The Wilson Effect was coined in 2013 after psychologist Dr. Ronald Wilson of the University of Louisville, who wrote (e.g. in 1983) that "mental development", basically IQ, change as we age. In childhood, he believed, nearly half of IQ variance can be explained by genetics, the rest was environment, while in adults this number climbs to about 80 percent.
This had an air of truthiness - many older people feel like they'd have been far better athletes when they were young if they had the confidence they had as adults and IQ tests were always weird. Kids who learned how to take tests or had been interested in the 'puzzle' form that parts of IQ tests had performed better.
Today, IQ is the BMI of intellect; it is probably okay for population-level statistics but no one should be making policy decisions based on that data. And people have had the knives out for Wilson for decades. If genetics mattered most, it led to arguments that if some people are 'born dumb' then entire groups marrying among that group would be dumber. If environment mattered less, a lot lot of groups that made their livelihoods getting government to give them money to improve the learning environment - subsidized tablets, subsidized internet, etc. - were out of business.
On the first part, evolution does not work that way, any more than two of you flipping a coin and getting heads means if you flip the next one together it will be heads. On the second part, if cultural engineering worked that way and it was proper for a political party in the 1980s to declare a college education 'a right' and make student loans unlimited, we wouldn't have 5,000 Ph.D.s working as janitors and 2,000,000 people with doing jobs that don't require anything more than high school.
There is a whole lot more data now so even if it is a small longitudinal study of IQ, both of which place it firmly in the EXPLORATORY rather than the science pile, the results are interesting. In a 2024 paper using two IQ measures, they found that the IQ of biological twins raised in separate homes converged over time - even controlling for age at adoption, contact with each other, and age of separation - while "virtual" twins, not biologically related but the same age and raised together diverged with increased age. '
'This was possible due to the Communist dictatorship in China mandating a One Child Policy. Twins were separated by the government. Another smaller group was in Denmark, sometimes of women in psychiatric asylums. The Virtual Twins data were from the Fullerton Twins study begun by Dr. Nancy L. Segal at California State University, Fullerton in 1991.
This does not settle the nature versus nurture debate but it is another interesting finding.
Citation: Nancy L. Segal, Elizabeth Pratt-Thompson, Developmental trends in intelligence revisited with novel kinships: Monozygotic twins reared apart v. same-age unrelated siblings reared together, Personality and Individual Differences, Volume 229, 2024, 112751, ISSN 0191-8869, DOI: 10.1016/j.paid.2024.112751
