Several examples may help clarify the relationships between heredity, environment, and characteristics such as IQ. The first example involves a highly heritable characteristic, height. In this example, a farmer has two fields, one rich in nutrients (field A) and the other barren (field B). The farmer takes seeds from a bag that has considerable genetic variety, plants them in the two fields, and cares for the two fields of crops equally well. After several weeks, the plants are measured. The farmer finds that within field A, some plants are taller than others in the same field. Because all these plants had the same growing environment, the variation could be attributed to the genetic differences in the seeds planted. The same would be the case with the plants in field B.
The farmer also finds differences between the two fields. The plants in field A are taller than the plants in field B, because of the richer soil in which they grew. The difference in the average heights of the plants is attributable to the quality of the growing environment, even though the genetic variation (heritability) within field A may be the same as that within field B. This same principle applies to IQ scores of different human groups.
Taking the example further, the farmer might call a chemist to test the soil. If the chemist was able to determine all the essential missing nutrients, the farmer could add them to the soil in field B for the next season. The second batch of plants would grow larger, with the average height being similar to the average height of plants in field A. Similarly, if one is comparing African Americans and Caucasians—or any racial groups—on a characteristic such as IQ test scores, it is important to understand that unless the groups have equivalent growing environments (social, political, economic, educational, and so on), differences between them cannot be easily traced to heredity.
As another example, one might take a set of identical twins who were born in Chicago, separate them at birth, and place one of the twins in the !Kung desert community in Africa. The life experiences of the twin in Africa would differ significantly from those of his Chicago counterpart because of the differences in diet, climate, and other relevant factors required for existence and survival in the two environments. The twin in Africa would have a different language and number system; drawing and writing would not likely be an important part of daily life. Therefore, if one were to use existing IQ tests, one would have to translate them from English to the !Kung language so that they could be understood. The translation might not truly capture the meaning of all the questions and tasks, which might interfere with the !Kung twin's understanding of what was being asked of him. More problems would arise when the !Kung twin is asked to interpret drawings or to copy figures, because he would not be very familiar with these activities.
It is likely that the !Kung twin would perform poorly on the translated IQ test, because it does not reflect what is emphasized and valued in his society. Rather, it is based on the schooling in society in which the Chicago twin lives. This does not mean that the !Kung twin is less intelligent than his Chicago twin. Similarly, the Chicago twin would do poorly on a test developed from the experience of !Kung culture, because the !Kung test would emphasize skills such as building shelter, finding water, and other activities that are not important for survival in Chicago. In this case, the !Kung test would not adequately measure the ability of the Chicago twin.
Studies done by psychologist Sandra Scarr show that evidence for a genetic basis for racial differences in IQ is far from clear. She looked at the IQ scores of African American children who were born into working-class families but were adopted and raised by white middle-class families. The IQ scores of these children were close to the national average and were almost 10 to 20 points higher than would have been expected had they remained in their birth homes.
Change in children's environments seems to be a critical factor in enhancing their ability to perform on the IQ tests, as seen in the research done by Scarr. Bronfenbrenner found similar results. He examined a dozen studies that looked at early intervention in children's lives; he found that whenever it was possible to change the environment positively, children's scores on IQ tests increased.
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