Can parenting affect children’s outcomes? Can you raise your child to be better, healthier, smarter, more successful?

There’s a lot of evidence, from twin and adoption studies, that behavioral traits are highly heritable and not much affected by adoptive parents or by the environment shared between siblings.

High heritability does not strictly imply that parenting doesn’t matter, for a few reasons.

  1. Changes across the entire population don’t affect heritability. For example, heights have risen as nutrition improves, but height remains just as heritable. So if parenting practices have changed over time, heritability won’t show whether those changes helped or hurt children.
  2. Family environment and genes may be positively correlated. For instance, if a gene for anxiety causes both anxiety in children and harshness in parents, then it may be that the parenting still contributes to the children’s anxiety. If parents who overcome their genetic predispositions are sufficiently rare, it may still be possible that choosing to parent differently can help.
  3. Rare behaviors won’t necessarily show up at the population level. Extremely unusual parenting practices can still be helpful (or harmful), if they’re rare enough to not be caught in studies. Extremely unusual outcomes in children (like genius-level achievement) might also not be caught in studies.
  4. Subtle effects don’t show up in studies that easily. A person who has to spend a lot of time in therapy unlearning subtle emotional harms from her home environment won’t necessarily show up as having a negative outcome on a big correlational study.

With those caveats in mind, let’s see what the twin and adoption studies show.

Personality

In a study of 331 pairs of twins reared together and apart, a negligible proportion of the variance in personality was due to shared family environment. About 50% of the variance in personality scores was due to genetics; average heritability was 0.48.[1]

Attachment Style

In a study of 125 early-adopted adolescents, secure-attached infants were more likely to grow into secure-attached teenagers (correlation 0.30, p<0.01), and mothers of secure adolescents were more likely to show “sensitive support” (high relatedness and autonomy in resolving disagreements with children) at age 14. (p < 0.03).[2]

Antisocial/Criminal Behavior

An adoption study found that adolescents whose adoptive parents had high levels of conflict with them (arguments, hitting, criticizing and hurting feelings, etc) were more likely to have conduct problems. Correlations were between 0.574 and 0.696. Effects persisted longitudinally (i.e. past conflict predicted future delinquency).[3]

A meta-analysis of 51 twin and adoption studies found that 32% of the variance in antisocial behavior was due to genetic influences, while 16% was due to shared environment influences.[4]

Drug Abuse

In a Swedish adoption study of 18,115 children, adopted children with biological parents who abused drugs were twice as likely to abuse drugs themselves, while there was no elevated risk for having an adoptive parent who abused drugs. However, adoptive _siblings _of adopted children with DA were twice as likely to abuse drugs as adoptive siblings of adopted children without DA. This implies that there is both environmental and genetic influence, but suggests that environmental influence may be more about peers than parents.[5]

Psychiatric Disorders

Having a mother (but not a father) with major depression was associated in adoptive children getting major depression, in a study of 1108 adopted and nonadopted adolescents. Odds ratio of having a mother with major depression was 3.61 for nonadopted children and 1.97 for adopted children. Odds ratio of externalizing disorders if you had a mother with depression was 2.23 for nonadopted children and 1.69 for adopted children.[6]

IQ

The Minnesota Study of Twins Reared Apart, which includes more than 100 pairs of twins, and found that 70% of the variance in IQ of monozygotic twins raised apart was genetic. No environmental factor (father’s education, mother’s education, socioeconomic status, physical facilities) contributed more than 3% of the variance between twins. Identical twins correlate about 70% in IQ, 53% on traditionalism, 49% on religiosity, 34% on social attitudes, etc. Identical twins reared apart are roughly as similar as identical twins reared together.[7]

According to a twin study, heritability on PSAT scores was 50-75%, depending on subscore.[8]

