A recent concern in psychological science is that many statistically significant findings, including some classic findings, do not replicate.
This problem is not unique to psychological science. The landmark article “Why Most Published Research Findings Are False” (Ioannidis, 2005b) was relevant to all scientific research.
In this context, the purpose of this article is to highlight 10 findings about the genetic and environmental origins of individual differences in behavior that have consistently replicated. On the basis of our decades of experience in the field of behavioral genetics and our experience in writing the major textbook in the field, we selected these 10 findings because in our opinion they are “big” findings, both in terms of effect size and their potential impact on psychological science. These findings are not novel precisely because we selected results that have been repeatedly verified. For this reason, each f the findings in our top 10 list has been re viewed else-where, and a few have been highlighted previously as “laws” of behavioral genetics, as will be noted later. Although not all of these findings are supported by formal meta-analyses, we expect that most behavioral geneticists will agree with the 10 findings on our list, but we also suspect they may wish to add to the list. What is novel about our article is that we bring together 10 reproducible findings from behavioral genetics and consider reasons specific to behavioral genetics that might explain why these results replicate and why others do not.
FINDING 1: ALL PSYCHOLOGICAL TRAITS SHOW SIGNIFICANT AND SUBSTANTIAL GENETIC INFLUENCE
Psychological domains traditionally focused on individual differences are those that have been studied most frequently with genetically sensitive designs, primarilythe twin method in which resemblance is compared in pairs of identical and fraternal twins: cognitive abilities and disabilities, psychopathology, personality, substance use and abuse, and health psychology. Traits in these domains have consistently shown significant genetic influence in adequately powered studies
Although ubiquitous genetic influence is now widely accepted, this finding should not be taken for granted because it was a battleground in psychology as recently as a few decades ago and remains controversial in some areas such as education.
As an example, a review of the world’s literature on intelligence that included 10,000 pairs of twins showed identical twins to be significantly more similar than fraternal twins (twin correlations of about .85 and .60, respectively), with corroborating results from family and adoption studies, implying significant genetic influence. We are not aware of a single adequately powered study reporting nonsignificant heritability.
Traits such as political beliefs, religiosity, altruism, and food preferences also have shown significant genetic influence. A recent metaanalysis of data drawn from 3,000 publications on nearly 18,000 traits of 15 million twin pairs showed that this finding is not limited to psychological traits
Significant and substantial genetic influence on individual differences in psychological traits is so widespread that we are unable to name an exception. The challenge now is to find any reliably measured behavioral trait for which genetic influence is not significantly different from zero in more than one adequately powered study.
Camden Wright
FINDING 2: NO TRAITS ARE 100% HERITABLE
As noted earlier, heritability estimates are substantial, typically between 30% and 50%, but this range of estimates is a long way from 100%. Again, we are unable to find any excep- tion in which the heritability of a behavioral trait is near 100%. However, this is not a limitation of the methods, because some traits, such as individual differences in height, yield heritability as high as 90%. It should be noted that behavioral traits are less reliably measured than physical traits such as height, and error of measurement contributes to nonheritable variance.
Although this finding might seem obvious and unsurprising, it is crucial because it provides the strongest available evidence for the importance of environmental influence after controlling for genetic influence. Because genetic influence is significant and substantial, one must control for genetic influence when investigating environmental influence.
Environmental research using genetically sensitive designs has led to three of the most important discoveries about the way the environment affects behavioral development
Austin Reyes
FINDING 3: HERITABILITY IS CAUSED BY MANY GENES OF SMALL EFFECT
Powerful but overlooked evidence that many genes affect complex traits including behavior comes from selection studies in nonhuman animal research. If only a few genes were responsible for the heritability of a trait, selected lines would separate after a few generations and would not diverge any further in later generations. In contrast, selection studies of complex traits show a linear response to selection even after dozens of generations of selection, as seen, for example in one of the largest and longest selection studies of behavior that included replicate selected and control lines
Although GWA studies have limited power to detect such minuscule effects even with samples in the tens or hundreds of thou- sands, these studies have tremendous power to detect larger effects. For example, a GWA study of 20,000 individuals has 99.9% power to detect an association with an effect size that accounts for 1% of the variance (i.e., a correlation of .10). This suggests that no such associations exist with effect sizes larger than 1% in the population. Some extremely rare mutations have large effects on individuals, but because they are rare, their effect on the population is small. If the largest effects are so small, the smallest effects are likely to be infinitesimal, which implies that heritability is caused by many genes of small effects.
Jack Campbell
FINDING 4: PHENOTYPIC CORRELATIONS BETWEEN PSYCHOLOGICAL TRAITS SHOW SIGNIFICANT AND SUBSTANTIAL GENETIC MEDIATION
Few of the thousands of reported correlations between traits such as these have been studied with genetically sensitive designs. However, when genetically informed designs are used, research consistently points to a finding with far-reaching implications: Phenotypic covariance between traits is significantly and substantially caused by genetic covariance, not just environmentally driven covariance.
