MedicalResearch.com: What are the main findings of the study?
Answer: We asked ourselves how well theories of intelligence actually predict empirical results. To this end, we reviewed and scrutinized the predictions from intelligence theories and collected relevant results that have been published in the scientific literature over the last decades. The results pertained to intelligence test scores from thousands of subjects across the world. We found that on essential aspects the empirical results were opposite of the predictions from the mainstream theories of intelligence, in which intelligence is interpreted as a biological trait.
MedicalResearch.com: Were any of the findings unexpected?
Answer: As a number of empirical results were in contrast to mainstream theories, these results were indeed unexpected. The most surprising finding was that the larger the genetic influences on completing an intelligence tests are, the larger is also the dependency on educational and cultural factors: Heritability estimates of clearly culture-depended knowledge and skills, such as vocabulary, spelling, and general knowledge, are the highest among all abilities that are measured by intelligence test batteries. Hence they are higher than heritability estimates of less culture-depended abilities such as working memory, spatial ability, reasoning, and processing speed. This pattern does not follow from mainstream theories of intelligence, in which intelligence is interpreted as a biological, largely genetically fixed trait.
MedicalResearch.com: What should clinicians and patients take away from your report?
Answer: There is no evidence at all that – although intelligence is highly heritable in adulthood – intelligence is largely fixed by the genes. Intelligence in childhood is also not that predictive of intelligence in adulthood. Intelligence changes, and these changes are normal. A child patient who has an IQ below average may develop as a normally intelligent adult. Also the other way around. Parents and teachers of children who’s IQ is exceptionally high should not be surprised when their children lose their advantage or do not meet high expectations. We really need to get rid of the reasoning that because intelligence is highly heritable, it is hardly malleable by environmental factors. To draw a parallel: Differences in vocabulary are the most heritable, but does this imply vocabulary is hardly malleable by environment? No. Simply educate people in a different language or raise them in a different culture and their vocabulary turns out radically different from our vocabulary. Yet, if you raise children in the same culture, individual differences in genetic makeup will eventually become clearly visible.
MedicalResearch.com: What recommendations do you have for future research as a result of this study?
Answer: Heritability estimates have been based on a model in which genetic and environmental influences are independent from another. There lies a problem, because those influences go together. We stress that relations between genetic and environmental influences on intelligence deserve more attention from the scientific community. We advance the development of models in which genetic and environmental influences depend on another. This allows researchers to study dynamical interactions between environment and genetic influences throughout the course of development and to study their effects on the maturation of intelligence.