Behavioural genetics

Part of a series on
Psychology
Outline History Subfields
Basic psychology Abnormal Affective neuroscience Affective science Behavioral genetics Behavioral neuroscience Behaviorism Cognitive/Cognitivism Cognitive neuroscience Social Comparative Cross-cultural Cultural Developmental Differential Ecological Evolutionary Experimental Gestalt Intelligence Mathematical Moral Neuropsychology Perception Personality Psycholinguistics Psychophysiology Quantitative Social Theoretical
Applied psychology Anomalistic Applied behavior analysis Assessment Clinical Coaching Community Consumer Counseling Critical Educational Ergonomics Feminist Forensic Health Humanistic Industrial and organizational Legal Media Medical Military Music Occupational health Pastoral Political Positive Psychometrics Psychotherapy Religion School Sport and exercise Suicidology Systems Traffic
Concepts Behavior Behavioral engineering Behavioral genetics Behavioral neuroscience Cognition Competence Consciousness Consumer behavior Emotions Feelings Human factors and ergonomics Intelligence Mind Psychology of religion Psychometrics
Lists Counseling topics Disciplines Organizations Outline Psychologists Psychotherapies Research methods Schools of thought Timeline Topics
Psychology portal
v t e

Behavioural genetics, also referred to as behaviour genetics, is a field of

Findings from behavioural genetic research have broadly impacted modern understanding of the role of genetic and environmental influences on behaviour. These include evidence that nearly all researched behaviours are under a significant degree of genetic influence, and that influence tends to increase as individuals develop into adulthood. Further, most researched human behaviours are influenced by a

The field of behavioural genetics, as founded by Galton, was ultimately undermined by another of Galton's intellectual contributions, the founding of the eugenics movement in 20th century society.^[3] The primary idea behind eugenics was to use selective breeding combined with knowledge about the inheritance of behaviour to improve the human species.^[3] The eugenics movement was subsequently discredited by scientific corruption and genocidal actions in Nazi Germany. Behavioural genetics was thereby discredited through its association to eugenics.^[3] The field once again gained status as a distinct scientific discipline through the publication of early texts on behavioural genetics, such as Calvin S. Hall's 1951 book chapter on behavioural genetics, in which he introduced the term "psychogenetics",^[7] which enjoyed some limited popularity in the 1960s and 1970s.^[8][9] However, it eventually disappeared from usage in favour of "behaviour genetics".

The start of behaviour genetics as a well-identified field was marked by the publication in 1960 of the book Behavior Genetics by John L. Fuller and William Robert (Bob) Thompson.^[1][10] It is widely accepted now that many if not most behaviours in animals and humans are under significant genetic influence, although the extent of genetic influence for any particular trait can differ widely.^[11][12] A decade later, in February 1970, the first issue of the journal Behavior Genetics was published and in 1972 the Behavior Genetics Association was formed with Theodosius Dobzhansky elected as the association's first president. The field has since grown and diversified, touching many scientific disciplines.^[3][13]

Methods

The primary goal of behavioural genetics is to investigate the nature and origins of individual differences in behaviour.^[3] A wide variety of different methodological approaches are used in behavioural genetic research,^[14] only a few of which are outlined below.

Animal studies

This section needs expansion. You can help by adding to it. (February 2020)

Investigators in animal behaviour genetics can carefully control for environmental factors and can experimentally manipulate genetic variants, allowing for a degree of causal inference that is not available in studies on

Machine learning and A.I. developments are allowing researchers to design experiments that are able to manage the complexity and large data sets generated, allowing for increasingly complex behavioural experiments. [24]

Human studies

Some research designs used in behavioural genetic research are variations on

Twin studies of monozygotic and dizygotic twins use a biometrical formulation to describe the influences on twin similarity and to infer heritability.[25]^[29] The formulation rests on the basic observation that the variance in a phenotype is due to two sources, genes and environment. More formally, ${\displaystyle Var(P)=g+(g\times \epsilon )+\epsilon }$, where ${\displaystyle P}$ is the phenotype, ${\displaystyle g}$ is the effect of genes, ${\displaystyle \epsilon }$ is the effect of the environment, and ${\displaystyle (g\times \epsilon )}$ is a

