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.
- “All psychological traits show significant and substantial genetic influence.” From abilities to personality to health, twin and adoption studies consistently reveal hereditary influence.
- “No traits are 100% heritable.” We are knitted of both nature and nurture.
- “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.”
- "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.
- “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.
- “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.
- “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.
- “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.
- “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.”
- “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.
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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.