What is intelligence? If this question is asked to many different people, the possible answers could be the level of understanding of complicated opinions, accustoming to the circumstances, learning and reasoning skills, capability of solving problems etc. There are a lot of definitions for intelligence; however no conceptualization has yet established as a fully satisfying definition (Neisser, Boodoo, Bouchard, Boykin, Ceci, Halpern, Loehlin, Perloff, Stenberg, & Urbina, 1996). As a matter of fact, when 24 well-known theorists were asked to define intelligence, they gave 24 different definitions (Sternberg & Detterman,as cited in Neisser et al., 1996). Hence, in order to prevent misunderstandings, this essay uses the word intelligent in the meaning of general cognitive ability.
Neisser et al.(1996) stated that some social and biological variables have some influences on intelligence. People vary from one another in their intelligences. What causes this variation? Mostly genetic factors or mostly environmental factors or both in equal weight? The proportion of influence on intelligence can be explained by IQ studies which will be discussed in further analysis. This paper aims to research the extent of the influence of environmental and genetic factors on intelligence by benefiting past studies and findings. The idea that genetic factors are slightly more influent on intelligence than environmental factors, is supported with evidences and explanations.
How do environmental factors affect intelligence? The social variables, which include occupation and schooling, have some influence on intelligence. Firstly, work environment may influence the intelligence of their workers. Kohn and Schooler (1973) had interviewed some 3,000 men who were farmers, managers, machinists, porters, etc. They argued that, having more complicated jobs provides more intellectual flexibility. Although this discussion was not fully clear in their study, it remains as a reasonable approach (Neisser et al., 1996). Secondly, Neisser et al. (1996) stated that, it is certain that schools help students to increase their intellectual skills in different extents such as systematic problem-solving, abstract thinking, categorization etc. The reason is that, intelligent tests measure the same skills with the schools. Schools affect intelligence in teaching different information and improving children 's intellectual skills. Not attending to school or attending to very poor schools decreases the score of intelligence. Pre-school programs has some positive effects on intelligence; however this effect is not permanent (Neisser et al., 1996).
IQ can be increased by learning better way of thinking, in other words, by environmental factors. Teaching intelligence can be teaching patterns in science, some heuristics in mathematics, some strategies, or metacognition. Perkins and Grotzer (1997) stated that training can help people to develop reflective intelligence with significant generality and persistence. This can be an evidence of the influence of environmental factors on IQ.
The Flynn effect can be considered as an environmental factor which affects intelligence. Flynn effect means that worldwide mean IQ increases over time (Flynn, 1999). This increase may derived from complex environment, cultural variations, improved education, or from some other reason which is not known yet. Wheeler (1942) stated that, intelligence tests were given to three thousand two hundred fifty-two children in forty mountain schools of East Tennessee, and the results are compared with a similar study made ten years ago. As a result, the IQ of an average mountain child has increased 10 points. This is an evidence of the influence of environmental factors on IQ.
Socioeconomic status influences IQ by either genetic or environmental factors. Turkheimer and colleagues (2003) argued that the proportions of IQ variance attributable to genes and environment vary with socioeconomic status. They found that among the families which have higher SES(rich), genetic factors are crucial. In contrast, among the families which have lower SES(poor), environmental factors are crucial (Turkheimer, Haley, Waldron, D 'Onofrio, &Gottesman, 2003). The suggestion can be derived from this experiment can be that, everyone is given some intellectual capacity by genes. However, environmental factors are necessary to discover or improve it.
The familial studies are crucial to understand the extent of genetic factors ' influence on intelligence. Bourchard and McGue (1981) stated that review of 111 familial studies of intelligence is made, and average correlations for given familial relationships are calculated. The result is that, if two family members have higher proportion of genes in common, then the average correlation between their IQ 's is higher. This piece of information is an evidence for the influence of genetic factors on IQ. In another study, intelligence has been found to be more highly correlated between monozygotic twins than between dizygotic twins. For example, the average intelligence correlation of monozygotic twins who reared together was 0.86. This high correlation could be related to genetic since they share the same genes, also it could be related to environment since they reared together. When they reared apart, the correlation became 0.75, still very high (Plomin & Spinath, 2004). This result suggests that genetic is definitely more important than environment. Another study (Plomin, Fulker, Corley, & DeFries, 1997) analyzed 245 adopted children, their biological and adoptive parents. The cognitive abilities, which are verbal, spatial, memory and speed of processing abilities of children, biological and adoptive parents are calculated through different IQ tests. The result suggests that while children grows up and gets older, the correlations between their cognitive abilities and their biological parents ' cognitive abilities are likely to increase from 0.1 at age 2 to 0.3 at age 16. In contrast, the correlations between their cognitive abilities and their adoptive parents ' cognitive abilities are likely to converge to 0. It is noted that the heritability score is 0.56 for general cognitive ability whereas the environmental score is 0.01. It can be deducted that despite the fact of environmental factors have an important role in development of intelligence, actually genetic factors are more important.
