Behavioral Genetics

BEHAVIORAL GENETICS

Matt McGue and Irving I. Gottesman

Department of Psychology

University of Minnesota

Correspondence: Matt McGue

Department of Psychology/Elliott Hall

University of Minnesota

Minneapolis, MN 55455 USA

Voice: 1.612.626.2079

FAX: 1.612.626.2079

E-mail: [email protected]

Submitted: Wiley-Blackwell Encyclopedia of Clinical Psychology

Abstract length of 166 words; Manuscript length: 4599 words and

18 pages; 2 figures, 1 table, 24 references, and 3 recommended

readings

ABSTRACT

Behavior Genetics 2

Behavioral genetics’ roots were planted by Francis Galton in the late 19th century, but its early

efforts were discredited by its association with the Eugenics Movement in the early 20th

century. Twin, adoption and family studies helped re-establish the credibility and importance of

behavioral genetic research and have supported three general conclusions: 1) genetic

influences on individual differences in behavior are pervasive, 2) the relevant environmental

influences are predominantly of the non-shared rather than the shared variety, and 3) the

importance of genetic factors appears to increase with age. With the heritability of behavioral

phenotypes well-established, the field has moved to trying to identify the specific genetic

variants that underlie these heritable effects and exploring how these variants interact with

environmental factors. Initial efforts in these areas have, however, been hampered by the

typically small sample sizes of the relevant research in psychology. The establishment of large-

scale consortia and the use of meta-analytic methods are now beginning to reveal the complex

genetic architecture of behavioral phenotypes.

Behavior Genetics 3

The 3 October 2013 issue of Nature ran an article with the provocative title, “Taboo

Genetics” (Hayden, 2013). The article discussed genetics research that held the potential to

generate wide-spread controversy. As it turns out, all of the examples considered were

behavioral. And so it goes for the field of behavioral genetics, again. On the one hand it is

deemed worthy of feature articles in the most prominent scientific journals. On the other, there

is concern that its research findings challenge our most cherished beliefs about the nature and

origin of the behavioral differences among us.

Although some of the controversy surrounding behavioral genetic research may be

warranted, much is based on misunderstandings of the field and its methodologies. Behavioral

genetics can be defined as:

The area of psychology concerned with the application of genetic

methods and research designs to study the nature and origins of

individual differences in behavior.

From this perspective, behavioral geneticists are not singularly concerned with documenting

the heritable basis of behavior, although that is certainly one aim of a behavioral genetic

analysis. Rather, behavioral geneticists seek to use the powerful tools of genetics – twin and

adoption studies as well as molecular genetic investigations – to characterize the nature of

individual differences in behavior. The power of genetic methods is that they provide a

foundation for strong causal inference in a field of psychology, individual differences

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psychology, that has been primarily observational. Some of the most influential findings from

behavioral genetic research concern the nature of environmental rather than genetic

influences.

An historical perspective provides an overview of the field. Behavioral genetics was

founded in the late 19th century but discredited by the early 20th century because its association

with the eugenics movement. For much of the ensuing 30 years, a radical environmental model

dominated within psychology. The prominence of purely environmental accounts within

psychology began to erode, however, as behavioral genetic research re-emerged in the latter

half of the 20th century. This research supports three general conclusions about the origins of

individual differences in behavior, provided in the second section below. We conclude with a

discussion of the major research questions that define behavioral genetics in the early 21st

century.

THE EARLY HISTORY OF BEHAVIORAL GENETICS

Philosophers, humanists and scholars have long speculated about the origin of individual

differences in behavior – Why is it that some achieve great intellectual distinction, are

gregarious and outgoing or live a life free of mental illness, while others struggle with the

simplest intellectual challenges, are sullen and withdrawn, or suffer debilitating psychiatric

illness? Central to answering these questions is whether the behavioral differences among us

can be attributed to inborn natural factors (what we would today call genetics) versus life

circumstances (the environment). The notion that natural factors might contribute to the

inheritance of behavior dates back millennia, at least to the inception of selective breeding and

the domestication of animals (Loehlin, 2009). The Greeks speculated about the basis for the

Behavior Genetics 5

inheritance of behavioral characteristics, which played a central role in the design of Plato’s

ideal society. Shakespeare may have been the first to use the felicitous terms nature and

nurture to capture the essence of the debate. In The Tempest, the miscreant Caliban is

described as, ``A devil, a born devil, on whose nature nurture can never stick''.