Cognitive abilities have been studied most systematically from a multivariate genetic perspective. This research consistently shows that the phenotypic correlations among cognitive abilities are mediated significantly and substantially by genetic factors called generalist genes. For example, a multivariate genetic analysis of intelligence, reading, mathematics, and language in over 5,000 pairs of 12-year-old twins showed that genetic factors consistently accounted for more than half of the phenotypic correlations, ranging from 53% to 65%, with a mean of 61% and a mean 95% confidence interval of between 53% and 67%. These findings have received support from multivariate GCTA. One implication of this finding is that the phenotypic structure of domains is similar to their genetic structure, as has been shown, for example, for cognitive abilities and personality.
Adam Kelly
FINDING 5: THE HERITABILITY OF INTELLIGENCE INCREASES THROUGHOUT DEVELOPMENT
Although the effects of experiences could be reasonably expected to accumulate as time goes by (as some developmental theorists propose) the heritability of intelligence has been shown consistently to increase linearly throughout the life course in more than three decades of research in longitudinal as well as crosssectional analyses and in adoption as well as twin studies. For example, an analysis of cross-sectional data for 11,000 pairs of twins—larger than all previous twin studies combined—showed that the heritability of intelligence increases significantly from 41% in childhood (age 9) to 55% in adolescence (age 12) and to 66% in young adulthood. The nonoverlapping standard errors in Figure 3 suggest that the increases in heritability across the three ages are significant, and model fitting confirmed that the increases are significant. A meta-analysis of results from longitudinal twin and adoption studies also showed increases in heritability from infancy through adolescence. Some evidence suggests that heritability might increase to as much as 80% in later adulthood independent of dementia; other results suggest a decline to about 60% after age 80
Why does heritability of intelligence increase throughout development? Increasing heritability could be due to new genetic influences coming online, a process called innovation, which would seem reasonable given the changes in brain structure and function that occur during development. However, the next finding, about age-to-age genetic stability, suggests a less obvious reason for the developmental increase in heritability.
Aiden Harris
FINDING 6: AGE TO AGE STABILITY IS MAINLY DUE TO GENETICS
Longitudinal genetic studies consistently show that phenotypic correlations from age to age are largely due to genetic stability. In other words, genetic effects contribute to continuity (the same genes affect the trait across age), whereas age-to-age change is primarily the provenance of environmental factors.
For personality, the first report of a longitudinal genetic analysis over an age span of a decade showed that 80% of the phenotypic stability was mediated genetically, which has been confirmed in recent meta-analyses. For psychopathology, results are similar for diverse traits related to psychopathology such as, antisocial personality disorder, aggression
For intelligence similar results have been found. This finding creates an apparent paradox: How can the heritability of intelligence increase so substantially throughout development if genetic effects are stable? That is, how can the same genes largely affect intelligence across the life course and yet account for more variance as time goes by? Increasing heritability despite genetic stability implies some contribution from what has been called genetic amplification. In other words, genetic nudges early in development are magnified as time goes by increasing heritability, but the same genetic propensities continue to affect behavior throughout the life course. This amplification model has recently been supported in a meta-analysis of 11,500 twin and sibling pairs with longitudinal data on intelligence, which showed that a genetic amp model fit the data better than a model in which new genetic influences arise across time. Genotype-environment correlation seems the most likely explanation in which small genetic differences are amplified as children select, modify, and create environments correlated with their genetic propensities
Jacob Hughes
post more
Parker Brown
FINDING 7: MOST MEASURES OF THE ENVIRONMENT SHOW SIGNIFICANT GENETIC INFLUENCE
Although it might seem a peculiar thing to do, measures of the environment widely used in psychological science—such as parenting, social support, and life events can be treated as dependent measures in genetic analyses. If they are truly measures of the environment, they should not show genetic influence. To the contrary, in 1991, Plomin and Bergeman conducted a review of the first 18 studies in which environmental measures were used as dependent measures in genetically sensitive designs and found evidence for genetic influence for these measures of the environment. Significant genetic influence was found for objective measures such as videotaped observations of parenting
Why do measures of the environment show genetic influence? The reason appears to be that such measures do not assess the environment independent of the person. As noted earlier, humans select, modify, and create environments correlated with their genetic behavioral propensities such as personality and psychopathology. For example, in studies of twin children, parenting has been found to reflect genetic differences in children’s characteristics such as personality and psychopathology. Meta-analyses of parenting, the most frequently studied domain, have shown genetic influence that is driven by child characteristics as well as by parent characteristics. Shikishima, Hiraishi, Yamagata, Neiderhiser, and Ando (2012) compared parenting in Japan and Sweden and found that parenting in Japan showed more genetic influence than in Sweden, consistent with the view that parenting is more child centered in Japan than in the West.
Jason Jones
mmmmm more.