Years of Schooling

In the Wisconsin Longitudinal Survey, of 16481 children of which 610 were adopted, finds that adopted parental income has a significant positive effect on years of schooling. Adoptive father’s years of schooling had a significant effect, but adoptive mother’s years of schooling were not significant. In nonadopted families, parental IQ and years of schooling (both mother and father) have a statistically significant effect.[9]

Reading Achievement

The Colorado Adoption Study finds that heritability of reading usually explains about 40% of the variance in outcomes in reading achievement, while adoptive-sibling correlations (a measure of shared environment) explain less than 10% of the variance. The rest is non-shared environment. Unrelated sibling correlations are 0.05, while related sibling correlations are 0.26. Genetic correlations rise with age (from 0.34 at age 7 to 0.67 at age 16).[10]

In the Western Reserve Twin Study of 278 twin pairs, ages 6-12, IQ score variance was mostly due to heritability (37%-78%, depending on subscore) and not on shared environment (<8%). However, school achievement was more dependent on shared environment (65-73%) than heritability (19-27%).[11]

In a twin study, spelling ability has a heritability of 0.53.[12]

Language ability in toddlers, in a twin study, was found to be more dependent on shared environment than genetics: 71% of variance explained by shared environment, 28% explained by genetics. This was reversed in the case of reading ability in 7-10-year-olds, where 72% of variance was explained by genetics, while 20% was explained by shared environment. Maybe the effects of home environment fade out with age.[13]

Academic Achievement

A twin study of 2602 twin pairs found that 62% of variance in science test scores at age 9 was explained by heredity, compared to 14% shared environment. There was no difference between boys and girls in heritability.[14]

51-54% of variance in grades, in the Minnesota Twin Study, is due to heredity, in girls and boys respectively. Similar genetic contributions to IQ (52%, 37%), externalizing behavior (45%, 47%) and engagement (54%, 49%). Shared environment mattered less (26%). The majority (55%) of the change in grades after age 11 is due to “nonshared environment.”[15]

Income

The National Longitudinal Study of Youth which included full and half-siblings found IQ was 64% heritable, education was 68% heritable, and income was 42% heritable. Almost all the rest of income variation was non-shared environment (49%), leaving only 9% explained by shared environment.[16]

In a study of Finnish twins, 24% of the variance of women’s lifetime income and 54% of the variance of men’s lifetime income was due to genetic factors, and the contribution of shared environment is negligible.[17]

Corporal Punishment

In laboratory settings, corporal punishment is indeed effective at getting immediate compliance. In a meta-analysis of mostly correlational and longitudinal studies, the weighted mean effect size of corporal punishment was -0.58 on the parent-child relationship, -0.49 on childhood mental health, 0.42 on childhood delinquent and antisocial behavior, 0.36 on childhood aggression, 1.13 on immediate compliance. There were no large adult effects significant at a <0.01 level, but there was an effect size of 0.57 on aggression significant at a <0.05 level.

Bottom line is that corporal punishment is fairly bad for childhood outcomes, but doesn’t usually cause lasting trauma or adult criminal/abusive behavior; still, there are good evidence-based reasons not to do it.

What Parenting Can’t Affect

Personality, IQ, reading ability in teenagers, and income are affected negligibly by the “shared environment” contribution. Drug abuse is also very heritable and not much affected by parenting.

What Parenting Might Affect

Reading ability in children and grades in teenagers have a sizable (but minority) shared environment component; reading ability in toddlers is _mostly _affected by shared environment. Grades are generally less IQ-correlated than test scores, and are highly affected by school engagement and levels of “externalizing” behavior (disruptive behavior, inattention, criminal/delinquent activity.) Antisocial and criminal behavior has a sizable (but minority) shared environment component. You may be able to influence your kids to behave better and study harder, and you can definitely teach your kids to read younger, though a lot of this may turn out to be a wash by the time your kids reach adulthood.