The scientific founding of the field is generally dated to 1869. A decade after publication

of the Origin of the Species, Francis Galton, Charles Darwin’s cousin, published Hereditary

Genius (Galton, 1869), a large pedigree study of social and intellectual achievement. Galton

observed that the rate of eminence among the relatives of eminent individuals declined as a

function of the degree of genetic relationship between the two relatives. Although Galton

acknowledged the possibility that environmental factors might contribute to the diminishing

gradient of familial resemblance he observed, based on his and other research, he initiated the

modern Nature-Nurture debate when he famously concluded that “nature prevails enormously

over nurture when the differences of nurture do not exceed what is commonly to be found

among persons of the same rank of society and in the same country.” (Galton, 1883, p. 241)

Through empirical contributions, Galton and his followers were largely successful in

placing the nascent field of psychology on a Darwinian foundation. Their efforts were ultimately

and ironically undermined, however, by another of Galton’s intellectual contributions. In 1865,

Galton introduced the term eugenics (from the Greek eu=well and genics=born) to denote the

practice of using scientific findings on the inheritance of behavior to try to improve the human

species through selective breeding. Galton thought that what had worked to produce prized

race horses could also help to solve pressing social problems. The fortunes of early research on

the inheritance of behavior closely tracked that of the popularity of the Eugenics Movement.

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This can be seen in Figure 1, in which we have plotted the frequency with which American

parents named their baby boys Eugene along with frequency of publications in the

psychological literature that dealt with the inheritance of behavior. The popularity of both

increased substantially throughout the early 20th century until the Eugenics Movement was

discredited by its association with the Nazi corruption of scientific genetics. Early behavioral

genetic research became a secondary casualty, not recovering to its 1930s peak until well into

the 1970s.

Insert Figure 1 about here

MAJOR FINDINGS FROM THE FIELD OF HUMAN BEHAVIORAL GENETICS

With early behavioral geneticists discredited, a radical environmental perspective came

to dominate within academic psychology. By the 1970s, however, some psychologists had

returned to addressing the empirical questions laid out by Galton and his followers more than a

half century earlier. In 1970, the Behavior Genetics Association was established and the journal

Behavior Genetics was launched. Seminal twin studies on personality and psychopathology,

supported by similar research with adopted individuals, implicated the importance of genetic

factors. Accumulating evidence from twin, family and adoption studies suggested that IQ was

moderately to strongly heritable. Over the past 40 years, a substantial and coherent behavioral

genetic literature has emerged and established three general conclusions (Plomin, DeFries,

Knopik, & Neiderhiser, 2013): 1) genetic influences on individual differences in human behavior

Behavior Genetics 7

are pervasive, 2) environmental factors are also important but the major source of

environmental influence appears to be factors that create differences rather than similarities

among reared-together relatives, and 3) for many psychological traits the relative importance

of genetic factors appears to increase with age, especially during the transition from

adolescence to early adulthood.

Establishing the heritability of behavior

The most common research design in human behavioral genetics has been the classical

twin study, that is, the comparison of the similarity of reared-together monozygotic (MZ) to

reared-together, same-sex dizygotic (DZ) twins. MZ twins are effectively genetically identical;

DZ twins share on average half their segregating genes. Consequently, the basic logic of a twin

study is to infer the existence of genetic influences when MZ twins are phenotypically more

similar than DZ twins. There have been hundreds of twins studies of behavior published in the

past few decades and these studies consistently report a familiar pattern: For most behavioral

traits MZ twins are more similar than DZ twins, sometimes substantially so. We illustrate this

pattern in Table 1 with meta-analytic findings from twin studies of two prototypic behavioral

traits, schizophrenia and IQ.

Insert Table 1 about here

The consistent observation of greater MZ than DZ twin similarity may reflect the

existence of genetic influences. Alternatively, it may reflect some limitation of the twin method.

