In 2011, a large group of researchers published the results of a genome-wide analysis of 549,692 single nucleotide polymorphisms (SNPs) involving 3511 unrelated adults. (An SNP represents a difference in a single DNA building block, called a nucleotide.) (See: Davies, G., A. Tenesa, A. Payton, J. Yang, S.E. Harris, D. Liewald, et al. 2011. “Genome-wide association studies establish that human intelligence is highly heritable and polygenic.” Molecular Psychiatry. 16. pp. 996–1005.) They reported:
Our results unequivocally confirm that a substantial proportion of individual differences in human intelligence is due to genetic variation, and are consistent with many genes of small effects underlying the additive genetic influences on intelligence. … [Furthermore] purely genetic (SNP) information can be used to predict intelligence.
This research estimated the heritability of IQ to be about 0.5, confirming the results of the studies involving twins. Its conclusion that general intelligence is polygenic, i.e., it derives from a combination of many genes supports the concept of intelligence as a multi-faceted characteristic. In his 2016 book, The Gene: An Intimate History, Siddhartha Mukherjee wrote:
While some combination of genes and environments can strongly influence g, this combination will rarely be passed, intact, from parents to their children. Mendel’s laws virtually guarantee that the particular permutation of genes will scatter apart in every generation. … . Intelligence, in short, is heritable (i.e., influenced by genes), but not easily inheritable (i.e., moved down intact from one generation to the next).
Mukherjee is arguing that since g is polygenic, and is derived from a random selection from two parents, a specific combination of genes in either parent has a low probability of passing intact to the offspring. For example, imagine that each of your parents has n alleles that combine at your conception to form your intellectual potential. Think of these alleles as a hand of n playing cards, most of which are face cards if your parents are intelligent. During your conception n/2 cards are drawn from each parent. If you draw mainly face cards from both parents, you will have more intellectual potential than either parent. However, if you draw many of your parents’ pip cards (non face cards) you will have less intellectual potential than either parent. (This scenario was the basis of the 1988 movie, Twins, featuring dizygotic (fraternal) twins played by Arnold Schwarzenegger, who received all the best alleles, and Danny DeVito, who received the “genetic garbage.”) Studies showing that the IQs of siblings have only a moderate correlation (about 0.47) support this model of inheritance. Traits that are defined by a complex combination of genes are less likely to be passed from one generation to the next intact. Hence height, governed by fewer alleles, has high heredity, while intelligence and temperament, governed by more alleles, have a lower heredity. The bottom line is this: your intellectual potential is determined by your DNA however, your intelligence can be enhanced by appropriate environmental influences. These findings have been supported by recent research in neurology, though the extent to which we can change intelligence during adulthood is still a topic of much debate.
