Studies on the heritability of IQ express the ratio of IQ variance between individuals in a study population related to the genetic variable in the population, from the similarity of IQ in closely related persons. As environmental factors can correlate with genetic factors, it provides a maximum estimate of genetic versus environmental impact for phenotypic variation in IQ in this population. “Inheritance”, in this sense, refers to the “genetic contribution to a variable within a population and a given environment”. In other words, inheritance is a mathematical prediction that shows the upper limit on how much of a trait variable can be attributed to genes. There has been considerable debate in the academic community about the heritability of IQ since research on the topic began in the late 19th century. Intelligence in the normal range is a polygenic trait, meaning it is affected by more than one gene.
The heritability of IQ for adults is between 58% and 77% (some recent estimates indicate that it is as high as 80% and 86%. Genome-wide association studies are inherited, where 50% of the genetic variation that contributes to inheritance is 20%. IQ, thought to be very weakly related to genetics in children, is strongly correlated with genetics for adolescents and adults. This phenomenon is known as the Wilson Effect. Recent studies suggest that family and parenting characteristics do not significantly contribute to the change in IQ scores, but unsuitable environmental conditions before birth, malnutrition and various diseases can cause harmful effects.
Inheritance and warnings
“Inheritance” is defined as the ratio of the variance in a trait based on a genetic variant in a population defined in a given setting. Inheritance takes a value from 0 to 1; An inheritance of 1 indicates that all variations in the trait in question are inherently genetic and a heritability of 0 indicates that none of the variation is genetic. The determination of many traits can be considered primarily genetic under similar environmental factors. For example, a 2006 study found that when looking only at changes in height in families where the environment should be very similar, the height of adults has a heritability of 0.80. Other traits have lower heritability, indicating a lower environmental impact. For example, twin studies on the inheritance of depression were calculated as 0.42 for women and 0.29 for men in the same study. Contrary to popular belief, two parents with a high IQ will not necessarily give birth to children of equal or higher intelligence. In fact, according to the concept of regression towards the mean, a parent with a high IQ is more likely to give birth to children with a closer to average IQ.
There are a few points to consider when interpreting inheritance:
Inheritance measures the rate of variation in a trait related to genes, not a trait that can be caused by genes. Thus, if the environment with a particular trait changes in a way that affects all members of the population equally, the average value of the trait will also change without any change in hereditary value (because the variations or differences of individuals within the population will remain the same). This clearly happened for height: the degree of inheritance of height is high, but average stature continues to increase. Thus, even in developed countries, a high heritability trait does not mean that average group differences are due to genes. Some have gone further and used height as an example to suggest that “even high-level inherited traits can be strongly manipulated by the environment, so that heritability has little to do with controllability.”
• It is a common mistake to assume that an inheritance figure is not necessarily modifiable. The value of inheritance can change if the influence of the environment (or genes) in the population has changed drastically. As the environmental changes faced by different individuals increase, the heritability value decreases. On the other hand, if everyone had the same environment, inheritance would be 100%. Populations in developing countries generally have a more diverse environment than in developed countries. This means that inheritance rates in developing countries are lower. Another example is phenylketonuria, which causes mental impairment for anyone who has previously had this genetic disorder and therefore 100% heritable. Today, this can be prevented by following a modified diet that results in reduced inheritance.
A high heritability of a trait does not mean that environmental influences such as learning are not involved. For example, the size of the vocabulary is quite significantly inherited (and highly correlated with general intelligence), although every word in an individual’s vocabulary is learned. In a society where words are readily accessible to everyone, especially for individuals motivated to find them, the number of words individuals learn depends significantly on their genetic predisposition and hence inheritance is high.
• Since inheritance increases during childhood and adolescence, and even increases substantially between the ages of 16-20 and adulthood, one should be cautious when drawing conclusions from studies in which participants are not followed until adulthood. In addition, there may be differences regarding the effects on g factor and non-g factors, such as affecting g and environmental interventions disproportionately affecting g factors.
Several studies in the United States have found the heritability of IQ between 0.7 and 0.8 in adults and 0.45 in children. Considering that an individual gains experience with age, it can be expected that genetic influences on traits such as IQ will gain less importance. However, the opposite has also happened. Heredity criteria are as low as 0.2 in infancy, around 0.4 in middle childhood and as high as 0.8 in adulthood. One explanation argues that people with different genes tend to seek different environments that amplify the effects of these genes. The brain undergoes developmental morphological changes, suggesting that age-related physical changes may also contribute to this effect.
