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  » Types of Intelligence

  » Intelligence, Heredity, and Environment
        Evidence for Nature
        Evidence for Nurture
        Comments on Research

  » Neuropsychological Testing
        Normal Intelligence
        Abnormal Examination             and Brain Trauma

  » Spectroscopy Data

  » Disorders related to Intelligence

  »  Gender Differences
       Self-Estimated              Intelligence
       Anatomical Differences
       Gray vs. White Matter

  » Artificial Intelligence
        A Timeline of AI
        Ancient History of AI
        Modern History of AI
        The Future of AI

  » Age and Intelligence
        Areas of Function
        Effects of Lesions

  » References

Evidence for ‘Nature’

In this section, our goal is to mention some studies which provide evidence in favor of the ‘hereditary’ nature of intelligence. Recall that the ‘nature’ side of the debate argues that a person maintains his mental ability only based on what he is born with genetically. Moreover, defending this side of the debate exclusively would be establishing that a person’s environment plays no role in determining his mental ability.

Scientists have known for years that traits such as eye color and hair color are determined by specific genes encoded in each human cell. In a way, the ‘nature’ theory takes things a step further to say that more abstract traits, including intelligence, personality, aggression, and sexual orientation, are also encoded in an individual's DNA. There are some very obvious reasons for an individual to be convinced that genetics play a large part in a person’s intelligence. When considering the biology of heredity, it is obvious that genes provide humans with their own physical equipment, which is in essence, their basis. Genes and chromosomes are passed on from each generation to the next. Therefore, without heredity, humans would have nothing to pass on biologically to their offspring; hence the environmentalist idea of genetics being purposeless is clearly incorrect.

www.micro.utexas.edu/.../ genetics/genetics.html

The hypothesis about the hereditary nature of any trait, including intelligence, can be tested thanks to certain events that occur naturally, such as the birth of monozygotic/identical twins, that is, twins born from the same cell (ovule). It was mentioned earlier that Francis Galton primarily used twin studies to collect evidence when hypothesizing his hereditarian explanation of the phenomenon of intelligence.  First used by Galton, twin studies have always been the major source of collecting evidence by the investigators of the ‘nature’ camp. These studies are rendered on sets of twins; these include both identical twins and fraternal twins. As noted earlier, they are conducted to determine the comparative influence of heritability and environment. In fact, there are largely three types of studies (including twin studies) that you will regularly see in the research area of hereditarians:
  • Family studies. Children share 50% their genes with each of their parents.

  • Twin Studies. As we saw this proves an interesting area of research, as there is a tendency to compare variables among Monozygotic (identical) twins who share 100 percent of their genes, and dizogotic (fraternal) twins who share 50 percent of their genes (the same percentage as non-twin siblings).
  • Adoption Studies. The influence of the environment and genes is often compared in adoption studies. Therefore variables are often compared between siblings or twins reared apart, to examine the relative influence of genes and the environment. For example, if two twins show similar behaviors, despite being raised in different environments, this may suggest that genes may be important in that behavior.

The motivation of studies involving fraternal twins is; if genetics didn't play a part, then fraternal twins, reared under the same conditions, would be alike by means of their intelligence and other traits, regardless of differences in their genes. But, while studies making use of IQ tests show they do more closely resemble each other than do non-twin brothers and sisters, they also show these same striking similarities when reared apart. This shows that genetics play a major role in human intelligence. A similar idea/logic can be seen in studies involving identical twins: In the case of identical twins, both twins have exactly the same hereditary load, so that differences between them can only be attributed to environmental factors. In general, in the case of intelligence tests, it can be stated that, as indicated by several researches, genetic inheritance is responsible of about 50% of a child’s variation with respect to the average, but this value increases with age. Identical twins show a higher degree of correlation in their scores, even when they are reared apart. It can then be stated, therefore, that heredity has a great influence in the intelligence of a person

As noted earlier, adoption studies form another set of research tool for the investigators of the ‘nature’ camp; yet, they are also used by the ‘nurture’ camp. Child adoptions are beneficial for the investigators of the ‘nature’ camp as they give the chance to see the effects of bringing up, in the same family, children with different inheritance, as well as bringing up children with the same inheritance in different families. Of particular interest to the study of hereditary traits are the cases of monozygotic twins that are reared apart. Even identical twins that are reared apart have more similar IQ's than fraternal twins that are reared together. Hereditarians claimed this as powerful support for genetic influences on intelligence.

Another piece of evidence in favor of hereditarian view of intelligence comes from cases where the intelligence levels of adopted children and biological sons of the same family are compared. On the contrary of what was found in the case of identical twins that are reared apart, there is no significance relation between the intelligence of adopted children and that of the biological sons of the same family, although all of these kids grew up in the same environment (i.e. with same education and economic condition). According to this last finding, it can then be stated, again, that not environment but heredity has a great influence in the intelligence of a person.

Below is the figure which summarizes the results/conclusions given above, and much more. The data represented in the figure is taken from a study conducted by Jensen (1969): ‘Concept of Heritability’, as an argument for the heritability of intelligence.

In general, in behavior genetic research, researchers would use the data to compute heritability estimates (h2), which are estimates of the proportion of variance accounted for by genetic factors. Jensen, in this study, found that 80% variance between IQ and individuals can be explained by genetics. The math/statistics part of the research process is left out for the sake of simplicity here. However, it is important to mention that the two sources of variation in a trait can be separated into independent genetic (G) and environmental (E) components that add to l00% of the variance that is accounted for. On the basis of this fact, the heritability coefficient (h2) can be subtracted from 100% to yield an estimate of the environmental contribution to variance. Therefore, if we computed that if genetics accounted for 25% of the variance for a personality trait, we would estimate that the environmental factors account for 75% of the variance.