${\displaystyle g}$

${\displaystyle a^{2}}$

${\displaystyle d^{2}}$

${\displaystyle i^{2}}$

${\displaystyle \epsilon }$

${\displaystyle c^{2}}$

${\displaystyle e^{2}}$

${\displaystyle g}$

${\displaystyle \epsilon }$

${\displaystyle Var(P)=(a^{2}+d^{2}+i^{2})+(c^{2}+e^{2})}$

Decomposing the genetic and environmental contributions to twin similarity.^[25]

Type of relationship	Full decomposition	Falconer's decomposition
Perfect similarity between siblings	${\displaystyle 1.0=a^{2}+d^{2}+i^{2}+c^{2}+e^{2}}$	${\displaystyle 1.0=a^{2}+c^{2}+e^{2}}$
Monozygotic twin correlation(${\displaystyle r_{MZ}}$)	${\displaystyle r_{MZ}=a^{2}+d^{2}+i^{2}+c^{2}}$	${\displaystyle r_{MZ}=a^{2}+c^{2}}$
Dizygotic twin correlation (${\displaystyle r_{DZ}}$)	${\displaystyle r_{DZ}={\frac {1}{2}}a^{2}+{\frac {1}{4}}d^{2}+(k)i^{2}+c^{2}}$	${\displaystyle r_{DZ}={\frac {1}{2}}a^{2}+c^{2}}$
Where ${\displaystyle k}$ is an unknown (probably very small) quantity.

The simplified Falconer formulation can then be used to derive estimates of ${\displaystyle a^{2}}$, ${\displaystyle c^{2}}$, and ${\displaystyle e^{2}}$. Rearranging and substituting the ${\displaystyle r_{MZ}}$ and ${\displaystyle r_{DZ}}$ equations one can obtain an estimate of the additive genetic variance, or heritability, ${\displaystyle a^{2}=2(r_{MZ}-r_{DZ})}$, the non-shared environmental effect ${\displaystyle e^{2}=1-r_{MZ}}$ and, finally, the shared environmental effect ${\displaystyle c^{2}=2r_{DZ}-r_{MZ}}$.

The Human Genome Project has allowed scientists to directly genotype the sequence of human DNA nucleotides.^[31] Once genotyped, genetic variants can be tested for association with a behavioural phenotype, such as mental disorder, cognitive ability, personality, and so on.^[32]

• Candidate Genes. One popular approach has been to test for association candidate genes with behavioural phenotypes, where the candidate gene is selected based on some a priori theory about biological mechanisms involved in the manifestation of a behavioural trait or phenotype.^[33] In general, such studies have proven difficult to broadly replicate^{[34][35][36][37]} and there has been concern raised that the false positive rate in this type of research is high.^[33][38]

• Genome-wide association studies In
  polygenic (involving many small genetic effects).^{[39][40][41][42][43]}

  

  Genetic variants identified to be associated with some trait or disease through GWAS may be used to improve disease risk predictions. However, the genetic variants identified through GWAS of common genetic variants are most likely to have a modest effect on disease risk or development of a given trait. This is different from the strong genetic contribution seen in Mendelian conditions
  or for some rare variants that may have a larger effect on disease.

• SNP heritability and co-heritability Recently, researchers have begun to use similarity between classically unrelated people at their measured
  single nucleotide polymorphisms (SNPs) to estimate genetic variation or covariation that is tagged by SNPs, using mixed effects models implemented in software such as genome-wide complex trait analysis (GCTA).^[46][47] To do this, researchers find the average genetic relatedness over all SNPs between all individuals in a (typically large) sample, and use Haseman–Elston regression or restricted maximum likelihood to estimate the genetic variation that is "tagged" by, or predicted by, the SNPs. The proportion of phenotypic variation that is accounted for by the genetic relatedness has been called "SNP heritability".^[48] Intuitively, SNP heritability increases to the degree that phenotypic similarity is predicted by genetic similarity at measured SNPs, and is expected to be lower than the true narrow-sense heritability to the degree that measured SNPs fail to tag (typically rare) causal variants.^[49] The value of this method is that it is an independent way to estimate heritability that does not require the same assumptions as those in twin and family studies, and that it gives insight into the allelic frequency spectrum of the causal variants underlying trait variation.^[50]

Other behavioural genetic designs include discordant twin studies,[51] children of twins designs,^[56] and Mendelian randomization.^[57]

General findings

There are many broad conclusions to be drawn from behavioural genetic research about the nature and origins of behaviour.^[3][58] Three major conclusions include:^[3]

1. all behavioural traits and disorders are influenced by genes
2. environmental influences tend to make members of the same family more different, rather than more similar
3. the influence of genes tends to increase in relative importance as individuals age.