A 1994 article on Behavior Genetics, based on a study of Swedish monozygotic and dizygotic twins, suggested that a high value of 0.80 may be found in general cognitive ability. However, it varies according to the feature, showing 0.60 for verbal tests, 0.50 for spatial and processing speed tests, and 0.40 for memory tests. In contrast, studies in other populations predict an average heritability of 0.50 for general cognitive ability.
In 2006, The New York Times Magazine listed about three-quarters as a figure that was edited by the majority of the studies.
Shared family atmosphere
See also: Environment and intelligence
Family structure has some effect on children’s IQ levels, making up about a quarter of the variances. However, adoption studies show that adoptive siblings are not more similar to strangers in IQ until adulthood; In addition, full adult siblings show an IQ correlation of 0.24. On the other hand, some studies of separated twins (for example, Bouchard, 1990) find a significant shared environmental impact, with at least 10% of them seeing post-adulthood. Judith Rich Harris suggests that this may be due to bias assumptions in the methodology of classic twin and adoption studies.
There are settings that family members have in common (eg characteristics of the house). This shared family environment accounts for 0.25-0.35 of the variation in childhood IQ. At the end of adolescence it is very little (in some studies zero). There is a similar effect for other psychological traits. These studies did not examine the effects of extreme environments such as in families where abuse occurred.
In the report of the American Psychological Association “Intelligence: Knowns and Unknowns” (1995), it is stated that there is no doubt that normal child development requires a certain minimum level of careful care. Children who grow up in severely deprived, negligent, or abusive environments are negatively affected in many aspects of development, including the intellectual aspects. However, beyond that, the role of family experience is in serious dispute. There is no doubt that variables such as home resources and parents’ use of language are associated with child IQ scores, but genetic and environmental factors also have an effect on these correlations. But how much of this variance in IQ is due to differences between families compared to the different experiences of different children in the same family? Recent twin and adoption studies show that while the impact of shared family environment is important in early childhood, it becomes quite insignificant in adolescence. These findings show that differences in families’ lifestyles do not make a long-term difference for skills measured by intelligence tests, regardless of what they may be for many aspects of children’s lives.
Non-partner family environment and non-family environment
Although parents treat their children differently, this type of different treatment explains only a small amount of uncommon environmental impact. According to one view, children react differently to the same environment depending on different genes. More likely effects may be those of peers and other experiences outside of the family. For example, siblings raised in the same household may have different friends and teachers, and may even have different illnesses. This factor may be one of the reasons that IQ score correlations between siblings decrease with age.
Malnutrition and diseases
Certain single gene diseases, genetic disorders can seriously affect intelligence. Phenylketonuria is an example and there are many publications demonstrating that it reduces phenylketonuria capacity by an average of 10 IQ points. Meta-analyzes have found that environmental factors such as iodine deficiency can result in large decreases in average IQ; It has been shown that iodine deficiency causes an average decrease of 12.5 IQ points.
Hereditary and Socioeconomic Status
The APA report “Intelligence: The Knowns and Unknowns” (1995) stated:
“We should point out that low-income and non-white families are poorly represented in current adoption studies and most twin samples, so it is not yet clear whether these studies apply to the population as a whole. It is possible that intra-family differences between all income and ethnicity have more lasting consequences for psychometric intelligence.
A study by Capron and Duyme (1999) examined the impact of socioeconomic status (SES) on French adopted children between the ages of four and six. The kids’ IQ initially averaged around 77, which means that decline begins. Many have been abused or neglected from infancy, then transferred from one foster family or institution to another. Nine years after their adoption, at an average age of 14, they took IQ tests again, and it turned out that they all showed better results. This amount of improvement they showed was directly related to the socioeconomic status of the adoptive family. “The average IQ score of children adopted by farmers and workers was 85.5, while their average score in middle-class families was 92. The average IQ score of young people raised in wealthy families increased by more than 20 points to 98. “
Stoolmiller (1999) claimed that the environments in previous adoption studies were limited. Adopting families, for example, tend to be more similar in socio-economic status than the general population, suggesting that the role of family environment shared in previous family studies should not be ignored. Adjustments for the constraints on adoption studies have shown that socio-economic status can account for 50% of the variance in IQ.
