This question is at the root of a “nature vs. nurture debate that has raged for the past three centuries and has now been illuminated by recent research. The intelligence that we develop through adulthood derives from the DNA we inherited from our parents (nature) and the experiences we accumulate in our voyage through life (nurture). The next two sections provide a brief perspective on the debate and summarize some of what research has revealed.
Historical Background
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.”
What some of the Research has revealed
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.
What we still don’t know
Although cognitive development through education and other experiences can increase our IQ, we still don’t know by how much, because it seems to vary substantially across individuals. Furthermore, there is no single gene that accounts for intelligence, but rather a large number of SNPs (single nucleotide polymorphisms) in a person’s DNA. (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.)
We also do not know how brain structure differs in people of different IQ levels, or why some brains seem to process information more efficiently than others. Such questions are currently under investigation by neurologists using sophisticated processes such as tomography and MRI scanning to observe brain function in vivo. For information on the inheritability of intelligence, visit Why can people have different IQ levels even if they have the same genetics and upbringing? – Intelligence and IQ
