The Etiology of IQ


Author: Adekola Taylor
November, 2013

Intelligence is the ability to acquire knowledge, ability to apply the knowledge acquired in solving problems and ability to engage in abstract reasoning. The attempt of psychologists in measuring intelligence brought about the concept of IQ. Intelligence Quotient forms an average value of one's intellectual capacity and population reference; it does not calculate one's intelligence. Formerly, Intelligence Quotient (IQ) was a quotient or a ratio: IQ = MA/CA x 100 [CA is the chronological age, MA is the mental age]. It is the score one gets from several standardized tests created to measure intelligence. Nowadays, scores are calibrated against norms of actual population scores (Boeree, 2003).

Intelligence is a potential that does not exist as a separate entity from observable behaviour; it may be considered to exist only to the extent that it manifests itself through behaviour. Over the years, there have been a lot of controversies among the psychologists as to this. Is IQ more determined by the genetic factors or by interaction with the environment factors? As a matter of fact, only few psychologists would deny that both environmental and genetic factors are significant. However there is still a propensity for those who are concerned with the developmental aspects of man to ascribe to the notion that the genetic factors are of more influence than that of the environmental factors. That is to say the "hereditarians" would primarily subscribe to the idea that observed differences in IQ are as a result of differences in the genetic potential while the "environmentalists" would subscribe to the idea that the observed differences in IQ are as a result of differences in the environment.

It is crystal clear that the ability to behave intelligently is increased during the stages of physical development and growth. This affirms the view of the hereditarians that intellectual growth is related to or affected by biological or genetic factors. Other evidence favouring the hereditarian point of view is from the studies with twins. IQ development were assessed in 483 same-sex twin pairs periodically from ages 1 - 16, results indicate an increasing influence of heritability across development and a decreasing influence of shared environment, with large and increasing age to age stability of genetic influences. The results further demonstrate the early stability in etiological influences on IQ and have potential implications for gene-finding efforts (Brant et al., 2009).

The evidence that there are genetic and environmental factors in the etiology of IQ is suggested by two studies: the twin studies and the adoption studies. The twin study involves the comparison of the differences within monozygotic (MZ) twin pairs, who share 100% of their genes, implicating environmental influence only while differences within dizygotic (DZ) twin pairs who share 50% of their genetic material reflect both genetic and environmental influences. In this study, the proportion of phenotypic variance associated with additive genetic factors is estimated by doubling the difference in correlation between the MZTs and DZTs, while the contribution of shared environmental factors is estimated by subtracting the heritability estimate from the MZT correlation, and the contribution of non shared environmental factors and measurement error is estimated by subtracting the MZT correlation from 1.0 (Luciano et al., 2004; Mcgue & Bouchard, 1998).

Adoption studies are also evidence based studies that have also provided evidence for genetic factors in the etiology of IQ. An adoption study determines the degree to which adopted persons take after both their adopted relatives, an indication of shared environment factors and their biological relatives, an indication of genetic factors. In this study, children given up for adoption before the age of 6 months continue to resemble their biological mother in IQ. Two recent American studies suggest that the correlation with the biological parents has been considerably higher than with the adoptive parents (, 2013). In conclusion both the genetic and environmental factors have profound effects in the etiology of IQ.

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Boeree, George (2003).Intelligence and IQ. Retrieved January 25, 2013 from http : //

Brant, Angela M., Haberstick, Brett C., Corley, Robin P., Wadsworth, Sally J., DeFries, John C.& Hewitt John K. (2009). The development etiology of high IQ. Behavior Genetics, 39(4):393 - 405. Retrieved from 1007%2Fs10519-009-9268-x?LI=true

Luciano Michelle, Wright, Margaret J. & Martin, Nicholas G. (2004).Exploring the etiology of the association between birthweight and IQ in an adolescent twin sampleTwin Research, 7(1):62 - 71. Retrieved from http://genepi.qimr contents/p/staff/CV368.pdf

Mcgue Matt, Bouchard Thomas J. (1998). Genetic and environmental influences on human behavioral differences.Annu.Rev.Neurosci, (21):1 - 24. Retrieved from (2013). Chapter 7 Review of the evidence: intelligence.Genetic and Human Behaviour: The Ethical Context,pp 69 - 76. Retrieved from iour%20Chapter%207%20-%20Review%20of%20the%20evidence%20intelligence .pdf

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