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Twin studies are studies conducted on identical or fraternal twins. They aim to reveal the importance of environmental and genetic influences for traits,
phenotype In genetics, the phenotype () is the set of observable characteristics or traits of an organism. The term covers the organism's morphology (biology), morphology or physical form and structure, its Developmental biology, developmental proc ...
s, and disorders. Twin research is considered a key tool in behavioral genetics and in related fields, from biology to psychology. Twin studies are part of the broader methodology used in
behavior genetics Behavioural genetics, also referred to as behaviour genetics, is a field of scientific research that uses genetic methods to investigate the nature and origins of individual differences in behaviour. While the name "behavioural genetics" ...
, which uses all data that are genetically informative – siblings studies, adoption studies, pedigree, etc. These studies have been used to track traits ranging from personal behavior to the presentation of severe mental illnesses such as
schizophrenia Schizophrenia is a mental disorder characterized by continuous or relapsing episodes of psychosis. Major symptoms include hallucinations (typically hearing voices), delusions, and disorganized thinking. Other symptoms include social w ...
. Twins are a valuable source for observation because they allow the study of environmental influence and varying genetic makeup: "identical" or
monozygotic Twins are two offspring produced by the same pregnancy.MedicineNet > Definition of TwinLast Editorial Review: 19 June 2000 Twins can be either ''monozygotic'' ('identical'), meaning that they develop from one zygote, which splits and forms two em ...
(MZ)
twin Twins are two offspring produced by the same pregnancy.MedicineNet > Definition of TwinLast Editorial Review: 19 June 2000 Twins can be either ''monozygotic'' ('identical'), meaning that they develop from one zygote, which splits and forms two em ...
s share essentially 100% of their genes, which means that most differences between the twins (such as height, susceptibility to boredom, intelligence, depression, etc.) are due to experiences that one twin has but not the other twin. "Fraternal" or dizygotic (DZ) twins share only about 50% of their genes, the same as any other sibling. Twins also share many aspects of their environment (e.g., uterine environment, parenting style, education, wealth, culture, community) because they are born into the same family. The presence of a given genetic or phenotypic trait in only one member of a pair of identical twins (called discordance) provides a powerful window into environmental effects on such a trait. Twins are also useful in showing the importance of the unique environment (specific to one twin or the other) when studying trait presentation. Changes in the unique environment can stem from an event or occurrence that has only affected one twin. This could range from a head injury or a birth defect that one twin has sustained while the other remains healthy. The classical twin design compares the similarity of monozygotic (identical) and dizygotic (fraternal) twins. If identical twins are considerably more similar than fraternal twins (which is found for most traits), this implies that genes play an important role in these traits. By comparing many hundreds of families with twins, researchers can then understand more about the roles of genetic effects, shared environment, and unique environment in shaping behavior. Modern twin studies have concluded that almost all traits are partly influenced by genetic differences, with some characteristics showing a stronger influence (e.g.
height Height is measure of vertical distance, either vertical extent (how "tall" something or someone is) or vertical position (how "high" a point is). For example, "The height of that building is 50 m" or "The height of an airplane in-flight is ab ...
), others an intermediate level (e.g.
personality traits In psychology, trait theory (also called dispositional theory) is an approach to the study of human personality. Trait theorists are primarily interested in the measurement of ''traits'', which can be defined as habitual patterns of behaviour, ...
) and some more complex heritabilities, with evidence for different genes affecting different aspects of the trait – as in the case of
autism The autism spectrum, often referred to as just autism or in the context of a professional diagnosis autism spectrum disorder (ASD) or autism spectrum condition (ASC), is a neurodevelopmental condition (or conditions) characterized by difficulti ...
. The methodological assumptions on which twin studies are based, however, have been criticized as untenable.


