In an 1886 publication, Sir Francis Galton graphed the heights of 205 adults and 928 of their adult offspring. From these data, he asserted that the height of an offspring deviates from the average height in the adult population by only two thirds of the parent’s deviation from the average. In other words, children of tall parents will be taller than average, but not by as much as their parents. Similarly, children of short parents will be shorter than average, but not as short as their parents. Furthermore, the ratio of these deviations will be approximately 2 to 3. This principle is known today as regression to the mean.
This principle applies to all inherited characteristics, including intelligence, so an offspring of two highly intelligent people will likely be more intelligent than average, but not as intelligent as his or her parents. Furthermore, the more intelligent the parents, the greater will be the likely gap between the intelligence of the parents and the offspring. The reason for this lies in the mathematics of probability that is easily understood in the following “thought experiment.”
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.
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.) 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 fraternal 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).
In short, two profoundly gifted parents can expect each of their children to have a higher than average IQ, though not likely their level of giftedness. This is born out by the absence of genius in the offspring of famous geniuses.
