Biological Inheritance of Intelligence
Various studies on the biological inheritance of intelligence have been conducted by doctors, scientists, and geneticists, and have been published in several academic journals both online and offline. These studies all say one thing that somehow nature is partly, or even mostly, responsible for determining intelligence.
The main aim of this essay is to provide evidence for the aforementioned statements on the role of nature in shaping intelligence. Specifically, this essay will present the latest studies on the heritability of intelligence as well as evidence from twin studies. Moreover, this essay seeks to explain that even some studies whose findings do not exactly link nature and intelligence may also point out to such claims.
Evidence from Twin Studies
Twin studies provide the most qualified proof for the role of nature in determining intelligence. The reason is that, ideally, twins have an identical genetic make-up so that a phenomenon in one of the twins that is exhibited by the other twin is already a strong proof for the assumption that such a phenomenon is inherited. Various current twin studies will be presented and criticized in the following paragraphs to fulfill the aims of the essay.
Jacobs, Os, Derom, Thiery (2007) found out that the fact that the twins share the same placenta and have a common blood circulation possess high heritability estimates at 82 for the verbal IQ, 72 for vocabulary IQ, 73 for the performance IQ, and 83 for the total IQ. These findings led to the conclusion that general cognitive ability (represented by the symbol g) is one of the most inherited dimensions of human behavior (Jacobs et al., 2007). The results of this confirmed previous studies like those of Wainwright et al. (2005) which found that genetic g accounted for 32 to 72 of the phenotypic variances, and explained 56 of variation in Verbal IQ and 42 for Performance IQ in the academic skills scores. The results suggest that general academic ability is more likely a result of genetic influences rather than common environmental influences. Another earlier study conducted by Singer et al. (2004) found out that general intelligence had a high heritability estimate of 74. In the same year, Price et al. (2004) found that this general intelligence is most pronounced by middle childhood and that the same genetic factors are responsible for individual differences when it comes to verbal and nonverbal abilities. Still, a study in Russia in the same year explored the genetic and environmental influences in cognitive abilities (Zyrianova, 2004) and found that cognitive characteristics are for the most part under genetic influences with estimates of heritability at 53.
These five aforementioned studies suggest a genetic basis for intelligence, specifically the general cognitive ability g. These studies directly investigated the various cognitive and even noncognitive abilities of twins and, through the data acquired from intelligence tests, came up with the conclusion that intelligence has a strong genetic basis.
Other studies, however, did not consider all the cognitive abilities but instead focused on a particular ability such as vocabulary (Van den Berg et al., 2004) and mathematical ability (Ando, 2004) and found the same genetic basis for intelligence. Van Den Berg et al. (2004) found that heritability varied from 50 to as high as 63 for vocabulary test scores from 997 adult twins or siblings of twins. The study also found out that genetic factors are responsible for the stability in vocabulary knowledge over time, where heritability reached 76. Furthermore, the study did not detect any noticeable influence from shared environmental factors, and although the males generally outperformed the females, results were similar for both sexes. Another study in Japan in the same year focused not on vocabulary but on mathematical achievement and was able to specifically underline the particular role that genetic influence has on mathematical ability (Ando, 2004). The study found that mathematical performance is largely an interplay of both additive genetic and shared environmental factors. Through univariate genetic analysis, the study was able to exactly link the two components of mathematical performance verbal cognitive and spatial cognitive ability with the additive genetic factor to explain the former and the common environmental factor to explain the latter. Although the study on mathematical performance found an almost equal influence of both nature and nurture in the development of mathematical abilities, it established the idea that intelligence with numbers is never possible without a substantial amount of genetic factor. The shared environmental factor in the development of mathematical skills is an obvious influence of education and other visible external factors. However, the existence of the additive genetic factor, whose influence is obviously not visible, was ascertained by this study.
Other twin studies focused on specific aspects of intelligence such as memory (Volk et al., 2006 Singer et al., 2005), speed in perceptual discrimination and IQ (Luciano et al., 2004), and academic achievement (Chertkova, 2004). Nevertheless, the same thing was found that intelligence is mostly inherited. A study conducted by Volk et al. (2006) found that free recall of unrelated and categorized words, as well as cued recall of categorized words, was found to be moderately heritable at 55 and vocabulary score was highly heritable at 77. Singer, MacGregor, Cherkas, Spector (2005) found that memory ability has a heritability estimate of 45 which is relatively higher than the shared family environmental influences whose heritability was estimated only between 32 and 33. This study may not have established a strong genetic basis for memory. Nonetheless, it was able to somehow prove that the genes count more than the environment when it comes to shaping memory. Another study involving Australian and Dutch twins tried determining the heritability of the cognitive abilities of perceptual discrimination speed and IQ (Luciano et al., 2004). This particular study showed that genetic effects influenced IQ more than familial environmental effects. A Russian study conducted by Chertkova (2004) focused on the genetic influences in terms of academic achievement and found that achievement in humanitarian subjects or the social sciences were relatively more influenced genetically than achievement in the natural sciences. Based on the results of the study, the heritability of the Russian language was registered at 77 while that of Physics only at 26. Nevertheless the study was able to confirm that at least some of the genetic contribution to school achievement may only reflect the genetic contribution to general cognitive functioning (Chertkova, 2004). It can be seen from these four aforementioned studies as well as in the previously discussed study on mathematical performance that although there is admittedly an environmental factor in shaping cognitive abilities, still the role of the genes in determining intelligence cannot be emphasized more heavily.