History

Twins have been of interest to scholars since early civilization, including the early physician
Hippocrates Hippocrates of Kos (; grc-gre, Ἱπποκράτης ὁ Κῷος, Hippokrátēs ho Kôios; ), also known as Hippocrates II, was a Greek physician of the classical period who is considered one of the most outstanding figures in the history o ...
(5th century BCE), who attributed different diseases in twins to different material circumstances, and the stoic philosopher Posidonius (1st century BCE), who attributed such similarities to shared astrological circumstances.
Gustav III Gustav III (29 March 1792), also called ''Gustavus III'', was King of Sweden from 1771 until his assassination in 1792. He was the eldest son of Adolf Frederick of Sweden and Queen Louisa Ulrika of Prussia. Gustav was a vocal opponent of what ...
, King of
Sweden Sweden, formally the Kingdom of Sweden,The United Nations Group of Experts on Geographical Names states that the country's formal name is the Kingdom of SwedenUNGEGN World Geographical Names, Sweden./ref> is a Nordic countries, Nordic c ...
was the first to commission a medical study using identical twins. Gustav's father, Adolph Frederick had been an opponent of stimulating drinks such as tea and
coffee Coffee is a drink prepared from roasted coffee beans. Darkly colored, bitter, and slightly acidic, coffee has a stimulating effect on humans, primarily due to its caffeine content. It is the most popular hot drink in the world. Seeds of ...
, signing the ''Misuse and Excesses Tea and Coffee Drinking Edict'' in 1757. Both Gustav III and his father had read and been strongly influenced by a 1715 treatise from a
French French (french: français(e), link=no) may refer to: * Something of, from, or related to France ** French language, which originated in France, and its various dialects and accents ** French people, a nation and ethnic group identified with Franc ...
physician on the dangers of what would later be identified as
caffeine Caffeine is a central nervous system (CNS) stimulant of the methylxanthine chemical classification, class. It is mainly recreational drug use, used recreationally as a Nootropic, cognitive enhancer, increasing alertness and attentional perfor ...
in tea and coffee. After assuming the throne in 1771 the king became strongly motivated to demonstrate to his subjects that coffee and tea had deleterious effects on human health. To this end he offered to commute the death sentences of a pair of twin murderers if they participated in a primitive
clinical trial Clinical trials are prospective biomedical or behavioral research studies on human participants designed to answer specific questions about biomedical or behavioral interventions, including new treatments (such as novel vaccines, drugs, diet ...
. Both condemned men agreed and subsequently spent the rest of their lives in prison fulfilling the king's demands: that one twin drink three pots of coffee every day and the other three pots of tea. The tea drinking twin passed away first at the age of 83, long after Gustav III, who was assassinated in 1792. The age of the coffee-drinking twin at his death is unknown, as both doctors assigned by the king to monitor this study predeceased him. The ban on coffee and tea in Sweden was lifted in 1823. More recent study is from Sir Francis Galton's pioneering use of twins to study the role of genes and environment on human development and behavior. Galton, however, was unaware of the difference between identical and DZ twins. This factor was still not understood when the first study using psychological tests was conducted by
Edward Thorndike Edward Lee Thorndike (August 31, 1874 – August 9, 1949) was an American psychologist who spent nearly his entire career at Teachers College, Columbia University. His work on comparative psychology and the learning process led to the theory ...
(1905) using fifty pairs of twins. This paper was an early statement of the hypothesis that family effects decline with age. His study compared twin pairs age 9–10 and 13–14 to normal siblings born within a few years of one another. Thorndike incorrectly reasoned that his data supported for there being one, not two, twin types. This mistake was repeated by Ronald Fisher (1919), who argued An early, and perhaps first, study understanding the distinction is from the
German German(s) may refer to: * Germany (of or related to) **Germania (historical use) * Germans, citizens of Germany, people of German ancestry, or native speakers of the German language ** For citizens of Germany, see also German nationality law **Ge ...
geneticist
Hermann Werner Siemens Hermann Werner Siemens (August 20, 1891 ( Charlottenberg) -1969)Ruggieri, Martino. "Neurocutaneous Disorders: Phakomatoses & Hamartoneoplastic Syndromes." Springer-Werlag/Vien, 2008. was a German dermatologist who first described multiple skin di ...
in 1924. Chief among Siemens' innovations was the ''polysymptomatic similarity diagnosis''. This allowed him to account for the oversight that had stumped Fisher, and was a staple in twin research prior to the advent of molecular markers.
Wilhelm Weinberg Wilhelm Weinberg (Stuttgart, 25 December 1862 – 27 November 1937, Tübingen) was a German obstetrician-gynecologist, practicing in Stuttgart, who in a 1908 paper, published in German in ''Jahresheft des Vereins für vaterländische Naturkund ...
and colleagues in 1910 used the identical-DZ distinction to calculate respective rates from the ratios of same- and opposite-sex twins in a maternity population. They partitioned co-variation amongst relatives into genetic and environmental elements, anticipating the later work of
Fisher Fisher is an archaic term for a fisherman, revived as gender-neutral. Fisher, Fishers or The Fisher may also refer to: Places Australia *Division of Fisher, an electoral district in the Australian House of Representatives, in Queensland *Elect ...
and
Wright Wright is an occupational surname originating in England. The term 'Wright' comes from the circa 700 AD Old English word 'wryhta' or 'wyrhta', meaning worker or shaper of wood. Later it became any occupational worker (for example, a shipwright i ...
, including the effect of dominance on similarity of relatives, and beginning the first classic-twin studies. A study conducted by Darrick Antell and Eva Taczanowski found that "twins showing the greatest discrepancies in visible aging signs also had the greatest degree of discordance between personal lifestyle choices and habits", and concluded that "the genetic influences on aging may be highly overrated, with lifestyle choices exerting far more important effects on physical aging."


