IQ genetics or environment? A bit of both really but more genetics


IQ genetics or environment
IQ genetics or environment?

IQ genetics or environment? Let’s provide a bit of background to the question. Many studies have been published which have investigated the issue and it is undeniable that there is a strong correlation between the IQ of offspring with that of their birth parents, especially the birth mother. A particularly intereting series of studies involved the comparison of adoptive children’s IQs with that of their birth parents and that of the adoptive parents. In a classic 1979 study, Scarr and Weinberg found that the correlation between the IQ of the adopted child and that of the adoptive parents was 0.13, which is lower than the IQ correlation between two random people off the street. This compares to 0.32 as the correlation coefficient between the IQs of adoptive children with that of their birth mothers. To make matters worse, by the time that these adoptive children turned 18, the correlation coefficient between their IQ and that of their adoptive parents had dramatically fallen from an already low base of 0.13 to 0.06, which provides strong evidence that environmental factors (barring detrimental living conditions) have little bearing on the IQs of children over the longer term.

Several other types of clever studies including twins studies have also shed light on the question: IQ genetics or environment? Herrnstein and Murray also looked at kinship and adoption studies, including virtual twins (i.e. non-genetically related children that grow up concurrently in the same household). Segal (1997 and 2000) studied 90 virtual twins and found that the coefficient of correlation between these virtual siblings’ IQs was 0.27 (with a mean IQ difference of 15.4 points), which compares to 0.86 for identical twins reared together (6 points), and 0.47 for biological siblings reared together (14 points). So it is clear that that genetics again appear to over-power environmental factors.

Fischbein (1980) also found that heritability of IQ increases for the highest social classes (0.70), compared to 0.30 for the lowest social classes. Although some of the studies cited in this posting date back several decades, multiple studies have largely reconfirmed results in that IQ is largely heritable, with 50-60% of the variability in IQ being explained by genetics as opposed to environmental factors.

IQ genetics or environment? The fade out effect wades in

In 2015, a meta study by researchers at the University of Santa Barbara examined whether early intervention programmes designed to raise the IQ of young children had lasting effects over time. The lead researcher Protzko conduced 44 different trials involving 7,584 young children and found that the experimental group who had early intervention programmes lost their IQ gains after the programme was over (as opposed to the control group catching up).

It would appear that once the programmes has ceased, that the cohort of intervention children did not seek out intellectual pursuits that would match their newly (and temporary) boosted IQ level, and that they would revert to their normal activities, leading any gains to fade out over time. So while Head Start type of programmes may confer a temporary IQ benefit, these gains are unlikely to be permanent. perspective

Protzko’s latest study reinforces the notion that IQ is inherently genetic, and that environmental factors may of course have short-lived positive effects, but that barring the existence of corrosive and unhealthy environments (e.g. drug use, terrible diets, stress, violence) genetics remain the primary explanatory factor of intellectual ability and IQ standing, certainly as children reach adulthood.

As expressed before, this is not to say that parents should not invest in providing the best possible environments for their children: in fact, the silver lining in the study suggests that some well crafted temporary intervention programmes may in fact produce temporary boosts in IQ, which if parents are capable of sustaining via intellectually-challenging environments, may allow children to get through school with greater ease, which in itself is half the battle won when raising a child.

That said, providing a “good”, not necessarily outstanding environment is likely to be sufficient for your child to live up to its full IQ potential. So for those concerned about lower IQs, teaching your children good work ethic and perseverence, in addition to making sure they have a pleasant and happy environment, may represent a better solution than hoping for miracles from early start intervention programmes.

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IQ differences between siblings

IQ differences between siblings are not meaningful
IQ differences between siblings are not meaningful

Researchers at the University of Illinois have published the results of the largest-ever “within family” evaluation of IQ and personality difference among siblings. The study helps shed light on IQ differences between siblings.

This very large sample study (backed by the National Institute on Aging which is part of the National Institute of Health) looked at 377,000 high school students and evaluated IQ differences between siblings, as well as differences in personality which might be influenced by the chronological order of birth.

The results of the study are clear: IQ differences between siblings are in fact influenced by birth order. First-born children on average will have a statistically significant one IQ point advantage relative to their younger siblings. Although the IQ differences between siblings are statistically significant, they are hardly meaningful as a one point IQ differential cannot produce any perceptible differences in intelligence or cognitive ability.

The study found that first-born children tend to be more outgoing and conscientious, in addition to have slightly higher IQ scores on average.

IQ differences between siblings:’s take on the study

At, we believe (based on several comprehensive studies on IQ and intelligence) that genetics factors explain the majority of the variance in IQ scores generally. However, we do acknowledge that environmental factors may have an influence on IQ scores and cognitive ability generally. Here is our analysis on the study measuring IQ differences between siblings.

On the one hand, older siblings will tend to get more parental attention and contact than their younger siblings. First-time parents are probably more likely to be more cautious with their first child (e.g. maternal contact and attention, which have been shown to increase IQ – see breastfeeding post IQ and breastfeeding). In support of the findings, older children will also need to “figure it out” as they progress through life, learning by trial and error.

The contra argument is that younger siblings, if the age gap is not too large, will tend to play and learn from older siblings, resulting in a transfer of knowledge from the older sibling to the younger one. This accelerated learning could also be an IQ booster for younger siblings, allowing for ‘catch up’ to take place.

Importantly, the IQ differential of one point, although statistically significant is not material. Based on the above two arguments however, it would be interesting to drill down into the results to test whether older siblings display a higher level of fluid intelligence (as they “figure it out”) relative to younger siblings, who are more likely to use their sequential processing and crystallized intelligence to follow in the footsteps of their older siblings.

article provided by a leader in online fluid IQ testing.