Is IQ genetically pre-determined or is IQ formed and acquired by the environment? Scientists have long been intrigued by the origins of IQ, and in particular whether IQ is genetically inherited or whether a high IQ is the result of a stimulating environment. If IQ is in fact explained by environmental factors (i.e. nurture rather than nature) then this provides validation to millions of parents globally who would like to believe that all the time, effort and money that they are investing in creating stimulating environments for their children will pay off with higher IQs, and that greater education and smarts will help their children to do better in a competitive world. If IQ is found to be genetic however, then this calls into question the usefulness of head start programs for children and equality-driven movements that seek proportional representation of various ethnic groups in the workplace.
So is IQ genetic or environmental in origin? Several studies have helped shed light on the issue. The short answer is that genetics explain over 50% of variations in IQs, with the balance explained by environmental factors. That said, genetics are more likely to be the dominant explanatory factor and I will elaborate why. IQ or intelligence needs to be decomposed into its two principal components of (1) crystallized (Gc) vs (2) fluid intelligence (Gf) before re-visiting the question of whether IQ is genetic or environmental.
Several interesting studies have examined the IQs of adopted children, comparing these against the IQs of their adoptive parents and that of their biological parents. In their 1978 Texas adoptions study, Scarr & Weinberg measured that adopted eight year olds’ IQs had a coefficient of correlation of 0.13 (i.e. a lower coefficient of correlation than two random people off the street) with their adoptive parents, which compared to 0.32 with their birth mothers. By the age of 18, the coefficient of correlation with adoptive parents fell to 0.06 (which is virtually no relationship at all!) which compared to 0.48 with their adoptive mothers. In other words, the genetic factor became stronger as the adopted children approached adulthood. For the environment however, the correlation fell to near zero for the adopted children and their adoptive parents. Similar results were also obtained in several follow-up studies.
Is IQ genetic then? Really…
So the positive IQ effect of the home environment fades over time and genetics prevail. This give new meaning to the expression “the apple doesn’t fall far from the tree”. Counter-intuitively, crystallized IQ appears to be more heritable than fluid intelligence.
Because the explanatory power of environmental factors on IQ diminishes over time, it is for this reason that I will argue that genetics are likely to play a larger environmental factor than environment. 60% appears to be a reasonable estimate. Is IQ genetic? It would appear to be the case.
This said, it has been shown that crystallized intelligence peaks in our late 50s compared to fluid intelligence which peaks in our mid 20s. It has also been shown that crystallized intelligence correlates positively with every additional year of formal education. For this reason, creating a favourable home environment for children is highly advisable despite the importance of genetics.
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