Inheritance of intelligence

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).

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.

In 1869, Francis Galton replaced mere speculations by statistical data through his book, Hereditary Genius:

Despite the differences in methods and societies, there is a notable parallelism in the published statistics. The ITO-method by Li and Sacks (1954) allows from this set of data the estimation of the underlying number of genes and their allele frequencies.

The inheritance of cognitive deficits
There is no doubt that genes are in many cases the cause of an IQ below 100. The number of such genes already known is in the hundreds. For example, a mutation of the gene GDI1 is associated with an IQ below 70. The public has free and open access to the current data in the field through the OMIM database.

There are number of known cases where the homozygotes have severe cognitive deficits and the heterozygotes show a small decrease of IQ. In such metabolic pathways further SNPs of such genes are investigated whether they are influencing IQ or not. For example, one SNP of the gene ALDH5A1 causes an IQ difference of around 1.5 points.

Development of IQ
It is reasonable to expect that genetic influences on traits like IQ should become less important as one gains experiences with age. Surprisingly, the opposite occurs. Heritability measured in infancy are as low as 20%, around 40% in middle childhood, and as high as 80% in adulthood. Plomin et al. (2001, 2003); although this could also suggest that genetic influences have an effect on a persons predisposition to learn and develop IQ.

Literature

 * P. McGuffin: The quantitative and molecular genetics of human intelligence. Novartis Found Symp. (2000) 243-255.
 * G. Meisenberg: Genes for intelligence. A review of recent progress. Mankind Quarterly 36 (Winter 2005) 139-164.
 * A. Payton: Investigating cognitive genetics and its implications for the treatment of cognitive deficit. Genes Brain Behav. 5 Suppl 1(2006) 44-53.