Inference about genetic influences in twin studies requires the assumption that MZ twins are

Behavior Genetics 8

not environmentally more similar than DZ twins (i.e., the Equal Environmental Similarity [EES]

assumption). Behavioral geneticists have themselves challenged the EES assumption by

investigating the impact of zygosity confusion (e.g., whether DZ twins mistakenly thought to be

MZ twins are more similar than correctly specified DZ twins), chorionicty (i.e., whether MZ

twins are more similar than DZ twins because they are more likely to share a chorion and

therefore a placenta), or physical similarity (e.g., whether the greater physical similarity of MZ

twins might be the basis for their greater psychological similarity) on twin study findings. In all

cases, the EES is supported (Plomin et al., 2013). Nonetheless, there are ways in which MZ

twins do have more similar environments than DZ twins – they consistently show greater

similarity on environmental factors that reflect in part personal choice (e.g., being intellectually

or physically engaged) or the reactions their behaviors elicit from others (e.g., whether their

relationships with their parents or others is marked by conflict) (Plomin & Bergeman, 1991).

Rather than being taken as evidence against the EES assumption, however, this form of

environmental similarity is typically interpreted by behavioral geneticists to indicate gene-

environment correlational processes. That is, one mechanism by which genetic factors might

influence our behavior is by influencing the environments we help to create for ourselves

indirectly. In this way genetic influences on behavioral phenotypes are fundamentally different

from those on non-behavioral phenotypes (Scarr & McCartney, 1983).

Biometrical genetics, a field founded by Galton, seeks to quantify genetic and

environmental contributions to phenotypic differences through analysis of familial

resemblance. In the standard formulation, the phenotypic variance, an index of individual

differences, is assumed to be an additive function of genetic and environmental effects. Genetic

Behavior Genetics 9

effects are further decomposed into additive and non-additive genetic effects, the latter

including genetic dominance and epistasis (i.e., gene-gene interaction), while environmental

effects are decomposed into shared and non-shared environmental effects. The shared

environmental component corresponds to the effects of environmental factors shared by

reared-together relatives and thus is the environmental basis for their phenotypic similarity. It

includes the effects of factors such as neighborhood, schools, socioeconomic status, and

common aspects of parenting. Alternatively, the non-shared environmental component

corresponds to the effects of factors that are not shared by reared-together relatives and so is

the environmental basis for their behavioral dissimilarity. It includes the effects of factors such

as peers, accidents, and differential parental treatment. Variance associated with measurement

error is also included in the non-shared environmental effect.

The eminent British geneticist Douglas Falconer adapted the biometric model to the

twin design by assuming parsimoniously that all genetic effects are additive. In this case, the

phenotypic correlation for MZ and DZ twins as well as the total phenotypic variance, normed to

1.0, can be written as:

rMZ = a2 + c2

rDZ = 1/2a2 + c2

1.0 = a2 + c2 + e2

Where a2 is the proportion of phenotypic variance associated with additive genetic effects

(sometimes called the additive heritability), c2 is the proportion associated with shared

environmental effects, and e2 is the proportion associated with non-shared environmental

effects. Through simple algebra the three equations can generate estimates, sometimes called

Behavior Genetics 10

the Falconer estimates, of the three unknown variance components from observations made in

the standard twin study:

a2 = 2(rMZ - rDZ)

c2 = 2rDZ – rMZ

e2 =1.0 – rMZ

The Falconer estimates for schizophrenia (strictly speaking the biometric estimates are for the

liability for developing schizophrenia) and IQ are also given in Table 1. The findings for

schizophrenia and IQ illustrate what is found for many psychological and psychiatric

phenotypes. First, genetic factors exert a pervasive influence, with heritability estimates

typically ranging from moderate (as illustrated with IQ) to large (as illustrated with

schizophrenia liability). Second, MZ twin similarity is never perfect, so that non-shared

environmental factors, so-far unspecified, also contribute to individual differences. Finally, for

many behavioral phenotypes (illustrated by schizophrenia) the impact of shared environmental

influences is minimal. An important exception is IQ, for which the estimate is typically moderate

in magnitude.