Michael Miller
FINDING 8: MOST ASSOCIATIONS BETWEEN ENVIRONMENTAL MEASURES AND PSYCHOLOGICAL TRAITS ARE SIGNIFICANTLY MEDIATED GENETICALLY
If genetic factors affect environmental measures as well as behavioral measures, it is reasonable to ask the extent to which associations between environmental measures and behavioral measures are mediated genetically. For example, rather than assuming that correlations between parenting and children’s behavior are caused by the environmental effect of parenting on children’s behavior, one should consider the possibility that the correlation is in part due to genetic factors that influence both parenting and children’s behavior.
In twin studies, multivariate genetic analysis can be used to disentangle genetic and environmental effects from correlations between environmental measures and behavioral measures. The first study of this type indicated that two thirds of the correlation between maternal negativity and adolescent children’s antisocial behavior could be attributed to genetic factors. More than 100 studies have reported similar results.
Disentangling genetic and environmental influences on correlations between environmental and behavioral measures is important for three reasons. First, if these correlations are mediated genetically, interpretations that assume environmental causation are wrong, which has important implications for intervention. Second, genetically sensitive designs can be used to identify causal effects of the environment free of genetic confound. Third, genetic mediation of the association between environmental measures and behavioral traits is not just a nuisance that needs to be controlled. It suggests a general way of thinking about how genotypes develop into phenotypes, from a passive model of imposed environments to an active model of shaped experiences in which humans select, modify, and create experiences in part based on their genetic propensities.
Joseph Long
FINDING 9: MOST ENVIRONMENTAL EFFECTS ARE NOT SHARED BY CHILDREN GROWING UP IN THE SAME FAMILY
It is reasonable to think that growing up in the same family makes brothers and sisters similar psychologically, which is what developmental theorists from Freud onwards have assumed. However, for most behavioral dimensions and disorders, it is genetics that accounts for similarity among siblings. Although environmental effects have a major impact, the salient environmental influences do not make siblings growing up in the same family similar. The message is not that family experiences are unimportant but rather that the relevant experiences are specific to each child in the family. This finding was ignored when it was first noted and controversial when it was first highlighted, but it is now widely accepted because it has consistently replicated
It seems likely that nonshared environmental effects are due to many experiences of small effect, analogous to Finding 3 (“Heritability is caused by many genes of small effect”). That is, rather than asking whether a monolithic factor like parental control is primarily responsible for nonshared effects, it might be necessary to consider many seemingly inconsequential experiences that are tipping points in children’s lives. The gloomy prospect is that these could be idiosyncratic stochastic experiences. However, the basic finding that most environmental effects are not shared by children growing up in the same family remains one of the most far-reaching findings from behavioral genetics.
It is important to reiterate that the message is not that family experiences are unimportant but rather that the salient experiences that affect children’s development are specific to each child in the family, not general to all children in the family.
Nathan Wright
Annnnnnnnnnnd again?
Eli Gutierrez
FINDING 10: ABNORMAL IS NORMAL
A fundamental question about common psychological disorders is the extent to which genetic and environmental effects on disorders are merely the quantitative extremes of the same genetic and environmental factors that affect the rest of the distribution. Or are common disorders qualitatively different from the normal range of behavior? There are thousands of rare single-gene disorders such as phenylketonuria (PKU), which causes intellectual disability and has a frequency of about 1 in 10,000. This is the way in which people often think about disorders—as qualitatively different from the normal range of behavior. However, disorders studied by psychologists are much more common, including learning disabilities and psychopathology such as schizophrenia, autism, and hyperactivity.
In this way, these two quantitative genetic methods—DF extremes analysis and liability- threshold model fitting—lead to the conclusion that common disorders represent the extremes of the same genetic influences responsible for heritability throughout the distribution.
Stated more provocatively, there are no common disorders, just quantitative traits—the abnormal is normal. This finding supports the recently adopted National Institute of Mental Health Research Domain Criteria strategy that focuses on dimensional models of psychopathology rather than diagnostic categories.
There is also a less obvious implication. This “risk” label misses the point that because these polygenic scores are distributed normally, their distrib. has a positive end as well as a negative end. This opens up opportunities for considering positive genetics - how children flourish rather than flounder and about resilience rather than vulnerability
Plomin, R., Defries, J. C., Knopik, V. S., & Neiderhiser, J. M. (2016). Top 10 Replicated Findings From Behavioral Genetics. Perspectives on Psychological Science, 11(1), 3-23. doi:10.1177/1745691615617439
Jonathan Anderson
well this bumped. an interesting thread. i guess there was no real question.
Landon Bell
tl;dr
Alexander Perez
So when do you guys think the nature vs nurture debate will stop being so controversial?
I honestly think this should be taught at schools. Or at least the simplified version, "50% from your parents and 50% from your culture". It's amazing the amount of people I've found whose view of the world is completely distorted because they don't know about this.