Plomin and Bergeman suggested the idea of nature of nurture (Plomin & Spinath, 2004). This idea considers that genetic factors may determine the individual 's behaviors and the relation with his environment. This situation results in a correlation between genetics and environment (Kendler & Eaves, as cited in Plomin & Spinath, 2004). Despite the fact of environment has a significant role on intelligence, some of its influences may be caused by genetic factors. Therefore, this can be another suggestion to support that genetic factors are more important on intelligence.
Genetic and environmental factors are evaluated from several aspects by benefiting past studies and explanations. The influence of environmental factors is explained through mentioning the subjects of social variables,teaching intelligence, Flynn effect, and SES. The influence of genetic factors is explained through expressing some past familial studies. As a result, it can be deducted that, although environmental factors which can improve intelligence, have an important influence on intelligence, actually genetic factors are slightly more effective on intelligence than environmental factors.
References
Bourchard T.J.,and McGue M., (1981).Familial Studies of Intelligence: A Review. Science, 212. 1055-1059
Flynn J.R., (1999). Searching for Justice: The discovery of IQ gains over time.American Psychologist, 54. 5-20
Gleitman H., Reisberg D., and Gross J. (2007).Psychology.London.W. W. Norton & Company.
Neisser U.,Boodoo G.,Bouchard T.J., Boykin A.W., Ceci S.J., Halpern D.F., Loehlin J.C., Perloff R. and Sternberg R.J., Urbina S., (1996).Intelligence: Knowns and Unknowns.American Psychologist 51, 77-101
Perkins D.N. and Grotzer T.A., (1997).Teaching Intelligence.American Psychologist, 52. 1125-1133
Plomin R., Fulker D.W., Corley R., & DeFries J.C., (1997).Nature, nurture, and cognitive development from 1 to 16 years: A Parent-offspring Adoption Study.Psychological Science, 8. 442-447
Plomin R., and Spinath F.M., (2004).Intelligence: Genetics, Genes and Genomics.Journal of Personality and Social Psychology, 86. 112-129
Turkheimer E., Haley A., Waldron M., D 'Onofrio B., &Gottesman I.I., (2003). Socioeconomic status modifies heritability of IQ in young children.Psychological Science, 14. 623-628
Wheeler L., (1942).A comparative study of the intelligence of East Tennessee mountain children.The Journal of Educational Psychology, 33.
References: Bourchard T.J.,and McGue M., (1981).Familial Studies of Intelligence: A Review. Science, 212. 1055-1059 Flynn J.R., (1999). Searching for Justice: The discovery of IQ gains over time.American Psychologist, 54. 5-20 Gleitman H., Reisberg D., and Gross J. (2007).Psychology.London.W. W. Norton & Company. Neisser U.,Boodoo G.,Bouchard T.J., Boykin A.W., Ceci S.J., Halpern D.F., Loehlin J.C., Perloff R. and Sternberg R.J., Urbina S., (1996).Intelligence: Knowns and Unknowns.American Psychologist 51, 77-101 Perkins D.N. and Grotzer T.A., (1997).Teaching Intelligence.American Psychologist, 52. 1125-1133 Plomin R., Fulker D.W., Corley R., & DeFries J.C., (1997).Nature, nurture, and cognitive development from 1 to 16 years: A Parent-offspring Adoption Study.Psychological Science, 8. 442-447 Plomin R., and Spinath F.M., (2004).Intelligence: Genetics, Genes and Genomics.Journal of Personality and Social Psychology, 86. 112-129 Turkheimer E., Haley A., Waldron M., D 'Onofrio B., &Gottesman I.I., (2003). Socioeconomic status modifies heritability of IQ in young children.Psychological Science, 14. 623-628 Wheeler L., (1942).A comparative study of the intelligence of East Tennessee mountain children.The Journal of Educational Psychology, 33.
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