If behavioral geneticists were limited to studies of reared-together twins only, the

conclusion that genetic factors influence individual differences in a broad array of behavioral

phenotypes would be uncertain at best. Findings from twin studies have, however, been

constructively replicated in adoption studies, which have consistently reported that adopted

individuals are more psychologically similar to the birth relatives they did not know than the

adoptive relatives with whom they were reared. Moreover, behavioral geneticists have been

able to undertake investigations of what represents the combination of the two most common


behavioral genetic research designs by studying the behavioral similarity of twins who were

separated not long after birth and reared without knowledge of each other. These studies have

shown that twins continue to be psychologically similar even when they are reared apart,

further implicating the importance of genetic factors (Bouchard, Lykken, McGue, Segal, &

Tellegen, 1990).

The nature of environmental influence

Behavioral genetic research provides some of the best evidence for the existence of

environmental as well as genetic influences on complex human behaviors. This research

suggests, however, that the major source of environmental influence is of the non-shared

rather than shared variety. The inference that non-shared environmental influences are much

stronger than shared environmental influences emerged initially from studies of reared-

together twins. That is, MZ twins are never perfectly concordant for psychopathology or

perfectly correlated for psychological characteristics. Since MZ twins are genetically identical,

this lack of perfect phenotypic similarity must be due to non-genetic, specifically non-shared

environmental, differences between the two members of a pair. Alternatively, for many

psychological traits the MZ correlation is at least twice the DZ correlation, in which case

Falconer’s formulae, given above, yields an estimate of c2 that is effectively zero (since negative

estimates of a variance component are non-admissible).

The inference that shared environmental influences on psychological traits are minimal

is further supported by adoption studies. For example, individuals who are reared-together but

are not genetically related (i.e., adopted siblings) show minimal similarity in personality and

psychopathology, although they have modest similarity in their IQs and level of substance


use/abuse (Buchanan, McGue, Keyes, & Iacono, 2009). Similarly, studies of reared-apart twins

report that the degree of twin similarity is only slightly less among reared-apart twins as

compared to reared-together twins (Bouchard et al., 1990). Again, IQ is a notable exception.

The IQ correlation for reared-apart MZ twins is about .20 less than the corresponding

correlation for reared-together MZ twins, implicating an effect of the shared environment.

The finding that for many psychological traits non-shared environmental influences are

much more powerful than shared environmental influences is unexpected and in some quarters

controversial. One aspect of the controversy concerns the impact parents have on the children

they rear. Although the finding of minimal shared environmental influences certainly implies a

need to reevaluate standard socialization models within the social sciences, it does not

necessarily imply that parental influences are trivial. Parents do not always treat their children,

even their twin children, the same, and there are traits, such as IQ, where biometrical estimates

suggest a moderate degree of shared environmental influence. Behavioral genetic research

does not necessarily imply that parents do not matter even if it shows that growing up in the

same home has minimal impact on similarity for many psychological traits.

If the non-shared environment is the major source of environmental influence for most

psychological traits, then what are the specific factors contributing to this effect? Several large

research projects have been undertaken with the goal of identifying the source of non-shared

environmental effects. The yield from these studies has been disappointing. The failure to

identify the specific contributors to the non-shared environmental component has led some

researchers to claim that the relevant factors are largely idiosyncratic (Turkheimer & Waldron,

2000). That is, the environmental factors that underlie, for example, bipolar disorder or an


extraverted personality are indeed unique to the individual. If this is the case, then it will be

very difficult to identify systematically and study these idiosyncratic influences on behavior.