Genetic influences on behaviour are pervasive

It is clear from multiple lines of evidence that all researched behavioural traits and

The nature of this environmental influence, however, is such that it tends to make individuals in the same family more different from one another, not more similar to one another.[3] That is, estimates of shared environmental effects (${\displaystyle c^{2}}$) in human studies are small, negligible, or zero for the vast majority of behavioural traits and psychiatric disorders, whereas estimates of non-shared environmental effects (${\displaystyle e^{2}}$) are moderate to large.^[11] From twin studies ${\displaystyle c^{2}}$ is typically estimated at 0 because the correlation (${\displaystyle r_{MZ}}$) between monozygotic twins is at least twice the correlation (${\displaystyle r_{DZ}}$) for dizygotic twins. When using the Falconer variance decomposition (${\displaystyle 1.0=a^{2}+c^{2}+e^{2}}$) this difference between monozygotic and dizygotic twin similarity results in an estimated ${\displaystyle c^{2}=0}$. The Falconer decomposition is simplistic.^[25] It removes the possible influence of dominance and epistatic effects which, if present, will tend to make monozygotic twins more similar than dizygotic twins and mask the influence of shared environmental effects.^[25] This is a limitation of the twin design for estimating ${\displaystyle c^{2}}$. However, the general conclusion that shared environmental effects are negligible does not rest on twin studies alone. Adoption research also fails to find large (${\displaystyle c^{2}}$) components; that is, adoptive parents and their adopted children tend to show much less resemblance to one another than the adopted child and his or her non-rearing biological parent.^[3] In studies of adoptive families with at least one biological child and one adopted child, the sibling resemblance also tends to be nearly zero for most traits that have been studied.^[11][61]

The figure provides an example from personality research, where twin and adoption studies converge on the conclusion of zero to small influences of shared environment on broad personality traits measured by the Multidimensional Personality Questionnaire including positive emotionality, negative emotionality, and constraint.^[62]

Given the conclusion that all researched behavioural traits and psychiatric disorders are heritable, biological siblings will always tend to be more similar to one another than will adopted siblings. However, for some traits, especially when measured during adolescence, adopted siblings do show some significant similarity (e.g., correlations of .20) to one another. Traits that have been demonstrated to have significant shared environmental influences include internalizing and

Genetic effects on human behavioural outcomes can be described in multiple ways.[25] One way to describe the effect is in terms of how much variance in the behaviour can be accounted for by alleles in the genetic variant, otherwise known as the coefficient of determination or ${\displaystyle R^{2}}$. An intuitive way to think about ${\displaystyle R^{2}}$ is that it describes the extent to which the genetic variant makes individuals, who harbour different alleles, different from one another on the behavioural outcome. A complementary way to describe effects of individual genetic variants is in how much change one expects on the behavioural outcome given a change in the number of risk alleles an individual harbours, often denoted by the Greek letter ${\displaystyle \beta }$ (denoting the slope in a regression equation), or, in the case of binary disease outcomes by the odds ratio ${\displaystyle OR}$ of disease given allele status. Note the difference: ${\displaystyle R^{2}}$ describes the population-level effect of alleles within a genetic variant; ${\displaystyle \beta }$ or ${\displaystyle OR}$ describe the effect of having a risk allele on the individual who harbours it, relative to an individual who does not harbour a risk allele.^[65]

When described on the ${\displaystyle R^{2}}$ metric, the effects of individual genetic variants on complex human behavioural traits and disorders are vanishingly small, with each variant accounting for ${\displaystyle R^{2}<0.3\%}$ of variation in the phenotype.

In the case of genetically simple and rare diseases such as Huntington's, the variant ${\displaystyle R^{2}}$ and the ${\displaystyle OR}$ are simultaneously large.

