During most of the 20th century, there were extensive debates about the extent to which intelligence was determined by the genes we inherit. The popular consensus was that intelligence is mainly a result of environmental experience. To some extent, this tacit assumption was wishful thinking by those who hoped to promote equity and inclusion, yet others, such as parents or teachers who observed young children, were able to see significant differences in mental function even between siblings. Those who believed that intelligence was mainly determined by environment were called “nurturists” and the others were referred to as “heditarians.” The emotionally-charged discussions between these two groups became known as the “nurture vs. nature” debates.
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 fact, they cited the Flynn Effect (the increase in average IQ during the late decades of the 20th century) as evidence that IQ scores can be increased by environmental influences. At least 5 subsequent studies involving samples of identical twins in the United States and Europe, who were reared apart, yielded heritabilities of IQ between 0.68 and 0.71, confirming the findings in the Minnesota study. In the face of this evidence, John Sedgwick, an advocate for the idea that environment plays a strong role in intelligence, conceded:
As for IQs genetic component, enough twins studies agree that heritability accounts for somewhere in the vicinity of 50% to 70% of intelligence, with 60% the most likely figure, which of course still leaves ample room for environmental influence.
Further studies showed that as siblings age, their IQ’s seem to correlate more closely than during their youth. Several different explanations have emerged to explain why this is so. It is generally believed that our intellectual development in early youth (say 5 years of age) is much more responsive to environmental influences than it is through adolescence. This could be a result of the prodigious creation and pruning of neurons that occur during these stages. Cognitive functions that are stimulated in early youth, are enhanced and those not used are soon purged. However, as the brain passes into puberty, it becomes less responsive to environmental influences and begins to create its own worlds. Different environmental experiences between two adolescent siblings have a decreasing influence on their intellectual differences and the similarity in their genetic makeup becomes the dominant factor in how their intelligence evolves. This makes sense from an anthropological standpoint. Nature plays a dominant role in acclimatizing the infant to its environment in the early development of basic cognition, as manifest in face recognition, language acquisition and walking. As the child moves into puberty, cognitive functions such as pattern recognition, abstract thinking and generalization–heritable faculties associated with neural efficiency–begin to evolve. Hence those of similar DNA will be more alike in intelligence than they were in the early years when environment played a larger role.