What Parenting Can Affect

Having a mother — even an adoptive mother — with major depression puts children at risk for major depression, drug abuse, and externalizing behavior. Conflict at home also predicts externalizing behavior in teenagers. Mothers of teenagers who treat them well are more likely to have teenagers who have loving and secure relationships with them. Basically, if I were to draw a conclusion from this, it would be that it’s good to have a peaceful and loving home and a mentally healthy mom.

Father’s income and family income, but not mother’s income, predicts years of schooling; I’m guessing that this is because richer families can afford to send their kids to school for longer. You can, obviously, help your kids go to college by paying for it.

References

[1]Tellegen, Auke, et al. “Personality similarity in twins reared apart and together.” Journal of personality and social psychology 54.6 (1988): 1031.

[2]Klahr, Ashlea M., et al. “The association between parent–child conflict and adolescent conduct problems over time: Results from a longitudinal adoption study.” Journal of Abnormal Psychology 120.1 (2011): 46.

[3]Klahr, Ashlea M., et al. “The association between parent–child conflict and adolescent conduct problems over time: Results from a longitudinal adoption study.” Journal of Abnormal Psychology 120.1 (2011): 46.

[4]Rhee, Soo Hyun, and Irwin D. Waldman. “Genetic and environmental influences on antisocial behavior: a meta-analysis of twin and adoption studies.” Psychological bulletin 128.3 (2002): 490.

[5]Kendler, Kenneth S., et al. “Genetic and familial environmental influences on the risk for drug abuse: a national Swedish adoption study.” Archives of general psychiatry 69.7 (2012): 690-697.

[6]Tully, Erin C., William G. Iacono, and Matt McGue. “An adoption study of parental depression as an environmental liability for adolescent depression and childhood disruptive disorders.” American Journal of Psychiatry 165.9 (2008): 1148-1154.

[7]Bouchard, T., et al. “Sources of human psychological differences: The Minnesota study of twins reared apart.” (1990).

[8]Nichols, Robert C. “The national merit twin study.” Methods and goals in human behavior genetic (1965): 231-244.

[9]Plug, Erik, and Wim Vijverberg. “Does family income matter for schooling outcomes? Using adoptees as a natural experiment.” The Economic Journal 115.506 (2005): 879-906.

[10]Wadsworth, Sally J., et al. “Genetic and environmental influences on continuity and change in reading achievement in the Colorado Adoption Project.” Developmental contexts of middle childhood: Bridges to adolescence and adulthood (2006): 87-106.

[11]Thompson, Lee Anne, Douglas K. Detterman, and Robert Plomin. “Associations between cognitive abilities and scholastic achievement: Genetic overlap but environmental differences.” Psychological Science 2.3 (1991): 158-165.

[12]Stevenson, Jim, et al. “A twin study of genetic influences on reading and spelling ability and disability.” Journal of child psychology and psychiatry 28.2 (1987): 229-247.

[13]Harlaar, Nicole, et al. “Why do preschool language abilities correlate with later reading? A twin study.” Journal of Speech, Language, and Hearing Research 51.3 (2008): 688-705.

[14]Haworth, Claire MA, Philip Dale, and Robert Plomin. “A twin study into the genetic and environmental influences on academic performance in science in nine‐year‐old boys and girls.” International Journal of Science Education 30.8 (2008): 1003-1025.

[15]Johnson, Wendy, Matt McGue, and William G. Iacono. “Genetic and environmental influences on academic achievement trajectories during adolescence.” Developmental psychology 42.3 (2006): 514.

[16]Rowe, David C., Wendy J. Vesterdal, and Joseph L. Rodgers. “Herrnstein’s syllogism: Genetic and shared environmental influences on IQ, education, and income.” Intelligence 26.4 (1998): 405-423.

[17]Hyytinen, Ari, et al. “Heritability of lifetime income.” (2013).

[18]Gershoff, Elizabeth Thompson. “Corporal punishment by parents and associated child behaviors and experiences: a meta-analytic and theoretical review.” Psychological bulletin 128.4 (2002): 539.