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.”
The Argument that Genetics is the main contributor to Intelligence
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.
The Argument that Environment is the main contributor to Intelligence
From the early 1930s and beyond, IQ tests were periodically restandardized. This meant that the researchers had to administer the IQ tests to a large number of people and set the average performance at 100. However, it was discovered that people were achieving scores substantially higher than 100 on the same IQ tests as those involved in the earlier standardization. This upward drift in IQ of about one standard deviation (15 points) every two generations is now known as the Flynn effect, in recognition of James R. Flynn’s discussion of its potential causes and implications.
To further complicate the issue, recent studies have revealed that while IQ scores increased between 1930 and 2000 (positive Flynn effect), IQ scores declined in subsequent decades (negative Flynn effect) in some countries–a decline that some researchers attribute to environmental effects such as declining educational standards and passive technologies.
In 2018, Bernt Bratsberg and Ole Rogeberg published “Flynn effect and Its Reversal are Both Environmentally Caused.” Proceedings of the National Academy of Sciences of the United States of America, accessible in PDF format at: http://www.pnas.org/content/early/2018/06/05/1718793115. In that publication, they presented the graph below suggesting that the increase in IQ has reversed and is in decline.
In 2020, Leehu Zysberg of Gordon College of education in Haifa, Israel published “The Reversal of the Flynn Effect,” http://www.pnas.org/content/early/2018/06/05/1718793115 , in which he presents the results of the world-wide PISA (Program for International Student Assessment) studies to verify that student performance in language and especially mathematics are in decline in the OECD countries. This raises a variety of questions. If IQ is mainly genetic, then why is there such variance in IQ over such a short period of time? If IQ is more malleable than once thought, what are the environmental experiences can enhance IQ? Are these effects merely statistical anomalies arising from comparing samples over different cohorts of significantly different populations? There is a great deal of speculation about whether humans are becoming more or less intelligent, and the answers when they come will be derived from mathematically rigorous statistical studies.
General Conclusions
Several studies of identical twins raised apart, along with evidence from the discovery of SNP’s associated with intelligence (See: Why can people have different IQ levels even if they have the same genetics and upbringing? – Intelligence and IQ ) suggest that genetics is a major component of intelligence. However, we don’t know to what extent environmental influences can expand intelligence. A rich learning environment at home and at school can go a long way to enhancing a person’s ability to think rationally. As Einstein once observed, “The value of a college education is not the learning of many facts, but the training of the mind to think.”
