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The subject of the inheritance of intelligence is the genetics of mental abilities. Research in this field is facing a number of difficulties, because intelligence as a trait is a fuzzy concept. The genetics of intelligence is thus operationally restricted to the genetics of IQ or the underlying g factor. Nevertheless, intelligence is embedded in the personality as a whole and its development during the lifespan of an individual. Not only is the number of social and biological factors influencing the development of a person a large one, but also there are a number of underlying genes with many minor and very few major effects.
Interindividual differences in the learning ability are also known from mice, dogs and other animals, and the achievements of pure strains can be improved by breeding. In such a way also behaviour genetics is contributing to our knowledge of the inheritance of mental traits. There is an open question to which degree differences of animal behaviour have any meaning for differences in human intelligence.
The main interest of the broader public is focused on the role played by nature versus nurture in the development of IQ. However, the science of genetics is trying to discover the genes contributing to IQ differences. The area, along with the entire field of intelligence research, has been strongly criticised by some academics and the media (see The Mismeasure of Man).
Methods and results
The similarity of relatives with respect to their intelligence
In the case of the inheritance of a certain level of IQ or a certain degree of giftedness, the relatives of probands with a high IQ exhibit a comparable high IQ with a much higher probability than the general population. In terms of correlation statistics, this means that theoretically the correlation of tests scores between monozygotic twins should be 1.00; practically the upper bound of this correlation is given by the reliability of the test.
Siblings and dizygotic twins share half of their genes and the correlation of their scores should be 0.50, half-siblings 0.25 and is halved by one additional decreasing degree of genetic relationship.
Bouchard and McGue (1981) have reviewed such correlations reported in 111 original studies. 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. From such data the heritability of IQ can be estimated, varying between 0.40 and 0.80.
|Uncles of the parents||5||-||-||5||1996|
|Cousins of parents||-||-||-||11||2250|
|"+": classified by occupation; 100%, if classified by test
"*": classified only by IQ; classification by occupation gives about 55%; n = 820.
"#": some cousins were still too young and did not have full opportunity to become distinguished
100 famous Famous men (n = 43) of science and the percentage of their famous male relatives.
Genetical Psychology Monographs 77 (1968) 3-93.
The mean IQ (transformed to 100;15) of the sample of probands was 146 (n = 724); the cut-off score IQ 137.
The American Naturalist 56 (1922) 504-547; 57 (1923) 74-88, 137-152, and 326-344.
In 1915 questionnaires were filled in by 956 distinguished American men of science and their relatives.
Highly gifted males (mean IQ 135 +/- 9) and their relatives in professions, typically associated with an IQ above 123.</font>
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