Examples

Examples of prominent twin studies include the following: *
Maudsley Bipolar Twin Study The Maudsley Bipolar Twin Study is an ongoing twin study of bipolar disorder running at the Institute of Psychiatry, King's College London since 2003. The study is investigating possible differences between people with a diagnosis of bipolar disor ...
* Minnesota Twin Family Study *
Twins Early Development Study The Twins Early Development Study (TEDS) is an ongoing longitudinal twin study based at King's College London. The main goal of TEDS is to use behavioural genetic methods to find out how nature (genes) and nurture (environments) can explain why p ...
* NASA Twins Study


Methods

The power of twin designs arises from the fact that twins may be either
monozygotic Twins are two offspring produced by the same pregnancy.MedicineNet > Definition of TwinLast Editorial Review: 19 June 2000 Twins can be either ''monozygotic'' ('identical'), meaning that they develop from one zygote, which splits and forms two em ...
(identical (MZ): developing from a single fertilized egg and therefore sharing all of their
allele An allele (, ; ; modern formation from Greek ἄλλος ''állos'', "other") is a variation of the same sequence of nucleotides at the same place on a long DNA molecule, as described in leading textbooks on genetics and evolution. ::"The chrom ...
s) – or dizygotic (DZ: developing from two fertilized eggs and therefore sharing on average 50% of their polymorphic alleles, the same level of genetic similarity as found in non-twin siblings). These known differences in genetic similarity, together with a testable assumption of equal environments for identical and fraternal twins creates the basis for the twin design for exploring the effects of genetic and environmental variance on a phenotype. The basic logic of the twin study can be understood with very little mathematics beyond an understanding of the concepts of
variance In probability theory and statistics, variance is the expectation of the squared deviation of a random variable from its population mean or sample mean. Variance is a measure of dispersion, meaning it is a measure of how far a set of numbe ...
and thence derived
correlation In statistics, correlation or dependence is any statistical relationship, whether causal or not, between two random variables or bivariate data. Although in the broadest sense, "correlation" may indicate any type of association, in statistic ...
.


Classical twin method

Like all behavior genetic research, the classical twin study begins from assessing the variance of a behavior (called a
phenotype In genetics, the phenotype () is the set of observable characteristics or traits of an organism. The term covers the organism's morphology (biology), morphology or physical form and structure, its Developmental biology, developmental proc ...
by geneticists) in a large group, and attempts to estimate how much of this is due to: * genetic effects (
heritability Heritability is a statistic used in the fields of breeding and genetics that estimates the degree of ''variation'' in a phenotypic trait in a population that is due to genetic variation between individuals in that population. The concept of her ...
); * shared environment – events that happen to both twins, affecting them in the same way; * unshared, or unique, or nonshared environment – events that occur to one twin but not the other, or events that affect either twin in a different way. Typically these three components are called A (additive genetics) C (common environment) and E (unique environment); hence the acronym ''ACE''. It is also possible to examine non-additive genetics effects (often denoted D for dominance ( ADE model); see below for more complex twin designs). The ACE model indicates what proportion of variance in a trait is heritable, versus the proportion due to shared environment or un-shared environment. Research is carried out using SEM programs such as
OpenMx OpenMx is an open source program for extended structural equation modeling. It runs as a package under R. Cross platform, it runs under Linux, Mac OS and Windows.S. Boker, M. Neale, H. Maes, M. Wilde, M. Spiegel, T. Brick, J. Spies, R. Estabrook, ...
, however the core logic of the twin design is the same, as described below: Monozygotic (identical – MZ) twins raised in a family share both 100% of their genes, and all of the shared environment. Any differences arising between them in these circumstances are random (unique). The correlation between identical twins provides an estimate of ''A'' + ''C''. Dizygotic (DZ) twins also share C, but share on average 50% of their genes: so the correlation between fraternal twins is a direct estimate of ½''A''+''C''. If ''r'' is
correlation In statistics, correlation or dependence is any statistical relationship, whether causal or not, between two random variables or bivariate data. Although in the broadest sense, "correlation" may indicate any type of association, in statistic ...
, then ''r''mz and ''r''dz are simply the correlations of the trait in identical and fraternal twins respectively. For any particular trait, then: :''r''mz = ''A'' + ''C'' :''r''dz = ½''A'' + ''C'' ''A'', therefore, is twice the difference between identical and fraternal twin correlations: the additive genetic effect (
Falconer's formula Heritability is the proportion of variance caused by genetic factors of a specific trait in a population. Falconer's formula is a mathematical formula that is used in twin studies to estimate the relative contribution of genetic vs. environmental f ...
). ''C'' is simply the MZ correlation minus this estimate of ''A''. The random (unique) factor ''E'' is 1 − ''r''mz: i.e., MZ twins differ due to unique environments only. (Jinks & Fulker, 1970; Plomin, DeFries, McClearn, & McGuffin, 2001). Stated again, the difference between these two sums, then, allows us to solve for ''A'', ''C'', and ''E''. As the difference between the identical and fraternal correlations is due entirely to a halving of the genetic similarity, the additive genetic effect 'A' is simply twice the difference between the identical and fraternal correlations: :''A'' = 2 (''r''mz − ''r''dz) As the identical correlation reflects the full effect of A and C, E can be estimated by subtracting this correlation from 1 :''E'' = 1 − ''r''mz Finally, C can be derived: :''C'' = ''r''mz − ''A''


