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 they 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 difference in their IQs, when tested at the end of their separation, could be entirely attributable to environmental factors. 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 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 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 spite of the recognition that genes contribute significantly to intelligence, new research in epigenetics and neuroplasticity is revealing that our environment plays a strong role in how those genes are expressed. Certain types of stimulations can “switch on” particular genes while not impacting others. The environment in which the human brain develops has a very significant effect upon its development and its ultimate capabilities.
We can summarize these findings by comparing the myth of the blank slate (tabula rasa) proposed by John Locke with the reality revealed by scientific investigation.
Myth: We all begin life as a blank slate and our cognitive ability, that we call intelligence, is the capacity for learning and reasoning that is acquired exclusively from our experiences.
Truth: The genes we inherit define our cognitive potential and the interaction of those genes with our environment determines our capacity for learning and reasoning that ultimately manifests as our intelligence.
The key to reaching our intellectual potential is to immerse ourselves in intellectually rich experiences and education, especially during our formative years. It is during these formative years that our brain experiences periods of prodigious growth in neural connections and in purging unused abilities.
