In 1690, English philosopher John Locke stated that we all come into the world as intellectually “blank slates” (Tabula Rasa, in Latin) and acquire knowledge and intelligence that are written on these slates by our experiences. The political view underpinning Locke’s assertion was that if, indeed, we all enter the world as blank slates, and differ only in our experiences, then we’re all intellectual equals at birth and social classes are not justifiable on the basis of differences in ability.
This view was challenged by Sir Francis Galton in his 1869 publication Hereditary Genius, in which he asserted that physical characteristics and intelligence are mainly inherited. His assertion stimulated a controversy among psychologists that spanned succeeding centuries as the “nurture vs. nature debate.”
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. However, the contribution of genetics to intelligence was incontestable.
In 1993, psychologist Anders Ericsson challenged the concept of innate talent, asserting:
Individual differences, even among elite performers, are closely related to assessed amounts of deliberate practice. Many characteristics once believed to reflect innate talent are actually the result of intense practice extended for a minimum of 10 years. Analysis of expert performance provides unique evidence on the potential and limits of extreme environmental adaptation and learning.
In his bestselling book Outliers: The Story of Success, Macolm Gladwell popularized Ericssons’ research, asserting that talent is a myth and exceptional performance is merely the result of about 10,000 hours of deliberate practice. This perception of the origins of high performance struck a responsive note with the general public, because it enables the average person to assume that they too could win a Nobel Prize in Physics if they merely worked harder.
It was time for biology to weigh in on the question, “To what extent is our intelligence based on the genes we inherit?” 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. Of course environmental influences such as education, study and curiosity contribute to general intelligence, but our genetic material is a strong factor in determining our intellectual potential.
Over 2700 years ago, Homer (the Greek poet, not Homer Simpson) observed:
So it is that the gods do not give all men gifts of grace–neither good looks nor intelligence nor eloquence.
Not everyone can attain the same level of intelligence, but education, hard work, and a healthy lifestyle can play a strong role in helping a person reach the potential of their genetic inheritance. See also: What is intelligence & Can it be inherited? – Intelligence and IQ
