Is Intelligence Primarily Inherited
Intelligence has been defined as problem-solving skills and the ability to learn from and adapt to life’s everyday experiences (Santrock, 2008). Whether such skills and abilities are attributable to genetic or hereditary influences (nature) or experiential or environment influences (nurture) is debatable. The genetic influence is strongly supported in the literature, and it has been suggested that as much as 70% of an individual’s intelligence might be inherited. On the other hand, there is strong support for the impact of environmental factors such as the home, school and mass media that also shape intelligence. The studies seem to be inconclusive and limitations have been identified in them, thus our position on this issue is that inheritance is largely responsible for intelligence but the environment is crucial to its development.
Support for the inheritability of intelligence is provided by studies of identical twins, studies of adopted children and experiments with rats. On the other hand there are studies that support the impact of environmental influences on intelligence such as the quality of the home, the impact of schooling and the changes in IQ scores within a relatively short time span. An analysis of these studies show that each has its strengths, flaws and limitations and further research is needed before any conclusive remarks can be made on the issue of the primary influence on intelligence.
In the 1960s Arthur Jensen was one of the main proponents that intelligence was primarily inherited and that environment played only a minimal role in the development of intelligence (Santrock, 2008). Much of the research in support of this theory focused on identical twins. The reasoning behind twin studies was that that siblings IQ scores become more similar as the siblings become more similar genetically and that IQ scores and patterns of developmental change in IQ are more alike for identical twins than for fraternal twins. Jensen identified a very high positive correlation (.82) in the IQ scores among twins. Subsequent studies of identical twins who were reared apart and tested decades later supported Jensen’s theory when it was found that such twins had similar IQ scores and also similar life experiences, e.g., the Jim twins and the giggle sisters.
Support for the inheritance of intelligence was also highlighted in the studies of adopted children. Correlations were computed in the Colorado Adoption Project which included adopted children as well as their biological and foster parents. At every age the correlation between the child’s IQ and that of the biological parent was greater than the correlation between the child’s IQ and that of the foster parents’.
In the study of rats, intelligent rats were bred and dull rats were bred. Each group reproduced intelligent and dull rats respectively. However, when the intelligent and dull rats were interbred the off springs demonstrated signs of both intelligence and dullness.
In support of the environmental factors on intelligence the home has been highlighted. Children with high test scores seem to come from homes that are well organized and have plenty of appropriate play materials. Research has also indicated a correlation between children’s IQ and how much the parents communicated with them in their first three years. The more parents communicated with their children the higher was the child’s IQ score (Hart & Risley, 1995 in Santrock, 2008).
Another environmental influence on intelligence is the school. Studies have shown that students who were deprived of formal education had lower intelligence than students who were not (Ceci & Gilstrap, 2000 in Santrock). Another possible effect of education is the rise in IQ score worldwide (Flynn, 1999 in Santrock) e.g., more people today are classified with higher IQs on tests such as the Stanford-Binet. Within a relatively short time, approximately 25 years, the number of people who score at the superior intelligence level has increased from 3% to 25%. Researchers conclude that 25 years is too short a period to have such significant genetic changes and that the changes in the scores are due to environmental factors such as better education and greater exposure to information such as mass media and advances in technology, especially computer technology and the Internet.
Further support for the environmental impact on intelligence can be found in the intervention programs such as Project Head Start and the Abecedarian Project. The former teaches preschool youngsters basic school readiness skills and social skills and offer guidance to parents which allegedly leads to higher test scores. The Abecedarian Project conducted by Craig Ramey and associates provided a study in which intervention occurred in 111 low income families. The study claimed as much as an 80 points increase in the child’s IQ as compared to the parent’s and grandparent’s IQ score, suggesting that IQ score can be increased by proper intervention strategies.
Limitations have been identified in the studies. For example, in the twin studies no consideration was given to the prenatal experience. Studies in reading, hearing and taste seem to suggest that the fetus, especially in the final two months, is aware of the environment. Thus, perhaps the commonality in experience, e.g., with the Jim twin’s commonality with names of wives, sons and dogs may be attributable to some experience within the prenatal stage. Similarly, the experience with the giggle girls where each experienced suffocation might have been a birth experience, perhaps the doctor’s use of forceps in delivery.
We do not have all the information to make firm conclusions. Further studies need to be done perhaps in the common prenatal experience. Heredity does influence intelligence but it is not the sole determinant of it. Genes and environment influence each other to shape the individual’s level of intelligence.