<P> A 2009 review article identified over 50 genetic polymorphisms that have been reported to be associated with cognitive ability in various studies, but noted that the discovery of small effect sizes and lack of replication have characterized this research so far . Another study attempted to replicate 12 reported associations between specific genetic variants and general cognitive ability in three large datasets, but found that only one of the genotypes was significantly associated with general intelligence in one of the samples, a result expected by chance alone . The authors concluded that most reported genetic associations with general intelligence are probably false positives brought about by inadequate sample sizes, but see . Arguing that common genetic variants explain much of the variation in general intelligence, they suggested that the effects of individual variants are so small that very large samples are required to reliably detect them . Genetic diversity within individuals is heavily correlated with IQ . </P> <P> A novel molecular genetic method for estimating heritability calculates the overall genetic similarity (as indexed by the cumulative effects of all genotyped single nucleotide polymorphisms) between all pairs of individuals in a sample of unrelated individuals and then correlates this genetic similarity with phenotypic similarity across all the pairs . A study using this method estimated that the lower bounds for the narrow - sense heritability of crystallized and fluid intelligence are 40% and 51%, respectively . A replication study in an independent sample confirmed these results, reporting a heritability estimate of 47% . These findings are compatible with the view that a large number of genes, each with only a small effect, contribute to differences in intelligence . </P> <P> The relative influence of genetics and environment for a trait can be calculated by measuring how strongly traits covary in people of a given genetic (unrelated, siblings, fraternal twins, or identical twins) and environmental (reared in the same family or not) relationship . One method is to consider identical twins reared apart, with any similarities which exists between such twin pairs attributed to genotype . In terms of correlation statistics, this means that theoretically the correlation of tests scores between monozygotic twins would be 1.00 if genetics alone accounted for variation in IQ scores; likewise, siblings and dizygotic twins share on average half of their alleles and the correlation of their scores would be 0.50 if IQ were affected by genes alone (or greater if, as is undoubtedly the case, there is a positive correlation between the IQs of spouses in the parental generation). Practically, however, the upper bound of these correlations are given by the reliability of the test, which is 0.90 to 0.95 for typical IQ tests </P> <P> If there is biological inheritance of IQ, then the relatives of a person with a high IQ should exhibit a comparably high IQ with a much higher probability than the general population . In 1982, Bouchard and McGue reviewed such 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, between half - siblings, 0.31, and between cousins, 0.15 . </P>

The heritability index for intelligence is approximately .50 meaning that
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