Heritability of behavioral phenotypes increases with age

Our discussion thus far has not been informed by a necessary developmental

perspective. Yet all of the behavioral phenotypes that have been investigated change in their

expression with age, raising the question whether genetic and environmental contributions

change with age as well. Meta-analyses of twin studies spanning childhood through early

adulthood do find that the heritability of psychological traits as diverse as externalizing

psychopathology and social attitudes increases with age (Bergen, Gardner, & Kendler, 2007). IQ

provides a useful example. Twin and adoption studies both imply that IQ is moderately

heritable with a moderate shared environmental effect. The magnitudes of these effects are

not, however, constant over age. In a study of more than 11,000 pairs of twins representing

four separate countries, the heritability of IQ was estimated to increase from .41 in childhood

to .55 in adolescence to .66 in adulthood. Over the same developmental period, the

contribution of shared environmental influences decreased from .31 in childhood to .16 in early

adulthood (Haworth et al., 2010). The increase in heritable effects along with the decline in

shared environmental effects is typically explained in terms of a shifting pattern of gene-

environment correlation. Early in life, parents have a major influence on the environments their

children experience, and they are likely to exert this influence in a way that is compatible with

their own genetically influenced dispositions (inducing a passive gene-environment correlation

between the parents’ genes and the environments they provide). With age the child will exert

greater control of her or his experiences, and in this case that influence is likely to reflect the


child’s genetically influenced dispositions (inducing an active gene-environment correlation

between the child’s genes and the experiences she or he helps to create).

THE CURRENT FRONTIER OF BEHAVIORAL GENETIC RESEARCH

Identifying the Specific Genetic Variants that Underlie the Heritability of Behavior

The heritability of behavior implies that differences in the sequence of DNA bases we

inherit contribute to individual differences in our behavior. Identifying these sequence

differences has, however, been anything but easy. Since 1990, much of the relevant research

has taken a candidate-gene approach. In this approach researchers identify a specific gene or

set of genes thought to be relevant for the phenotype in question (e.g., serotonin system genes

for depression, dopamine system genes for schizophrenia and impulsivity) and then determine

whether genetic variants in the targeted genes are associated with the relevant phenotypes. A

candidate-gene study is relatively simple to design and execute inexpensively. Unfortunately,

this approach has yielded a very limited number of replicable findings, for non-behavioral and

behavioral phenotypes alike. While several factors have contributed to the poor track record of

candidate-gene research, the most salient is that candidate-gene studies have typically been

under-powered and a statistically significant finding in an under-powered study is more likely to

be a false than a true positive result (Ioannidis, 2005). A study by Chabris and colleagues (2012)

illustrates the issue. These researchers sought to replicate associations between general

cognitive ability and 12 specific genetic variants identified in an earlier review as being

supported by candidate-gene research. They failed, however, to replicate any of the

associations in a combined replication sample of nearly 10,000 participants even though they

had power to detect effects accounting for as little as 0.1% of phenotypic variance for many of


the variants they studied. Given that the original studies included samples much smaller in size

than 10,000, it is safe to conclude that the original reports were almost certainly false-positive

findings. General cognitive ability is not unique in this regard, candidate-gene findings for

major forms of mental illness, personality and non-behavioral phenotypes have all been

similarly difficult to replicate.

Fortunately, an alternative strategy for identifying the specific genetic factors

influencing complex phenotypes has emerged over the past few years. In a genomewide

association study (GWAS) the whole genome rather than a specific gene or set of genes is

surveyed. GWAS involves 500,000 or more genetic markers distributed throughout the genome,

whose assay has been made possible by extraordinary advances in high-throughput genotyping

and the availability of commercially manufactured chips. Although the scale is large, the basic

analytical approach in a GWAS is relatively straightforward – each of the markers is associated

with the target phenotype, and, to control for multiple testing, a stringent p-value threshold is

used before an association is declared to be significant (usually 5*10-8).

Since its inception in 2007, GWAS has resulted in the identification of thousands of

genetic associations with hundreds of different phenotypes

(http://www.genome.gov/gwastudies), including behavioral phenotypes such as schizophrenia,

bipolar disorder, autism and dementia. The first round of GWAS research has made evident two

essential characteristics of the genetic architecture of complex phenotypes (Visscher, Brown,

McCarthy, & Yang, 2012). First, for any given phenotype the number of relevant genetic

variants is likely to be large, numbering at least in the hundreds but more likely in the

thousands. Second, almost as a corollary to the first, the magnitude of the effect of any specific

http://www.genome.gov/gwastudies


variant will be very small, accounting for 0.3% of the variance or less for quantitative

phenotypes or having a genetic odds ratio (OR) of 1.3 or less for categorical phenotypes.