Modern modeling

Beginning in the 1970s, research transitioned to modeling genetic, environmental effects using
maximum likelihood In statistics, maximum likelihood estimation (MLE) is a method of estimating the parameters of an assumed probability distribution, given some observed data. This is achieved by maximizing a likelihood function so that, under the assumed stat ...
methods (Martin & Eaves, 1977). While computationally much more complex, this approach has numerous benefits rendering it almost universal in current research. An example structural model (for the heritability of height among Danish males) is shown: Model A on the left shows the raw variance in height. This is useful as it preserves the absolute effects of genes and environments, and expresses these in natural units, such as mm of height change. Sometimes it is helpful to standardize the parameters, so each is expressed as percentage of total variance. Because we have decomposed variance into A, C, and E, the total variance is simply A + C + E. We can then scale each of the single parameters as a proportion of this total, i.e., Standardised–A = A/(A + C + E). Heritability is the standardised genetic effect.


Model comparison

A principal benefit of modeling is the ability to explicitly compare models: Rather than simply returning a value for each component, the modeler can compute confidence intervals on parameters, but, crucially, can drop and add paths and test the effect via statistics such as the AIC. Thus, for instance to test for predicted effects of family or shared environment on behavior, an AE model can be objectively compared to a full ACE model. For example, we can ask of the figure above for height: Can C (shared environment) be dropped without significant loss of fit? Alternatively, confidence intervals can be calculated for each path.


Multi-group and multivariate modeling

Multivariate modeling can give answers to questions about the genetic relationship between variables that appear independent. For instance: do IQ and long-term memory share genes? Do they share environmental causes? Additional benefits include the ability to deal with interval, threshold, and continuous data, retaining full information from data with missing values, integrating the latent modeling with measured variables, be they measured environments, or, now, measured molecular genetic markers such as SNPs. In addition, models avoid constraint problems in the crude correlation method: all parameters will lie, as they should, between 0–1 (standardized). Multivariate, and multiple-time wave studies, with measured environment and repeated measures of potentially causal behaviours are now the norm. Examples of these models include extended twin designs, simplex models, and growth-curve models. SEM programs such as
OpenMx OpenMx is an open source program for extended structural equation modeling. It runs as a package under R. Cross platform, it runs under Linux, Mac OS and Windows.S. Boker, M. Neale, H. Maes, M. Wilde, M. Spiegel, T. Brick, J. Spies, R. Estabrook, ...
and other applications suited to constraints and multiple groups have made the new techniques accessible to reasonably skilled users.


Modeling the environment: MZ discordant designs

As MZ twins share both their genes and their family-level environmental factors, any differences between MZ twins reflect E: the unique environment. Researchers can use this information to understand the environment in powerful ways, allowing
epidemiological Epidemiology is the study and analysis of the distribution (who, when, and where), patterns and determinants of health and disease conditions in a defined population. It is a cornerstone of public health, and shapes policy decisions and evid ...
tests of causality that are otherwise typically confounded by factors such as gene–environment covariance, reverse causation and
confounding In statistics, a confounder (also confounding variable, confounding factor, extraneous determinant or lurking variable) is a variable that influences both the dependent variable and independent variable, causing a spurious association. Con ...
. An example of a positive MZ discordant effect is shown below on the left. The twin who scores higher on trait 1 also scores higher on trait 2. This is compatible with a "dose" of trait 1 causing an increase in trait 2. Of course, trait 2 might also be affecting trait 1. Disentangling these two possibilities requires a different design (see below for an example). A null result is incompatible with a causal hypothesis. Take for instance the case of an observed link between depression and exercise (See Figure above on right). People who are depressed also reporting doing little physical activity. One might ''hypothesise'' that this is a
causal Causality (also referred to as causation, or cause and effect) is influence by which one event, process, state, or object (''a'' ''cause'') contributes to the production of another event, process, state, or object (an ''effect'') where the ca ...
link: that "dosing" patients with exercise would raise their mood and protect against depression. The next figure shows what empirical tests of this hypothesis have found: a null result. Longitudinal discordance designs As may be seen in the next Figure, this design can be extended to multiple measurements, with consequent increase in the kinds of information that one can learn. This is called a cross-lagged model (multiple traits measured over more than one time). In the longitudinal discordance model, differences between identical twins can be used to take account of relationships among differences across traits at time one (path A), and then examine the distinct hypotheses that increments in trait1 drive subsequent change in that trait in the future (paths B and E), or, importantly, in other traits (paths C & D). In the example, the hypothesis that the observed
correlation In statistics, correlation or dependence is any statistical relationship, whether causal or not, between two random variables or bivariate data. Although in the broadest sense, "correlation" may indicate any type of association, in statistic ...
where depressed persons often also
exercise Exercise is a body activity that enhances or maintains physical fitness and overall health and wellness. It is performed for various reasons, to aid growth and improve strength, develop muscles and the cardiovascular system, hone athletic ...
less than average is causal, can be tested. If exercise is protective against depression, then path D should be significant, with a twin who exercises more showing less depression as a consequence.


