Twin studies are one of a family of designs in behavior genetics which aid the study of individual differences by highlighting the role of environmental and genetic causes on behavior. Twins are invaluable for studying these important questions because they disentangle the sharing of genes and environments. If we observe that children in a family are more similar than might be expected by chance, this may reflect shared environmental influences common to members of family —class, parenting styles, education etc.— but they will also reflect shared genes, inherited from parents. The twin design compares the similarity of identical twins who share 100% of their genes, to that of dizygotic or fraternal twins, who share only 50% of their genes. By studying many hundreds of families of twins, researchers can then understand more about the role of genetic effects, and the effects of shared and unique environment effects. Behavioural genetics (behavioral genetics) is the field of biology that studies the role of genetics in animal behaviour. ... DNA, the molecular basis for inheritance. ... Fraternal twins at two weeks old. ...

Modern twin studies have shown that almost all traits are in part influenced by genetic differences, with some characteristics showing a strong influence (e.g., height), others an intermediate level and some more complex heritabilities, with evidence for different genes affecting different elements of the trait - for instance autism. Autism is a brain development disorder characterized by impairments in social interaction and communication, and restricted and repetitive behavior, all exhibited before a child is three years old. ...

History

Francis Galton laid the foundations of behavior genetics as a branch of science.

While twins have been of interest to scholars since early civilization, such as the early physician Hippocrates (5th c. BCE), who attributed similar diseases in twins to shared material circumstances, and the stoic philosopher Posidonius (1rst c. BCE), who attributed such similarities to shared astrological sex circumstances, the modern history of the twin study derives from Sir Francis Galton's pioneering use of twins to study the role of genes and environment on human development and behavior. This image has been released into the public domain by the copyright holder, its copyright has expired, or it is ineligible for copyright. ... This image has been released into the public domain by the copyright holder, its copyright has expired, or it is ineligible for copyright. ... This article does not cite any references or sources. ... Behavioural genetics (behavioral genetics) is the field of biology that studies the role of genetics in animal behaviour. ... Part of a scientific laboratory at the University of Cologne. ... Fraternal twin boys in the tub The term twin most notably refers to two individuals (or one of two individuals) who have shared the same uterus (womb) and usually, but not necessarily, born on the same day. ... Hippocrates of Cos II or Hippokrates of Kos (ca. ... “Era Vulgaris” redirects here. ... The bust of Posidonius as an older man depicts his character as a Stoic philosopher. ... Francis Galton Sir Francis Galton FRS (February 16, 1822 - January 17, 1911) was an English explorer, statistician, anthropologist, creator of modern eugenics (he coined the term), and investigator of the human mind. ...

Criticism

Many scientists from multiple disciplines including Statistical Genetics, Statistics and Psychology question the validity of twin study research and maintain that conclusions reached via this method are ambiguous in nature or may actually be meaningless, altogether. It has been shown that the Statistical underpinnings and sampling that these studies base themselves are of questionable value. Statistical critics maintain that heritability estimates used for most twin studies rest on restrictive assumptions which are usually not tested, and if they are, are often found to be violated by the data. For example, Peter Schonemann, a noted statistician and Psychometrics expert has shown that Heritability estimates often produce unusually high results for things that have nothing to do with genes. For example, The narrow heritability’s of HR 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. While using the statistical models published in Loehlin and Nichols (1976) the question “Did you wear sunglasses after dark?” is 130% heritable for males and 103% for females. ^[1][2] For a List of scientists, see: List of anthropologists List of astronomers List of biologists List of chemists List of computer scientists List of economists List of engineers List of geologists List of inventors List of mathematicians List of meteorologists List of physicists Scientist pairs List of scientist pairs See... DNA, the molecular basis for inheritance. ... Psychology (from Greek: ψυχή, psukhÄ“, spirit, soul; and λόγος, logos, knowledge) is both an academic and applied discipline involving the scientific study of mental processes and behavior. ... For Wikipedia statistics, see m:Statistics Statistics is the science and practice of developing human knowledge through the use of empirical data expressed in quantitative form. ... Sampling may refer to: Sampling (signal processing), converting a continuous signal into a discrete signal Sampling (music), re-using portions of sound recordings in a piece Sampling (statistics), selection of observations to acquire some knowledge of a statistical population Sampling (case studies), selection of cases for single or multiple case... A critic (derived from the ancient Greek word krites meaning a judge) is a person who offers a value judgement or an interpretation. ... In genetics, heritability is the proportion of phenotypic variation in a population that is attributable to genetic variation among individuals. ... Look up Study in Wiktionary, the free dictionary To study means to acquire knowledge, often by memorization or reading. ... Peter H. Schönemann is a German born psychometrician and statistical expert. ... Psychometrics is the field of study concerned with the theory and technique of psychological measurement, which includes the measurement of knowledge, abilities, attitudes, and personality traits. ... In genetics, heritability is the proportion of phenotypic variation in a population that is attributable to genetic variation among individuals. ... This stylistic schematic diagram shows a gene in relation to the double helix structure of DNA and to a chromosome (right). ...