Reliable identification of genetic factors of this magnitude requires massive sample sizes, a

problem that is only compounded by the very low p-value threshold used in GWAS to account

for the multiple testing of a large number of markers. Therefore, gene-identification efforts for

behavioral as well as non-behavioral phenotypes currently involves the building of large multi-

study consortia and the use of meta-analytic techniques.

A GWAS of schizophrenia illustrates the approach (Ripke et al., 2013). This study, which

involved more than 21,000 cases with schizophrenia and 38,000 controls, identified 22 specific

genetic loci as being associated with schizophrenia (at the GWAS threshold of p < 5*10-8). The

genetic variant with strongest effect had an OR of only 1.2. In aggregate, the genetic variants

could account for approximately 6% of the variance in case-control status even though

estimates of the heritability for schizophrenia liability range from about 60%-80%. Thus a

combined sample of nearly 60,000 leaves much of the heritability of schizophrenia “missing”

and unaccounted for.

Several factors may be contributing to the missing heritability for schizophrenia (or for

that matter most every other human genetic trait that has been studied with GWAS) (Manolio

et al., 2009). Even a sample as large as the one used in the Ripke study will have limited

statistical power to detect genetic variants that have an extremely small effect on the risk of

developing schizophrenia (e.g., ORs less than 1.05). In the hope of identifying these small-effect

factors, geneticists continue to build larger and larger consortia. Alternatively, rather than

search for genetic effects on clinical phenotypes such as schizophrenia, researchers are seeking


to increase the sensitivity of their gene identification efforts through use of endophenotypes,

phenotypes that intervene between the primary gene product and the observed phenotype

(Gottesman & Gould, 2003). Second, GWAS only samples genetic variants that are reasonably

common in the population (e.g., with a frequency of 1% or more). Rare variants, that might

occur in small and geographically isolated subsamples of the population may also be

contributing to schizophrenia risk and will require strategies other than GWAS to detect them

(e.g., genome sequencing). Finally, GWAS only investigates a specific type a genetic variant,

what is called a single nucleotide polymorphism or SNP. SNPs refer to differences in the DNA

nucleotide base (A versus T versus G versus C) that exist at a specific genomic location. They are

used in GWAS because they are easy to genotype. Genomes can differ in other ways, however.

Copy number variants (CNVs) refer to relatively large regions of the genome (from 1000 to

several million DNA bases) that are either deleted or duplicated in some individuals. CNVs tend

to be rare, but there is growing evidence that both genomic duplications and genomic deletions

contribute to risk of neurodevelopmental disorders including schizophrenia and autism (St Clair,

2013).

Gene x Environment Interaction

There is broad recognition within the field of human genetics that the biometric

assumption of additivity of effects, which is used for example in deriving the Falconer

estimates, is not generally true. Gene x environment interaction (GxE) is likely to be pervasive.

GxE occurs when there is a statistical interaction between genetic background and

environmental circumstances and not merely when there is both a genetic and environmental

effect. Figure 2 illustrates a GxE where the magnitude of the genetic effect depends upon level


of environmental exposure. That is, in a protective environment, there is little effect associated

with inheriting the high-risk versus low-risk genotype, while in a provocative environment the

high-risk genotype is associated with a substantial increase in disease risk. This form of GxE

illustrates the diathesis-stress model of psychopathology. The diathesis (i.e., the high-risk

genotype) establishes a vulnerability; whether that vulnerability manifests as psychopathology,

however, will depend on the nature of environmental provocation. Alternatively, individuals

inheriting the low-risk genotype have little likelihood of developing the disorder, regardless of

their level of environmental exposure. They are not vulnerable.