Assumptions

It can be seen from the modeling above, the main assumption of the twin study is that of equal environments, also known as the equal environments assumption. This assumption has been directly tested. A special case occurs where parents believe their twins to be non-identical when in fact they are genetically identical. Studies of a range of psychological traits indicate that these children remain as concordant as MZ twins raised by parents who treated them as identical. Molecular genetic methods of heritability estimation have tended to produce lower estimates than classical twin studies, although this discrepancy could be due to modern SNP arrays not capturing the influence of certain types of variants (e.g., rare variants or repeat polymorphsism) or because twin studies overestimate heritability. A 2016 study determined that the assumption that the prenatal environment of twins was equal was largely tenable. Researchers continue to debate whether or not the equal environment assumption is valid.


Measured similarity: A direct test of assumptions in twin designs

A particularly powerful technique for testing the twin method was reported by Visscher ''et al.'' Instead of using twins, this group took advantage of the fact that while siblings on average share 50% of their genes, the actual gene-sharing for individual sibling pairs varies around this value, essentially creating a continuum of genetic similarity or "twinness" within families. Estimates of heritability based on direct estimates of gene sharing confirm those from the twin method, providing support for the assumptions of the method.


Sex differences

Genetic factors may differ between the sexes, both in gene expression and in the range of gene × environment interactions. Fraternal opposite sex twin pairs are invaluable in explicating these effects. In an extreme case, a gene may only be expressed in one sex (qualitative sex limitation). More commonly, the effects of gene-alleles may depend on the sex of the individual. A gene might cause a change of 100 g in weight in males, but perhaps 150 g in females – a quantitative gene effect. Environments may impact on the ability of genes to express themselves and may do this via sex differences. For instance genes affecting voting behavior would have no effect in females if females are excluded from the vote. More generally, the logic of sex-difference testing can extend to any defined sub-group of individuals. In cases such as these, the correlation for same and opposite sex DZ twins will differ, betraying the effect of the sex difference. For this reason, it is normal to distinguish three types of fraternal twins. A standard analytic workflow would involve testing for sex-limitation by fitting models to five groups, identical male, identical female, fraternal male, fraternal female, and fraternal opposite sex. Twin modeling thus goes beyond correlation to test causal models involving potential causal variables, such as sex.


Gene × environment interactions

Gene effects may often be dependent on the environment. Such interactions are known as ''G×E interactions'', in which the effects of a gene allele differ across different environments. Simple examples would include situations where a gene multiplies the effect of an environment: perhaps adding 1 inch to height in high nutrient environments, but only half an inch to height in low-nutrient environments. This is seen in different slopes of response to an environment for different genotypes. Often researchers are interested in changes in
heritability Heritability is a statistic used in the fields of breeding and genetics that estimates the degree of ''variation'' in a phenotypic trait in a population that is due to genetic variation between individuals in that population. The concept of her ...
under different conditions: In environments where
alleles An allele (, ; ; modern formation from Greek ἄλλος ''állos'', "other") is a variation of the same sequence of nucleotides at the same place on a long DNA molecule, as described in leading textbooks on genetics and evolution. ::"The chrom ...
can drive large phenotypic effects (as above), the relative role of genes will increase, corresponding to higher heritability in these environments. A second effect is ''G × E correlation'', in which certain alleles tend to accompany certain environments. If a gene causes a parent to enjoy reading, then children inheriting this allele are likely to be raised in households with books due to GE correlation: one or both of their parents has the allele and therefore will accumulate a book collection ''and'' pass on the book-reading allele. Such effects can be tested by measuring the purported environmental correlate (in this case books in the home) directly. Often the role of environment seems maximal very early in life, and decreases rapidly after
compulsory education Compulsory education refers to a period of education that is required of all people and is imposed by the government. This education may take place at a registered school or at other places. Compulsory school attendance or compulsory schooling ...
begins. This is observed for instance in reading as well as intelligence. This is an example of a G*Age effect and allows an examination of both GE correlations due to parental environments (these are broken up with time), and of G*E correlations caused by individuals actively seeking certain environments.


