Intelligence is genetic

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Intelligence is a psychological construct that refers to the ability to understand complex ideas, to adapt effectively to the environment, to learn from experience, and to engage in various forms of reasoning (Neisser et al., 1996). Intelligence is an important predictor of educational and occupational success (Schmidt & Hunter, 2004) and relates closely to positive life outcomes such as health and longevity (Deary, Whiteman, Starr, Whalley, & Fox, 2004). Understanding the neurobiological basis of intelligence is therefore an important aim of ongoing research across multiple scientific disciplines. This article provides a focused overview of the current state of research. Specifically, we emphasize robust and well-established empirical benchmark findings from electrophysiological, neuroimaging, and genetic research on intelligence (see Fig. 1 for a summary of those benchmark findings).

https://www.sciencedirect.com/science/article/abs/pii/S0160289622000460

Are the tests able to measure human intelligence and does expressing it in a single number—IQ score—make sense? Despite critiques of this reductionist approach to intelligence, the tests have proven their validity and relevance. First, results of IQ tests strongly correlate with life outcomes, including socioeconomic status and cognitive ability, even when measured early on in life (Foverskov et al., 2017). The increasing complexity and technology-dependent society imposes ever growing cognitive demands on individuals in almost every aspect of everyday life, such as banking, using maps and transportation schedules, reading and understanding forms, interpreting news articles. Higher intelligence offers many seemingly small advantages, but they accumulate to affect overall chances in life of individuals (Gottfredson, 1997). These are beneficial to socioeconomic status, education, social mobility, job performance, and even lifestyle choices and longevity (Lam et al., 2017).

Second, intelligence turns out to be a very stable trait from young to old age in the same individual. In a large longitudinal study of English children, a correlation of 0.81 was observed between intelligence at 11 years of age and scores on national tests of educational achievement 5 years later. This contribution of intelligence was evident in all 25 academic disciplines (Deary et al., 2007). Even at much later age, intelligence remains stable: a single test of general intelligence taken at age 11 correlated highly with the results of the test at the age of 90 (Deary et al., 2013).

Finally, one of the most remarkable findings of twin studies is that heritability of intelligence is extraordinarily large, in the range 50%–80% even reaching 86% for verbal IQ (Posthuma et al., 2001). This makes human intelligence one of the most heritable behavioral traits (Plomin and Deary, 2015). Moreover, with every generation, assortative mating infuses additive genetic variance into the population, contributing to this high heritability (Plomin and Deary, 2015).

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6384251/

Researchers have identified over 1,016 specific genes associated with intelligence, the vast majority of which are unknown to science.

An international team conducted a large-scale genetic association study of intelligence and discovered 190 new genomic loci and 939 new genes linked with intelligence, significantly expanding our understanding of the genetic bases of cognitive function.

Led by statistical geneticist Danielle Posthuma from Vrije Universiteit Amsterdam in the Netherlands, the researchers performed a genome-wide association study (GWAS) of almost 270,000 people from 14 independent cohorts of European ancestry.

https://www.sciencealert.com/scientists-have-discovered-almost-1-000-new-genes-associated-with-intelligence

The single-nucleotide polymorphism heritability for the extreme IQ trait was 0.33 (0.02), which is the highest so far for a cognitive phenotype, and significant genome-wide genetic correlations of 0.78 were observed with educational attainment and 0.86 with population IQ. Three variants in locus ADAM12 achieved genome-wide significance, although they did not replicate with published GWA analyses of normal-range IQ or educational attainment. A genome-wide polygenic score constructed from the GWA results accounted for 1.6% of the variance of intelligence in the normal range in an unselected sample of 3414 individuals, which is comparable to the variance explained by GWA studies of intelligence with substantially larger sample sizes. The gene family plexins, members of which are mutated in several monogenic neurodevelopmental disorders, was significantly enriched for associations with high IQ.

https://www.nature.com/articles/mp2017121

Cognitive ability differences form a hierarchy of variances. This grew from the finding that all cognitive tests are positively correlated; people who score well on one cognitive test tend to score well on all the others, no matter how different the cognitive skills being assessed appear to be. This finding has been replicated consistently since Charles Spearman discovered it in 1904 [4].

https://www.nature.com/articles/s41380-021-01027-y

As shown in Fig. 2, high intelligence, defined as the highest 5% of scores, is highly familial. For siblings of probands (i.e., those with a standardised score of 1.98, equivalent to a raw stanine score of 9), the distribution of intelligence is shifted sharply to the right of that of the rest of the population, with a mean score (0.81) approximately halfway between the proband score and the population mean (0). These results suggest a sibling "group correlation" (the ratio between the siblings' deviation from the population mean to the probands' deviation from the population mean) of 0.41. In other words, almost half of the difference between high intelligence and the rest of the population is familial in origin.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4286575/

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Analysis showed that the men born in 1962 had higher scores than those born in 1991.

Those born in 1991 scored five points lower than those born in 1975, and three points lower than those born in 1962.

This is the opposite of what happened during much of the 20th century when IQ scores rose by around 3% a year.

https://www.weforum.org/agenda/2018/06/iq-scores-have-been-falling-for-decades-new-study-finds/