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 (general intelligence) 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, of which about one-third are face cards if your parents are of average intelligence. During your conception, n/2 cards are drawn from each parent. If the cards you draw from both parents are mainly face cards, (conducive to high IQ) you will have more intellectual potential, and a higher IQ than either parent and probably any of your siblings–unless one of your siblings draws a richer set of face cards. Of course, you would need to involve yourself in deep learning and challenging intellectual pursuits in order to reach your potential IQ.
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. Of course, if you do attain a high IQ, you will find that it comes with some inherent dangers, as explained in http://What danger threatens those of high IQ? – Intelligence and IQ .
Recent research, such as the studies of hundreds of twins raised in different environments, reveals that upbringing has much less effect on IQ than genetics. Furthermore, the effect of a common upbringing on IQ seems to wane after siblings reach adulthood.