Norms of reaction

Studies in plants or in animal breeding allow the effects of experimentally randomized genotypes and environment combinations to be measured. By contrast, human studies are typically observational. This may suggest that
norms of reaction In ecology and genetics, a reaction norm, also called a norm of reaction, describes the pattern of phenotypic expression of a single genotype across a range of environments. One use of reaction norms is in describing how different species—especi ...
cannot be evaluated. As in other fields such as
economics Economics () is the social science that studies the production, distribution, and consumption of goods and services. Economics focuses on the behaviour and interactions of economic agents and how economies work. Microeconomics anal ...
and
epidemiology Epidemiology is the study and analysis of the distribution (who, when, and where), patterns and determinants of health and disease conditions in a defined population. It is a cornerstone of public health, and shapes policy decisions and evi ...
, several designs have been developed to capitalise on the ability to use differential gene-sharing, repeated exposures, and measured exposure to environments (such as children social status, chaos in the family, availability and quality of education, nutrition, toxins etc.) to combat this confounding of causes. An inherent appeal of the classic twin design is that it begins to untangle these confounds. For example, in identical and fraternal twins shared environment and genetic effects are not confounded, as they are in non-twin familial studies. Twin studies are thus in part motivated by an attempt to take advantage of the random assortment of genes between members of a family to help understand these correlations. While the twin study tells us only how genes and families affect behavior within the observed range of environments, and with the caveat that often genes and environments will covary, this is a considerable advance over the alternative, which is no knowledge of the different roles of genes and environment whatsoever.M. C. Neale and H. H. Maes. (1996). Methodology for genetics studies of twins and families. ''Journal''. Twin studies are therefore often used as a method of controlling at least one part of this observed variance: Partitioning, for instance, what might previously have been assumed to be family environment into shared environment and additive genetics using the experiment of fully and partly shared genomes in twins. No single design can address all issues. Additional information is available outside the classic twin design. Adoption designs are a form of natural experiment that tests norms of reaction by placing the same genotype in different environments. Association studies, e.g., allow direct study of allelic effects.
Mendelian randomization In epidemiology, Mendelian randomization (commonly abbreviated to MR) is a method using measured variation in genes to interrogate the causal effect of an exposure on an outcome. Under key assumptions (see below), the design reduces both reverse ca ...
of alleles also provides opportunities to study the effects of alleles at random with respect to their associated environments and other genes.


Extended twin designs and more complex genetic models

The basic or classical twin-design contains only identical and fraternal twins raised in their biological family. This represents only a sub-set of the possible genetic and environmental relationships. It is fair to say, therefore, that the heritability estimates from twin designs represent a first step in understanding the genetics of behavior. The variance partitioning of the twin study into additive genetic, shared, and unshared environment is a first approximation to a complete analysis taking into account gene–environment covariance and
interaction Interaction is action that occurs between two or more objects, with broad use in philosophy and the sciences. It may refer to: Science * Interaction hypothesis, a theory of second language acquisition * Interaction (statistics) * Interaction ...
, as well as other non-additive effects on behavior. The revolution in
molecular genetics Molecular genetics is a sub-field of biology that addresses how differences in the structures or expression of DNA molecules manifests as variation among organisms. Molecular genetics often applies an "investigative approach" to determine the ...
has provided more effective tools for describing the genome, and many researchers are pursuing molecular genetics in order to directly assess the influence of
alleles An allele (, ; ; modern formation from Greek ἄλλος ''állos'', "other") is a variation of the same sequence of nucleotides at the same place on a long DNA molecule, as described in leading textbooks on genetics and evolution. ::"The chrom ...
and environments on traits. An initial limitation of the twin design is that it does not afford an opportunity to consider both Shared Environment and Non-additive genetic effects simultaneously. This limit can be addressed by including additional siblings to the design. A second limitation is that gene–environment correlation is not detectable as a distinct effect. Addressing this limit requires incorporating adoption models, or children-of-twins designs, to assess family influences uncorrelated with shared genetic effects.


Continuous variables and ordinal variables

While concordance studies compare traits either present or absent in each twin,
correlation In statistics, correlation or dependence is any statistical relationship, whether causal or not, between two random variables or bivariate data. Although in the broadest sense, "correlation" may indicate any type of association, in statistic ...
al studies compare the agreement in continuously varying traits across twins.


Criticism

The twin method has been subject to criticism from statistical
genetics Genetics is the study of genes, genetic variation, and heredity in organisms.Hartl D, Jones E (2005) It is an important branch in biology because heredity is vital to organisms' evolution. Gregor Mendel, a Moravian Augustinian friar work ...
, statistics, and
psychology Psychology is the science, scientific study of mind and behavior. Psychology includes the study of consciousness, conscious and Unconscious mind, unconscious phenomena, including feelings and thoughts. It is an academic discipline of immens ...
, with some researchers, such as Burt & Simons (2014), arguing that conclusions reached via this method are ambiguous or meaningless. Core elements of these criticisms and their rejoinders are listed below.


Criticisms of fundamental assumptions

Critics of twin studies argue that they are based on false or questionable assumptions, including that monozygotic twins share 100% of their genes and the equal environments assumption. On this basis, critics contend that twin studies tend to generate inflated estimates of heritability due to biological confounding factors and consistent underestimation of environmental variance. Other critics take a more moderate stance, arguing that the equal environments assumption is typically inaccurate, but that this inaccuracy tends to have only a modest effect on heritability estimates.