Additional general findings

In response to general concerns about the replicability of psychological research, behavioural geneticists Robert Plomin, John C. DeFries, Valerie Knopik, and Jenae Neiderhiser published a review of the ten most well-replicated findings from behavioural genetics research.^[58] The ten findings were:

1. "All psychological traits show significant and substantial genetic influence."
2. "No behavioural traits are 100% heritable."
3. "Heritability is caused by many genes of small effect."
4. "Phenotypic correlations between psychological traits show significant and substantial genetic mediation."
5. "The heritability of intelligence increases throughout development."
6. "Age-to-age stability is mainly due to genetics."
7. "Most measures of the 'environment' show significant genetic influence."
8. "Most associations between environmental measures and psychological traits are significantly mediated genetically."
9. "Most environmental effects are not shared by children growing up in the same family."
10. "Abnormal is normal."

Criticisms and controversies

Behavioural genetic research and findings have at times been controversial. Some of this controversy has arisen because behavioural genetic findings can challenge

Qualitative research has fostered arguments that behavioural genetics is an ungovernable field without scientific norms or consensus, which fosters controversy. The argument continues that this state of affairs has led to controversies including race, intelligence, instances where variation within a single gene was found to very strongly influence a controversial phenotype (e.g., the "gay gene" controversy), and others. This argument further states that because of the persistence of controversy in behaviour genetics and the failure of disputes to be resolved, behaviour genetics does not conform to the standards of good science.^[82]

The scientific assumptions on which parts of behavioural genetic research are based have also been criticized as flawed.^[78] Genome wide association studies are often implemented with simplifying statistical assumptions, such as additivity, which may be statistically robust but unrealistic for some behaviours. Critics further contend that, in humans, behaviour genetics represents a misguided form of genetic reductionism based on inaccurate interpretations of statistical analyses.^[83] Studies comparing monozygotic (MZ) and dizygotic (DZ) twins assume that environmental influences will be the same in both types of twins, but this assumption may also be unrealistic. MZ twins may be treated more alike than DZ twins,^[78] which itself may be an example of evocative gene–environment correlation, suggesting that one's genes influence their treatment by others. It is also not possible in twin studies to eliminate effects of the shared womb environment, although studies comparing twins who experience monochorionic and dichorionic environments in utero do exist, and indicate limited impact.^[84] Studies of twins separated in early life include children who were separated not at birth but part way through childhood.^[78] The effect of early rearing environment can therefore be evaluated to some extent in such a study, by comparing twin similarity for those twins separated early and those separated later.^[59]

See also

References

Further reading

• S2CID 53200512
  .
• Crusio WE,
  ISBN 978-0-444-50239-1
  .
• Crusio WE, Sluyter F,
  ISBN 978-1-107-03481-5
  .
• Flint J, Greenspan RJ,
  ISBN 978-0-19-955990-9
  .
• Johnson W, Turkheimer E,
  PMID 20625474
  .
• Johnson W, Penke L, Spinath FM (2011). "Understanding Heritability: What it is and What it is Not" (PDF). European Journal of Personality. 25 (4): 287–294.
  S2CID 41842465. Archived
  (PDF) from the original on 16 December 2013. Retrieved 15 December 2013.
• Maxson SC (10 October 2012). "Chapter 1: Behavioral Genetics". In Weiner IB, Nelson RJ, Mizumori S (eds.). Handbook of Psychology (Volume 3: Behavioral Neuroscience). John Wiley & Sons.
  ISBN 978-0-470-89059-2. Archived
  from the original on 16 December 2013. Retrieved 15 December 2013.
• Spinath FM, Johnson W (2011). "Chapter 10: Behavior Genetics". In Chamorro-Premuzic T, von Stumm S, Furnham A (eds.). The Wiley-Blackwell Handbook of Individual Differences. United Kingdom: Blackwell Publishing Ltd.
  ISBN 978-1-4443-3438-8
  .

External links

Wikimedia Commons has media related to Behavioural genetics.

• McGue M (5 May 2014). "Introduction to Human Behavioral Genetics". Coursera. Retrieved 10 June 2014.
• McGue M (December 2020). "Introduction to Behavioral Genetics". University of Minnesota. Retrieved 28 June 2021.
• "Behavior Genetics Association". Behavior Genetics Association. Retrieved 8 May 2022.
• Institute for Behavioral Genetics at the University of Colorado Boulder, University of Colorado Boulder
• "Virginia Institute for Psychiatric and Behavioral Genetics". Virginia Commonwealth University.