This is a complex question because it presupposes that we can assign a precise percentage to how much of an individual’s intelligence is a result of their genetic makeup. When a psychologist says, “70% of intelligence is heritable and 30% is environmental,” that does not mean that 70% of your IQ score is attributable to your inherited genes and 30% to your environment.
This statement means that when a very large group of people were administered the same IQ tests, the IQ scores were spread widely around an average IQ score. However, people with identical genes (i.e., identical twins) who were separated at birth, differed very little in their IQ scores compared with people from different families. In formal statistical terms, this idea is expressed as, “70% of the variation in the IQ scores of the tested group is attributable to variation in genes, and 30% comes from differences in experiences.” This means, that on average, environment contributes about one-third of the variation in IQ among people. However, if two people are in the same environment, one person’s intelligence may be amplified much more by that environment than the intelligence of the other person. That is, the interaction of each person’s genetic makeup with a particular environment will be different.
In 1990, Thomas J. Bouchard et al. published a seminal article titled, Sources of Human Psychological Differences: The Minnesota Study of Twins Reared Apart in which he and his research team assembled 100 sets of identical twins who were separated early in life and raised apart. All participants in the study completed about 50 hours of medical and psychological assessment.
Since identical twins come from a single fertilized egg, they share virtually 100% of their alleles and can be considered to be genetically identical. Furthermore, since they were raised apart, the differences in their IQ, when tested at the end of their separation could be entirely attributable to environmental factors.
Since the twins in the Minnesota study were raised apart, no part of the correlation in the IQ scores of twins could be attributed to shared experiences. Hence, Bouchard et al. were able to estimate the difference in IQ attributable to genetics, using the correlation in the IQ scores of the twins. The researchers reported “about 70% of the variance in IQ was found to be associated with genetic variation.” The authors cautioned that this finding did not imply that IQ cannot be enhanced by rich experiences. In the years that followed, many researchers conducted similar studies with similar results establishing that the contribution of genetics to intelligence is incontestable, although the size of its importance varies across individuals.
Adding to the complexity of interpreting the percentage of intelligence attributable to genes is the fact that intelligence is polygenic, meaning that it is determined by a huge number of individual genes and is therefore manifest in different forms. See: What is intelligence? Is it measurable? – Intelligence and IQ
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.) Genome-wide association studies establish that human intelligence is highly heritable and polygenic – Molecular Psychiatry. 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.
Recent research in epigenetics suggests that environments can “switch on” and “switch off” certain genes, and this adds more complexity to the role of environment on intelligence at the individual level. While we must be cautious in how we express the contributions of environment and genetics to our intelligence, we do know that genetics play a very strong role in cognitive performance, especially when allowed to interact with a stimulating environment.