Criticisms of statistical methods

It has been argued that the
statistical Statistics (from German: '' Statistik'', "description of a state, a country") is the discipline that concerns the collection, organization, analysis, interpretation, and presentation of data. In applying statistics to a scientific, industr ...
underpinnings of twin research are invalid. Such statistical critiques argue that
heritability Heritability is a statistic used in the fields of breeding and genetics that estimates the degree of ''variation'' in a phenotypic trait in a population that is due to genetic variation between individuals in that population. The concept of her ...
estimates used for most twin studies rest on restrictive assumptions that are usually not tested, and if they are, they are often contradicted by the data. For example, Peter Schonemann has criticized methods for estimating
heritability Heritability is a statistic used in the fields of breeding and genetics that estimates the degree of ''variation'' in a phenotypic trait in a population that is due to genetic variation between individuals in that population. The concept of her ...
developed in the 1970s. He has also argued that the heritability estimate from a twin study may reflect factors other than shared
genes In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a ba ...
. Using the statistical models published in Loehlin and Nichols (1976), the narrow HR-heritability of responses to the question "did you have your back rubbed" has been shown to work out to .92 heritable for males and .21 heritable for females, and the question "Did you wear sunglasses after dark?" is 130% heritable for males and 103% for females. Critics also contend that the concept of "heritability" estimated in twin studies is merely a statistical abstraction with no relationship to an underlying entity in DNA.


Responses to statistical critiques

Before computers, statisticians used methods that were computationally tractable, at the cost of known limitations. Since the 1980s these approximate statistical methods have been discarded: Modern twin methods based on
structural equation modeling Structural equation modeling (SEM) is a label for a diverse set of methods used by scientists in both experimental and observational research across the sciences, business, and other fields. It is used most in the social and behavioral scienc ...
are not subject to the limitations and heritability estimates such as those noted above are mathematically impossible. Critically, the newer methods allow for explicit testing of the role of different pathways and incorporation and testing of complex effects.


Sampling: Twins as representative members of the population

Results of twin studies cannot be automatically generalized beyond the population they come from. It is therefore important to understand the particular sample studied, and the nature of twins themselves. Twins are not a random sample of the population, and they differ in their developmental environment. In this sense they are not representative. For example: Dizygotic (DZ) twin births are affected by many factors. Some women frequently produce more than one egg at each
menstrual The menstrual cycle is a series of natural changes in hormone production and the structures of the uterus and ovaries of the female reproductive system that make pregnancy possible. The ovarian cycle controls the production and release of eggs a ...
period and, therefore, are more likely to have twins. This tendency may run in the
family Family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its members and of society. Idea ...
either in the mother's or father's side of the family, and often runs through both. Women over the age of 35 are more likely to produce two eggs. Women who have three or more children are also likely to have dizygotic twins. Artificial induction of ovulation and in
vitro Vitro is the largest glass producer in Mexico and one of the world's largest organizations in the glass industry. Founded in 1909 in Monterrey, Mexico, this corporation has 30 subsidiaries in Mexico, United States, Brazil, Colombia, Bolivia, Cos ...
fertilization Fertilisation or fertilization (see spelling differences), also known as generative fertilisation, syngamy and impregnation, is the fusion of gametes to give rise to a new individual organism or offspring and initiate its development. Pro ...
-
embryo An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male spe ...
replacement can also give rise to fraternal and identical twins.


Response to representativeness of twins

Twins differ very little from non-twin siblings. Measured studies on the personality and intelligence of twins suggest that they have scores on these traits very similar to those of non-twins (for instance Deary et al. 2006).


Separated twin pairs as representative of other twins

Separated twin pairs, identical or fraternal, are generally separated by
adoption Adoption is a process whereby a person assumes the parenting of another, usually a child, from that person's biological or legal parent or parents. Legal adoptions permanently transfer all rights and responsibilities, along with filiation, fro ...
. This makes their families of origin non-representative of typical twin families in that they give up their children for adoption. The families they are adopted to are also non-representative of typical twin families in that they are all approved for adoption by children's protection authorities and that a disproportionally large fraction of them have no biological children. Those who volunteer to studies are not even representative of separated twins in general since not all separated twins agree to be part of twin studies.


Detection problems

There can be some issues of undetected behaviors in the case of behaviors that many people keep secret presently or in their earlier lives. They may not be as willing to reveal behaviors that are discriminated against or stigmatized. If environment played no role in the actual behavior, skewed detection would still make it look like it played a role. For environment to appear to have no role in such cases, there would have to be either a counterproductivity of intolerance in the sense of intolerance causing the behavior it is bigoted against, or a flaw in the study that makes the results scientifically useless. Even if environment does play a role, the numbers would still be skewed.How Black African and White British Women Perceive Depression and Help-Seeking: a Pilot Vignette Study, International Journal of Social Psychiatry March 2010


Terminology


Pairwise concordance

For a group of twins, ''pairwise concordance'' is defined as C/(C+D), where C is the number of concordant pairs and D is the number of discordant pairs. For example, a group of 10 twins have been pre-selected to have one affected member (of the pair). During the course of the study four other previously non-affected members become affected, giving a pairwise concordance of 4/(4+6) or 4/10 or 40%.


Probandwise concordance

For a group of twins in which at least one member of each pair is affected, ''probandwise concordance'' is a measure of the proportion of twins who have the illness who have an affected twin and can be calculated with the formula of 2C/(2C+D), in which C is the number of concordant pairs and D is the number of discordant pairs. For example, consider a group of 10 twins that have been pre-selected to have one affected member. During the course of the study, four other previously non-affected members become affected, giving a probandwise concordance of 8/(8+6) or 8/14 or 57%.


