show that siblings raised together are more similar than those that are raised apart (Bouchard & McGue 1981; Bouchared et al., 1990; Scarr, 1992, cited in Passer et al. 2007 p.333). Siblings who grow up in the same house generally go to the same schools, read many of the same books and share many of the same friends. Thus, the similarity of their intelligence may reflect the similarity of their genes or it may reflect the similarity of their experiences (Schacter et al. 2012, p.357). Also unrelated Adoptive children fared better when raised together in the same home by an adoptive parent as opposed to those being raised by an adoptive parent by themselves.
Education is a factor on the influence nature can have on intelligence. Many studies have shown that school attendance can raise IQ and the lack of attendance lowers it (Passer and Smith, 2007, p.334). As Mayer (2000) observed gains in intelligence are most likely to occur when certain educational goals are met. These include helping students to learn specific cognitive skills and problem solving approaches; encourage students to critically think and apply course content; and teaching students learning skills. (Passer and Smith, 2007, p.334). This is a more forward thinking approach to education and moves away from the traditional emphasis of repetition and learning of facts. This new style of education could also be a contributor to the rise in IQ.
The Flynn effect is another indicator to the influence of environment over intelligence. It refers to the rise in the average intelligent test score, about 0.3% each year. The average person today scores about 15 IQ points higher than the average person did 50 years ago (Dickens and Flynn, 2001; Flynn, 1984 cited in Schacter et al. 2012, p.365). The increase seems to be occurring to the same degree for both men and women and for different ethnic groups (Truscott & Frank, 2001 as cited in Passer et al. 2007, p. 329). This has been accredited to an all-round increase in our standards of living, better nutrition, better parenting, better schooling and better test-taking ability (Schacter et al. 2012,, p.365). Richer and more complex learning environments that require more complex coping may have increased mental abilities. Likewise, technological advances may have helped shape the kinds of analytical and abstract reasoning skills that boost performance on intelligence tests (Passer and Smith, 2007, p. 329)
Intervention in deprived areas has also shown an increase on IQ (Passer and Smith, 2007, p.334). Children removed from deprived environments and placed in middle or upper class homes seen an increase of 10 to 12 points (scar & Weinberg, 1977; Schiff & Lewontin, 1986 as cited in Passer et al. 2007, p. 334). When children were left in deprived environments their IQ did not improve and they deteriorated intellectually over time (Serpell, 2000 as cited in Passer and Smith, 2007, p.334).
In conclusion there are many varying factors which can have an impact on an individual’s intelligence which can be classified as either environmental or genetic. Genetics certainly plays an important role as we have seen with IQ levels of twins, identical and fraternal being higher than those who were reared apart. There is also a stronger correlation of IQ between parents and siblings. Environmental influences from family and their exposure to education and stimulating environments also have an impact. It is hard to rule one out from the other but both nature and nurture are key influences on intelligence. The many studies of monozygotic twins have been an invaluable source of data to this debate and provide the most useful research. These studies show both are influences but nature seems to come out on top as being the biggest influencer on intelligence. As per Buchard and Lykken’s research stated that human characteristics are determined by some combination of genetic and environmental influences. When the environment exerts less influence, differences must be attributed more to genes. When genetic influences are weaker than the difference is more inclined to the environment (Hock, 2009, p.24). No doubt the nature v nurture debate will still continue to divide opinion for years to come.