On the other hand, the effect of this was examined by Matt McGue and colleagues (2007) and they stated that “restrictive psychopathology of the parent and limitation in family socioeconomic status have no effect on the correlations of adopted siblings in terms of IQ”.
Turkheimer and colleagues (2003) argued that IQ variance rates based on genes and environment vary according to socioeconomic status. In a study conducted on seven-year-old twins, it was found that 60% of the variance in early childhood IQs in poor families was created by the common family environment and the contribution of genes was close to zero; however, he reported that in wealthy families the result was almost the opposite.
Contrary to Turkheimer (2003), a study by Nagoshi and Johnson (2005) concluded that in 949 Caucasian and 400 Japanese families included in the Hawaiian Family Cognition Study, the degree of inheritance of IQ does not change as a function of parental socioeconomic status.
Asbury et al. (2005) examined the impact of environmental risk factors on verbal and nonverbal skills in a national representative sample of 4-year-old British twins. Although there was no statistically significant interaction for nonverbal skills, it was found that the persistence of verbal ability was higher in low SES and high-risk environments.
Harden and colleagues (2007) studied adolescents, most of whom were 17 years old, and found that genetic influences accounted for about 55% of the variance in cognitive ability and about 35% of shared environmental influences among high-income families. Among the low-income families, the rates were inversely, but 39% genetic and 45% shared environment. “
Rushton and Jensen (2010) criticized most of these studies for being conducted with children or adolescents. They argued that inheritance increases during childhood and adolescence, and even increases significantly between the ages of 16-20 and adulthood. Therefore, the results should be drawn with caution, given that the genetic effect is derived from studies in which participants were not adults. Moreover, studies have not examined whether IQ gains due to adoption are above the general intelligence factor (g). When Capron and Duyme’s studies were reviewed, IQs resulting from adoption to high SES homes were on non-g factors. In contrast, the g values of the adopted children were mainly dependent on the SES values of their biological parents, suggesting that g is more difficult to change environmentally.
The most cited adoption projects that tried to predict the degree of inheritance of IQ were the Texas, Colorado and Minnesota studies that started in the 1970s. These studies showed that while the IQ of adoptive parents correlated with the IQ of the adopted children at the beginning, the correlation decreased and disappeared when the children reached puberty. It was actually correlation with the biological parent that explained most of the variation.
A 2011 study by Tucker-Drob et al. explained that at the age of 2, genes accounted for about 50% of the change in mental abilities of children raised in families with high socioeconomic status, but that the variation of genes in mental abilities in children raised in families with low socioeconomic status was insignificant. Although this gene-environment interaction is not evident at 10 months of age, it has been shown that the effect occurs in the early development course.
A 2012 study based on a representative sample of twins from the United Kingdom found no evidence for lower heritability in low SES families with longitudinal data on IQ from age two to fourteen. However, the study showed that the effects of shared family environment on IQ are generally greater in low SES families than in high SES families, and IQ causes greater variance in low SES families. Experts have noted that previous research has produced inconsistent results on whether SES controls IQ inheritance. Experts who come up with three explanations for the inconsistency; First, they explained that some studies may not have statistical power to detect interactions, secondly, that the age range studied may have varied between studies, and thirdly, the effect of SES may have changed in different demographics and different countries.
The 2017 King’s College London study suggests that genes make up about 50 percent of the differences between whether children are socially active or not.
Maternal (fetal) environment
A meta-analysis of previous 212 studies by Devlin et al. (1997) evaluated an alternative model for environmental impact and found that it matched the data better than the commonly used ‘family environments’ model. Common maternal (fetal) environmental effects, which are generally considered to be negligible, constitute 20% of the common variable between twins and 5% between siblings; The effects of genes are reduced accordingly, and the two heritability measures are below 50%. They argue that the common maternal environment may explain the striking correlation between IQs of twins, particularly separated adult twins. IQ heritability increases during early childhood, but it is unclear whether it remains stable afterwards.  These results have two meanings: a new model of the effect of genes and environment on cognitive function may be required; and interventions to improve the prenatal environment can lead to a notable increase in the IQ of the population.