There were also a number of twin studies that focused on problems and disorders but yielded results in favor of our thesis that intelligence is biologically inherited. Oliver Plomin (2007), in their longitudinal study on language, cognition and behavior problems, used multivariate genetic analyses which allowed them to conclude that genes are generalists and environments are specialists (Oliver Plomin, 2007). This simply means that similarity in performance across age and within and between abilities and disabilities in learning is largely due to genetic factors. On the other hand, differences as to performance are largely brought about by environmental factors. In another study, Reynolds, Fiske, Fratiglioni, Pedersen, Gatz (2006) explored the heritability of cognitive dysfunction and found out that 35 of the likelihood of ones having cognitive dysfunction is due to heritable influences and the remaining 65 is due to nonfamilial environmental influences. The two aforementioned studies showed that although the environment was a necessary factor, still the genetic remains to be of extreme significance. In the study by Oliver and Plomin, nature and nurture seem to possess an equal influence on learning. On the other hand, Reynolds et al. (2006) emphasizes the relatively more significant influence of the environment compared to genetic factors. Nevertheless, these findings do not at all change the fact that a part of intelligence is biologically inherited.
Other twin studies focused on aspects of behavior that determined intelligence such as extraversion. Pincombe, Luciano, Martin, Wright (2007) found in their study on the heritability of extraversion and its relationship with IQ that all extraversion facets were significantly influenced by genes. The heritability of activity was estimated at 25 while that of warmth was 54. The study also found out that there is a strong correlation between excitement-seeking and IQ and this is solely due to genetic influences. The study attributes such genetic effects to the influence of brain dopamine, a neurotransmitter that is known to have significance on both personality and cognition. The results of this particular study show that since certain aspects of the personality such as extraversion have a strong genetic basis, and that personality and intelligence are usually parallel, it therefore follows that intelligence too is heavily influenced by the genes.
Lastly, there were also twin studies that focused on suspected physical determinants of intelligence such as birthweight (Newcombe et al., 2007 Derom et al., 2004). Newcombe, Milne, Caspi, Poulton, Moffitt (2007) conducted a study based on the hypothesis that birthweight predicts IQ and found that an increase of 1000 g in birthweight was associated with a 3 point increase in the IQ. In an earlier study, Derom, Thiery, Van Gestel, Jacobs, Vlietinck, Fryns (2004) found that this phenomenon of correlation between birthweight and IQ is more pronounced in opposite-sexed twin pairs, especially with a female co-twin who is heavier than her twin brother. The findings of these two studies show that intelligence may indeed be genetically determined at birth and may even be physically indicated by certain physical attributes such as birthweight. The problem with these findings however is that birthweight may be due to several factors such as the volume of body fat or the weight of the bones, thus there is a need for a more in-depth study on what are the specific components or aspects of birthweight that more likely determine or predict intelligence.
Evidence from Family Studies
Family studies also provide some evidence for the genetic basis of intelligence. In a study undertaken by Thomas Bouchard of the University of Minnesota, the correlation between IQs is highest among identical twins and lowest among unrelated people. Identical twins reared together accounted for a percentage correlation 86 in their IQs. Identical twins reared apart accounted for 76 fraternal twins, 55 biological siblings reared together, 47 and unrelated people, 0 (Nature vs Nurture, 2005).
Family studies involving correlation of child-parent verbal ability scores also help establish the claim that intelligence is biologically inherited. The findings of these studies show that the verbal ability scores of children and their birth parents have a relatively higher correlation compared to those between adopted children and their birth parents, and even much higher than those between adopted children and their adoptive parents (Nature vs Nurture, 2005).
Conclusion
The heritability of intelligence has been one of the most highly-debated topics since the advent of neurology and psychoanalysis. However, it was not only until the latter half of the 20th century that twin studies and family studies have actually been conducted to investigate on the matter and to ascertain if the claim is true as well as to determine which part of intelligence was nature and which part was nurture. Twin studies proved the existence of a genetic basis for intelligence through direct experiments with all of the cognitive abilities or a specific ability at a time. Certain cognitive abilities which were equally or unequally influenced by both the genes and the environment still somehow proved the indispensability of the genes as a complement to the environment in the development of intelligence and cognitive abilities. Similarly, twin studies on behavior disorders, extraversion, and physical aspects such as birthweight also pointed out to the same conclusions. Lastly, family studies on IQ correlation among twins and verbal ability scores between children and their parents also proved that intelligence is all in the genes.
Categories:
0 comments:
Post a Comment