There has been significant controversy in the academic community about the heritability of IQ since research on the issue began in the late nineteenth century.

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.

Intelligence in the normal range is a polygenic trait, meaning that it is influenced by more than one gene, more specifically, over 500, and is thought to be 50% to 80% genetic in origin.(Wikipedia)

Recent studies suggest that family and parenting characteristics are not significant contributors to variation in IQ scores; however, poor prenatal environment, malnutrition and disease can have deleterious effects

Millions of parents globally would like to believe that 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.

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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 favorable home environment for children is highly advisable despite the importance of genetics.

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While it’s clear that parents have a large influence on their children’s intelligence, how they raise their children may be just as important as which genes they pass on. For example, a 2012 study from Washington University in St. Louis found that having a loving and nurturing mother significantly contributed to a child’s eventual intelligence. In the study, the team observed that children whose mothers nurtured them early in life had a larger hippo-campus, an area of the brain linked to learning and memory.

“I think the public health implications suggest that we should pay more attention to parents’ nurturing, and we should do what we can as a society to foster these skills because clearly nurturing has a very, very big impact on later development,” lead study author Dr. Joan L. Luby explained in a statement.

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Other research has suggested that playing a musical instrument early in life could be a strong predictor of academic success. In a 2014 study, researchers from the University of Toronto found that  musicians’ brains were more active than the non-musicians’ brains, and they performed better on cognitive tests.

The American Psychological Association’s report “Intelligence: Knowns and Unknowns” (1995) states that there is no doubt that normal child development requires a certain minimum level of responsible care. Severely deprived, neglectful, or abusive environments must have negative effects on a great many aspects of development, including intellectual aspects. Beyond that minimum, however, the role of family experience is in serious dispute. There is no doubt that such variables as resources of the home and parents’ use of language are correlated with children’s IQ scores, but such correlations may be mediated by genetic as well as (or instead of) environmental factors.

But how much of that variance in IQ results from differences between families, as contrasted with the varying experiences of different children in the same family? Recent twin and adoption studies suggest that while the effect of the shared family environment is substantial in early childhood, it becomes quite small by late adolescence.

These findings suggest that differences in the life styles of families whatever their importance may be for many aspects of children’s lives make little long-term difference for the skills measured by intelligence tests.

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