Bouchard and McGue reviewed the literature in 2003, suggesting that Devlin’s conclusions on the magnitude of heredity did not differ significantly from previous reports, and that their conclusions on prenatal effects contradict previous reports. And they stated that:
Chipuer et al. and Loehlin concluded that the postnatal period is the most important rather than the prenatal environment. Devlin et al. (1997a) points out that the prenatal setting contributes to twin IQ similarity, especially given the presence of a large empirical literature on prenatal effects. Price (1950) argued in a comprehensive review published 50 years ago that almost all MZ twin prenatal effects produce differences rather than similarities. From 1950 onwards, the literature on the subject was so large that not all bibliographies were published. It was eventually published in 1978 with an additional 260 references. Price reiterates his previous result. (Price, 1978). Research conducted after the 1978 review greatly reinforces Price’s hypothesis (Bryan, 1993; Macdonald et al .., 1993; Hall and Lopez-Rangel, 1996; see also Martin et al., 1997, box 2; Machin, 1996).
Dickens and Flynn model
Dickens and Flynn (2001) argued that the figure of “inheritance” has both direct effects on the IQ of the genotype, and the indirect effects of the genotype that change the environment and effects on IQ. That is, those with higher IQs tend to seek stimulating environments that further increase their IQ. The direct impact may be very small at first, but feedback loops can make big differences in IQ. In their model, the environmental stimulus can have a tremendous effect on IQ even in adults, but this effect deteriorates over time as the stimulus continues. This model can be adapted to include possible factors such as nutrition in early childhood and cause lasting effects.
The Flynn effect results in average intelligence test scores increasing at an annual rate of about 0.3% and the average person scoring 15 points higher on IQ today than 50 years ago. This effect can be explained by a generally more stimulating environment for all people. Experts say that programs that aim to increase IQ will be more likely to produce long-term IQ gains while participating in the program and motivating them to continue with that replication after leaving the program if they teach children how to replicate the type of cognitively challenging experiences that produce IQ gains outside of the program. suggests. Many of the advances have provided opportunities for better abstract reasoning, spatial relations and understanding. Some scientists have argued that such advances are due to better nutrition, proper parenting and school education, as well as the removal of the inferior, the exclusion of genetically weak people from bringing children into the world. However, Flynn and another group of scientists share the view that modern life requires solving many abstract problems that lead to an increase in IQ scores.
Effect of genes on IQ stability
Recent research has highlighted genetic factors underlying IQ stability and alteration. Genome-wide association studies have shown that genes associated with intelligence remain fairly constant over time. In particular, in terms of IQ stability, “genetic factors mediate phenotypic stability over the entire period [0 to 16], while most age variability has turned out to be due to non-shared environmental influences.” These findings have been replicated in the United Kingdom, the United States and the Netherlands. Also, researchers have shown that natural changes in IQ occur in individuals at varying times.
Influence of non-inherited parental genes
“The environment has a genetic component, for example, that the alleles in the parents influence their phenotype and, as a result, the outcome of the child,” says Kong. These results were obtained by a meta-analysis of educational status and polygenic scores of non-transmitted alleles. While the study is about educational attainment and not IQ, nevertheless these two are strongly linked.
Spatial ability component of IQ (intelligence level)
Spatial ability has been shown to be unidirectional (a single score is sufficient for all spatial abilities) and may be 69% inherited in 1,367 twin samples aged 19-21. Also, only 8% of spatial ability may be responsible for shared environmental factors such as school and family. While 24% of the genetically determined part of spatial ability is shared with verbal ability (general intelligence), 43% is only specific to spatial abilities.
Molecular genetic research
A 2009 review article identified 50 genetic polymorphisms reported to be associated with cognitive ability in various studies, but noted that small effect sizes and lack of repetition characterize this research so far. Another study attempted to replicate 12 reported links between specific genetic variables and general cognitive abilities across three large datasets, but in one example only one of the genotypes was found to be significantly associated with general intelligence, and this was an expected result by chance alone. Experts concluded that the genetic linkages reported for general intelligence were likely to be false positives due to insufficient sample size. Arguing that common genetic variables explain most of the variations in general intelligence, they suggested that the effects of individual variants are so small that very large samples are necessary to detect them reliably. Genetic diversity in individuals is highly correlated with IQ.