See also

* Behavioral genetics *" Burt Affair" * Gene-environment interaction * Gene-environment correlation *
Genome-wide complex trait analysis Genome-wide complex trait analysis (GCTA) Genome-based restricted maximum likelihood (GREML) is a statistical method for variance component estimation in genetics which quantifies the total narrow-sense (additive) contribution to a trait's herita ...
*
Heritability Heritability is a statistic used in the fields of breeding and genetics that estimates the degree of ''variation'' in a phenotypic trait in a population that is due to genetic variation between individuals in that population. The concept of her ...
*
Heritability of IQ Research on the heritability of IQ inquires into the degree of variation in IQ within a population that is due to genetic variation between individuals in that population. There has been significant controversy in the academic community about the ...
*
Human nature Human nature is a concept that denotes the fundamental dispositions and characteristics—including ways of thinking, feeling, and acting—that humans are said to have naturally. The term is often used to denote the essence of humankind, or ...
*'' Identical Strangers: A Memoir of Twins Separated and Reunited'' * Kaiser Wilhelm Institute of Anthropology, Human Heredity, and Eugenics * Michigan State University Twin Registry * Minnesota Twin Family Study *
Nature versus nurture Nature versus nurture is a long-standing debate in biology and society about the balance between two competing factors which determine fate: genetics (nature) and environment (nurture). The alliterative expression "nature and nurture" in English h ...
* Otmar Freiherr von Verschuer *
Quantitative genetics Quantitative genetics deals with phenotypes that vary continuously (such as height or mass)—as opposed to discretely identifiable phenotypes and gene-products (such as eye-colour, or the presence of a particular biochemical). Both branches u ...
* Differential susceptibility *'' Three Identical Strangers'' * Twin registry * TwinsUk


References


Further reading


Free courseware, textbook, software, and example scripts for twin research
* * R. Plomin, J. C. DeFries, V. S. Knopik and J. M. Neiderhiser. (2012). ''Behavioral Genetics''. Worth Publishers. London * Nancy L. Segal (2005) ''Indivisible by Two: Lives of Extraordinary Twins''. New York,
Harvard University Press Harvard University Press (HUP) is a publishing house established on January 13, 1913, as a division of Harvard University, and focused on academic publishing. It is a member of the Association of American University Presses. After the retir ...
. * ** * Am J Med Genet C Semin Med Genet. 2009 May 15;151C(2):136-41. Not really identical: epigenetic differences in monozygotic twins and implications for twin studies in psychiatry. Haque FN, Gottesman II, Wong AH.


Critical accounts

* * * Kamin, L. J. (1974). ''The Science and Politics of I.Q.'' Potomac, MD: Lawrence Erlbaum Associates. * * Joseph, J. (2003). The Gene Illusion: Genetic Research in Psychiatry and Psychology Under the Microscope. PCCS Books. :This book has been critically reviewed for th
American Psychological Association.
Hanson, D. R. (2005). 'The Gene Illusion Confusion: A review of The Gene Illusion: Genetic Research in Psychiatry and Psychology Under the Microscope by Jay Joseph' lectronic Versionbr>PsycCritiques
50, e14. * * *And in reply to this article see: ** {{cite journal , last1 = Freese , first1 = J , last2 = Powell , first2 = B , date = Jun 2003 , title = Tilting at Twindmills: rethinking sociological responses to behavioral genetics , pmid = 12866385 , journal = J Health Soc Behav , volume = 44 , issue = 2, pages = 130–135 , doi=10.2307/1519803, jstor = 1519803


External links


Academic bodies

Several academic bodies exist to support behavior genetic research, including the Behavior Genetics Association, the International Society for Twin Studies, and the
International Behavioural and Neural Genetics Society The International Behavioural and Neural Genetics Society (IBANGS) is a learned society that was founded in 1996. The goal of IBANGS is "promote and facilitate the growth of research in the field of neural behavioral genetics". Profile Mission Th ...
. Behavior genetic work also features prominently in several more general societies, for instance the
International Society of Psychiatric Genetics The International Society of Psychiatric Genetics (ISPG) is a learned society that aims to "promote and facilitate research in the genetics of psychiatric disorders, substance use disorders and allied traits". To this end, among other things, it o ...
.


Journals

Prominent specialist journals in the field include ''
Behavior Genetics Behavioural genetics, also referred to as behaviour genetics, is a field of scientific research that uses genetic methods to investigate the nature and origins of individual differences in behaviour. While the name "behavioural genetics" ...
,
Genes, Brain and Behavior ''Genes, Brain and Behavior'' (also known as ''G2B'') is a peer-reviewed online-only scientific journal covering research in the fields of behavioral, neural, and psychiatric genetics. It is published by Wiley-Blackwell on behalf of the Internat ...
'', and ''
Twin Research and Human Genetics ''Twin Research and Human Genetics'' is a peer-reviewed scientific journal published bimonthly by the Cambridge University Press. It is the official journal of the International Society for Twin Studies (ISTS) and the Human Genetics Society of ...
''. Twin Behavioural genetics sv:Tvillingstudier