Originally posted on March 1, 2016.
Amid concerns about the replicability of psychological science findings comes “a cause for celebration,” argue behavior geneticist Robert Plomin and colleagues (here). They identify ten “big” take-home findings that have been “robustly” replicated. Some of these are who-would-have-guessed surprises.
1. “All psychological traits show significant and substantial genetic influence.” From abilities to personality to health, twin and adoption studies consistently reveal hereditary influence.
2. “No traits are 100% heritable.” We are knitted of both nature and nurture.
3. “Heritability [differences among individuals attributable to genes] is caused by many genes of small effect.” There is no single “smart gene,” “gay (or straight) gene,” or “schizophrenia gene.”
4. "Correlations between psychological traits show significant and substantial genetic mediation.” For example, genetic factors largely explain the correlation found among 12-year-olds’ reading, math, and language scores.
5. “The heritability of intelligence increases throughout development.” I would have guessed—you, too?—that as people mature, their diverging life experiences would reduce the heritability of intelligence. Actually, heritability increases, from about 41% among 9-year-olds to 66% among 17-year-olds, and to even more in later adulthood, studies suggest.
6. “Age-to-age stability is mainly due to genetics.” This—perhaps the least surprising finding—indicates that our trait stability over time is genetically disposed.
7. “Most measures of ‘environment’ show significant genetic influence.” Another surprise: many measures of environmental factors—such as parenting behaviors—are genetically influenced. Thus if physically punitive parents have physically aggressive children both may share genes that predispose aggressive responding.
8. “Most associations between environmental measures and psychological traits are significantly mediated genetically.” For example, parenting behaviors and children’s behaviors correlate partly due to genetic influences on both.
9. “Most environmental effects are not shared by children growing up in the same family.” As Nathan DeWall and I report in Psychology, 11th Edition, this is one of psychology’s most stunning findings: “The environment shared by a family’s children has virtually no discernible impact on their personalities.”
10. “Abnormal is normal.” Psychological disorders are not caused by qualitatively distinct genes. Rather, they reflect variations of genetic and environmental influences that affect us all.
From this “firm foundation of replicable findings,” Plomin and colleagues conclude, science can now build deeper understandings of how nature and nurture together weave the human fabric.