To predict the degree of inheritance, a new molecular genetic method calculates all genetic similarity (as indexed by the cumulative effects of single nucleotide polymorphisms with all genotypes) between all pairs of individuals in a sample of unrelated unrelated individuals, and then this genetic similarity between all pairs by phenotypic similarity. relates. In a study using this method, the lower limits for the narrow heritability of crystalline and fluid intelligence were calculated to be 40% and 51%, respectively. A practice study in an independent sample confirmed these results and reported a 47% heritability estimate. These findings are consistent with the view that multiple genes, each with only a small effect, contribute to differences in intelligence.
Correlation between IQ and degree of genetic association
The relative impact of genetics and environment for a trait can be measured by determining how strong a trait is in a particular genetic (unrelated, sibling, fraternal, or identical twins) and environmental (whether or not bred in the same family) relationship. One method is to consider identical twins that are distinct from each other, with the similarities that exist between twins related to genotype. In terms of correlation statistics, this means that the correlation of test scores between monozygotic twins should have been 1.00 if genetics alone were calculated as the cause of variation in IQ scores. Similarly, siblings and dizygotic twins share an average of half of their alleles, and the correlation between their scores would be 0.50 if IQ alone was influenced by genes (or, of course, if there was a positive correlation between spouses’ IQs in parental formation). Practically, however, the upper limit of these correlations is provided by the reliability of the test; 0.90 to 0.95 for typical IQ tests.
If the IQ has biological inheritance, relatives of a person with a high IQ should exhibit a comparably high IQ, with a higher probability than the general population. In 1982, Bouchard and McGue reviewed the correlations reported in 111 original studies in the United States. The mean correlation of IQ scores between monozygotic twins was 0.86, between siblings 0.47, half-siblings 0.31, and cousins 0.15. 
The 2006 edition of Assessment of adolescent and adult intelligence by Alan S. Kaufman and Elizabeth O. Lichtenberger reports correlations of 0.86 for co-growing twins, 0.76 for twins raised separately, and 0.47 for siblings. While these numbers are not strictly fixed, when comparing data from the late 1970s before 1963, DeFries and Plomin studies found that the IQ correlation between cohabiting parents and their children dropped significantly from 0.50 to 0.35. The opposite has occurred for fraternal twins.
It should be noted that these consecutive studies included estimates of only two of the three factors related to each. The three factors are G, E and GxE. The GxE factor cannot be isolated, as there is no possibility to work in equal environments, similar to the use of identical twins for equal genetics. Hence, the predictions are actually G + GxE and E. This may seem absurd, but is confirmed by the stated assumption that GxE = 0. There is also a case where the values shown below have r correlations and not r (squared). Numbers less than one are smaller when squared. Next to the last number in the list below shows that there is less than 5% shared variance between parent and separated child.
• Same person (tested twice) .95
Identical twins – .86 grown together.
Identical twins – Growing up .76
Fraternal twins – .55 who grow up together
Fraternal twins – Separately grown .35
Biological siblings – grown together .47
Biological siblings – Separately grown .24
• Biological siblings – Growing up together— Adults .24
• Unrelated children – Growing up together – Children .28
• Unrelated children – Growing up together— Adults .04
• Cousins .15
Parent-child – Living together .42
Parent-child – Living separately .22
• Adopted parent-child – Living together .19
Although IQ differences between individuals have been shown to have a large inherited component, mean group-level inequalities in IQ (within-group differences) cannot necessarily have a genetic basis. The Flynn effect is an example where there is a large difference (past and present) between groups with little or no genetic difference. An analogy attributed to Richard Lewontin makes this point:
Suppose that two handfuls are taken from a sack containing a genetically different variety of maize and each is grown under carefully controlled and standardized conditions, but one group is not given some nutrients given to the other. After a few weeks, the plants are measured. In each group there is variability in growth due to genetic variability of maize. Given that the growing conditions are closely controlled, almost all variation in the height of plants in a group is due to differences in their genes. Thus, in the population, heritability will be very high. However, the difference between the two groups is entirely due to an environmental factor – different diets. Lewontin did not aim to paint the pots in one group white and the others black, and he did not go that far, but the situation is quite obvious. His message, for example, is that, in any case, the reason for differences between groups can, in principle, be quite different from the reasons for within-group variation.
Arthur Jensen agreed that this was technically correct, but also noted that a high inheritance increases the likelihood that genetics play a role in mean group differences.