Research on the heritability of intelligence quotient (IQ) inquires into the degree of variation in IQ within a population that is associated with genetic variation between individuals in that population. 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. Intelligence in the normal range is a polygenic trait, meaning that it is influenced by more than one gene,Alice Marcus. 2010. Human Genetics: An Overview. Alpha Science section 14.5 and in the case of intelligence at least 500 genes. Further, explaining the similarity in IQ of closely related persons requires careful study because environmental factors may be correlated with genetic factors. Outside the normal range, certain single gene genetic disorders, such as phenylketonuria, can negatively affect intelligence.
Estimates in the academic research of the heritability of IQ vary significantly by study and by study design. The general figure for heritability of IQ from behavioral genetic studies is about 0.5 across multiple studies in varying populations. The relationship between heritability and age is uncertain, though most researchers believe that there is an increase in heritability over the course of the lifespan and that this increase reflects the importance of gene-environment correlations. Recent genetic research has come to more equivocal results, with estimates of heritability lower than those derived from twin studies, causing what is known as the "missing heritability problem".
Although IQ differences between individuals have been shown to have a hereditary component, it does not follow that disparities in IQ between groups have a genetic basis. The scientific consensus is that genetics does not explain average differences in IQ test performance between racial groups.
Heritability and caveats
Heritability is a statistic used in the fields of breeding and genetics that estimates the degree of variation in a phenotypic trait in a population is explained by genetic variation between individuals in that population. The concept of heritability can be expressed in the form of the following question: "What is the proportion of the variation in a given trait within a population that is not explained by the environment or random chance?"
Estimates of heritability take values ranging from 0 to 1; a heritability estimate of 1 indicates that all variation in the trait in question is genetic and a heritability estimate of 0 indicates that none of the variation is genetic.
There are a number of caveats to consider when interpreting heritability: Heritability measures the proportion of variation in a trait that can be attributed to genes, and not the proportion of a trait caused by genes. Thus, if the environment relevant to a given trait changes in a way that affects all members of the population equally, the mean value of the trait will change without any change in its heritability (because the variation or differences among individuals in the population will stay the same). This has evidently happened for height: the heritability of stature is high, but average heights continue to increase. Thus, even in developed nations, a high heritability of a trait does not necessarily mean that average group differences are due to genes. Some have gone further, and used height as an example in order to argue that "even highly heritable traits can be strongly manipulated by the environment, so heritability has little if anything to do with controllability." A common error is to assume that a heritability figure is necessarily unchangeable. The value of heritability can change if the impact of environment (or of genes) in the population is substantially altered. If the environmental variation encountered by different individuals increases, then the heritability figure would decrease. On the other hand, if everyone had the same environment, then heritability would be 100%. The population in developing nations often has more diverse environments than in developed nations. This would mean that heritability figures would be lower in developing nations. Another example is phenylketonuria which previously caused intellectual disabilities in everyone who had this genetic disorder and thus had a heritability of 100%. Today, this can be prevented by following a modified diet, resulting in a lowered heritability. A high heritability of a trait does not mean that effects such as learning are not involved. Vocabulary size, for example, is substantially heritable (and highly correlated with general intelligence) although every word in an individual's vocabulary is learned. In a society in which plenty of words are available in everyone's environment, especially for individuals who are motivated to seek them out, the number of words that individuals actually learn depends to a considerable extent on their genetic predispositions and thus heritability is high. Contrary to popular belief, two parents of higher IQ will not necessarily produce offspring of equal or higher intelligence. Polygenic traits often appear less heritable at the extremes. A heritable trait is definitionally more likely to appear in the offspring of two parents high in that trait than in the offspring of two randomly selected parents. However, the more extreme the expression of the trait in the parents, the less likely the child is to display the same extreme as the parents. In fact, parents whose IQ is at either extreme are more likely to produce offspring with IQ closer to the mean (or average) than they are to produce offspring with high IQ. At the same time, the more extreme the expression of the trait in the parents, the more likely the child is to express the trait at all. For example, the child of two extremely tall parents is likely to be taller than the average person (displaying the trait), but unlikely to be taller than the two parents (displaying the trait at the same extreme). See also regression toward the mean.Schacter, Daniel, Daniel Gilbert, and Daniel Wegner. "Intelligence." Psychology. 2 ed. New York: Worth Publishers, 2010. 405-406. Print.