Insert Figure 2 about here

In their initial attempts to identify GxE for psychopathology, behavioral geneticists could

only measure the presumed diathesis indirectly, through for example a positive family history,

rather than directly, by assaying the relevant risk genotype. As a consequence, there was

limited progress in GxE research despite considerable interest and speculation. The Human

Genome Project enabled behavioral geneticists to begin to assess putative genetic risk factors

directly, and the result was an explosion of GxE research. A major breakthrough came in 2003,

when Caspi and colleagues (2003) investigated whether depression might be the combined

result of a high-risk genotype, in this case inheriting a specific allele in the gene that codes for

the serotonin transporter, and a high-risk environment, level of psychological stress. Consistent

with the diathesis-stress formulation, these researchers reported that those who had inherited

the high-risk genotype were more sensitive to psychological stress than those who had


inherited the low-risk genotype. At low levels of psychological stress, the high-risk genotype

was not associated with elevated risk of depression. At high levels of psychological stress,

however, rate of depression was significantly elevated among those carrying the high-risk

genotype but only modestly elevated in those with the low-risk genotype.

The Caspi et al. GxE paper is one of the most highly cited papers in psychology and

psychiatry over the past decade. Remarkably, however, the field continues to debate whether

the GxE they report is really there. While there are certainly studies that replicate Caspi’s

findings, there are also others that do not. Aggregating study findings does not seem to resolve

the matter as there are meta-analyses that both support as well as dispute the existence of the

GxE effect. There is a concern that GxE research today is where candidate-gene research was 20

years ago – i.e., dominated by underpowered studies prone to false positive results (Duncan &

Keller, 2011). Although there is no dispute that GxE exists for many behavioral phenotypes, the

reliable detection of those effects may require much larger samples than has been typical.

Within the past few years, another form of gene-environment interplay has emerged

and gained widespread attention. Epigenetics (roughly translated as “on top of the DNA

sequence”) refers to stable changes in gene expression that are not attributable to the DNA

sequence. By regulating access of the transcription machinery to the DNA sequence (e.g.,

through DNA methylation or histone modification), epigenetic processes can suppress or

enhance gene expression. Epigenetic processes have been known to play a significant role in

the ontology of various forms of cancer. Recent evidence suggests that the influence of factors

such as maternal nurturance, childhood maltreatment, and exposure to drugs of abuse on

behavior may also involve epigenetic mechanisms. Although the field is only at the initial stages


of inquiry, the study of epigenetic processes holds great promise for understanding how both

genes and the environment come to shape our behavior(Petronis, 2010).

Further Reading:

Baker, C. (2004). Behavioral genetics. Washington, D.C.: American Association for the

Advancement of Science.

Goldman, D. (2012). Our genes, our choices. Waltham, MA: Elsevier.

Kim, Y.-K. (Ed.). (2009). Handbook of behavior genetics. New York: Springer.


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Table 1. Illustrative findings from twin studies of psychiatric and psychological phenotypes

Study Phenotype

MZ Twins DZ Twins Falconer Estimates

#

Pairs Similarity*

#

Pairs Similarity* a2 c2 e2

(Cardno &

Gottesman,

2000)

Schizophrenia (DSM-III-R)

114 50.0% 97 4.1% .88 .00 .12

(Bouchard &

McGue, 1981)

IQ

4672

.85

5546

.58

.54

.31

.15

Note: MZ = monozygotic twins, DZ = dizygotic twins, a2 = additive genetic heritability, c2= shared

environmental effects, and e2 = non-shared environmental effect

* Twin similarity assessed as probandwise concordance for schizophrenia and correlation for

IQ.


Figure Captions

Figure 1: Frequency of American-born babies named Eugene and rate of publication of articles

dealing with the inheritance of behavior. Frequency of baby names was obtained through

records of the U.S. Social Security administration (http://www.ssa.gov/OACT/babynames/); rate

of publication was determined through PsychInfo by searching under inheritance or related

keywords (e.g., heritability).

Figure 2: Diathesis-Stress Model of Gene-Environment Interaction (GxE). In a GxE, the genetic

effect depends upon the environmental context. In the Diathesis-Stress form of this model, it is

assumed that individuals inherit a vulnerability to develop a specific form of psychopathology

(the diathesis or high-risk genotype). Whether the individual with the diathesis develops the

disorder will, however, depend on their environment. The high-risk genotype is only associated

with increased risk if the individual is reared in a high-risk environment.

http://www.ssa.gov/OACT/babynames/


Figure 1


Figure 2

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