Methodology
There are multiple types of studies that are designed to estimate heritability and other variance components, stemming from the field of biometrical genetics. The variance, or more simply the differences in a trait within a population, can be partitioned into specific variance components that are based on particular decomposition models for the phenotype. First pointed out by Ronald Fisher and Sewall Wright, different sources of variance differ in their contribution to the resemblance between types of relatives.By making certain assumptions about the relationship between genes and environment, the field of quantitative genetics has developed equations that describe the expected correlation between relatives in terms of specific variance components (shared environment, additive genetic, etc.).
One simple case is presented by twin studies. By making assumptions about genetic additivity and sources of environmental variation, researchers can estimate the heritability of a phenotype by comparing the resemblance of twins. Another method used to estimate heritability is adoption studies. Because adoptive parents and their adoptive children share a common environment, but not genetics, researchers in behavioral genetics have used data from adoptive families to estimate the heritability of IQ. Based on quantitative genetic theory, some researchers have taken a broader approach to analyzing familial resemblance. These researchers typically use path analysis to fit models using larger sets of familiar correlations, incorporating cultural transmission, adoption, other complex living arrangements, and generalized assortative mating. However, with the transition into the postgenomic era, issues with controversial assumptions in previous models analyzing familial resemblance may be superseded by genetic analysis. Methods have developed in genetic analysis to estimate the total heritability of a trait using large datasets with millions of genetic variants.
In the case of IQ, heritability estimates based on direct observation of molecular genetics have been significantly lower (around 10%) than those employing traditional methods (40–80%). This discrepancy has been termed the "missing heritability problem." While researchers such as Robert Plomin and Sophie von Stumm contend that this gap will likely disappear with the acquisition of more genetic data, Eric Turkheimer and Lucas J. Matthews argue that it reveals a deeper set of methodological problem inherent in the concept of heritability itself, which will not be easy to overcome.
Estimates Although individual family studies that estimate the heritability of IQ vary greatly, most family studies estimate the heritability of IQ in the range from 0.4 to 0.8 and reviews of the literature typically summarize classical family design research with an estimate of 0.5. Issues with the methodology of family studies have long plagued the research field, but some researchers now believe that genome-wide association studies can provide less biased estimates of heritability. These genomic studies provide estimates of heritability that are much lower than those derived from twin studies, leading to what is denoted as the "missing heritability problem". The latest results from genomic methods are between 0.10 to 0.20, and researchers have proposed various theories to explain the gap.
Twin and family research
Twin studies Twin studies are the most common method used to estimate the heritability of most traits. Because monozygotic twins derive from one fertilized egg, they are largely genetically identical, while dizygotic twins are expected to share 50% of their DNA. Twin studies compare the resemblance of monozygotic twins to the resemblance of dizygotic twins to estimate the heritability of a trait using various models.
Twin studies have shown greater resemblance between monozygotic twins than dizygotic twins. This has lead most researchers in behavioral genetics to conclude that the heritability of IQ is in a range between 0.4 to 0.8. However, it is not clear if all of the assumptions in twin studies hold true. Critics often focus on the equal environment assumption, which is the assumption that environmentally caused similarity is the same for both monozygotic and dizygotic twins. Genetic modeling has found that even in the complete absence of genetic factors for a trait, environmental similarity between monozygotic twins will cause heritability estimators to produce large estimates of genetic heritability. Researchers have produced different analyses of the equal environment assumption, some suggesting that the assumption is untenable for related traits like educational attainment.
Twins reared apart have also been the subject of significant debate. The 2006 edition of Assessing adolescent and adult intelligence by Alan S. Kaufman and Elizabeth O. Lichtenberger reports correlations of 0.86 for identical twins raised together compared to 0.76 for those raised apart and 0.47 for siblings. Theoretically, the correlation between twins reared apart is a direct estimate of heritability, as the only common factor between the twins would be the genetic variance. However, this makes an assumption of uncorrelated environments between twins, which may not be the case. For example, some researchers argue that twins experience similar education and this causes increased similarity in IQ. Thomas Bouchard argues that some previous analyses of the data on twins reared apart are an abuse of statistical theory that he calls "pseudoanalysis".
Adoption studies