The theory of correlation between relatives has been of vast importance in plant and animal breeding because it is possible to design and carry out experiments to estimate variance components in expressions for covariances between relatives. However, data on humans is observational and individuals are not randomly assigned to environments, so that estimation of heritability from such data is not on the same firm foundation as it is in plant and animal breeding contexts ^[3]Heritability estimates themselves are inaccurate, given the potential for gene-environment covariance and interaction, as well as other non-additive effects on behavior. The revolution in molecular genetics' has provided more effective tools for describing the genome, but doesn't permit separation of gene and environmental effects on traits. ^[4] The word theory has a number of distinct meanings in different fields of knowledge, depending on their methodologies and the context of discussion. ... Positive linear correlations between 1000 pairs of numbers. ... Kinship is the most basic principle of organizing individuals into social groups, roles, and categories. ... For other uses, see Plant (disambiguation). ... Selective breeding in domesticated animals is the process of developing a cultivated breed over time. ... In probability theory and statistics, the variance of a random variable (or somewhat more precisely, of a probability distribution) is a measure of its statistical dispersion, indicating how its possible values are spread around the expected value. ... In genetics, heritability is the proportion of phenotypic variation in a population that is attributable to genetic variation among individuals. ... For other uses, see Plant (disambiguation). ... Selective breeding in domesticated animals is the process of developing a cultivated breed over time. ...

Other critics’ note that in human populations twins are not a random sample of the population - They are not even a representative sample^{[5] [6]}. For example: Dizygotic (DZ) twin births are affected by many factors. Some women frequently produce more than one egg at each menstrual period and, therefore, are more likely to have twins. This tendency may run in the family 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 fertilization-embryo replacement can also give rise to DZ and MZ twins ^{[7] [8] [9][10][11] [12]} To study the effect of some environmental factors, an experimenter will need to bring up a random sample of genotypes in different environments. Similarly, to study the effect of the same environment on genotypes, one would need to bring up individuals of the same genetical composition in that environment.^[13][14][15] Fraternal twin boys in the tub The term twin most notably refers to two individuals (or one of two individuals) who have shared the same uterus (womb) and usually, but not necessarily, born on the same day. ... Look up egg in Wiktionary, the free dictionary. ... The menstrual cycle is the periodic change in a womans body that occurs every month between puberty and menopause and that relates to reproduction. ... a family of Ouagadougou, Burkina Faso in 1997 Family is a Western term used to denote a domestic group of people, or a number of domestic groups linked through descent (demonstrated or stipulated) from a common ancestor, marriage or adoption. ... Look up artificial in Wiktionary, the free dictionary. ... Look up induction in Wiktionary, the free dictionary. ... Ovulation is the process in the menstrual cycle by which a mature ovarian follicle ruptures and discharges an ovum (also known as an oocyte, female gamete, or casually, an egg) that participates in reproduction. ... ... Categories: Biology stubs ... For other uses, see Embryo (disambiguation). ... An environment is a complex of external factors that acts on a system and determines its course and form of existence. ... The genotype is the specific genetic makeup (the specific genome) of an individual, usually in the form of DNA. It codes for the phenotype of that individual. ... The genotype is the specific genetic makeup (the specific genome) of an individual, usually in the form of DNA. It codes for the phenotype of that individual. ... This article is about the general scientific term. ...

Methods

The power of twin designs arises from the fact that twins may be either monozygotic (MZ: developing from a single fertilized egg and therefore sharing all of their genes) – or dizygotic (DZ: developing from two fertilized eggs and therefore sharing on average 50% of their genes, 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 MZ and DZ twins (Bouchard & Propping, 1993) creates the basis for the twin design for exploring the effects of genetic and environmental variance on a phenotype (Neale & Cardon, 1992).

The basic logic of the twin study can be understood with very little mathematics beyond an understanding of correlation and the concept of variance. Positive linear correlations between 1000 pairs of numbers. ... In probability theory and statistics, the variance of a random variable (or somewhat more precisely, of a probability distribution) is a measure of its statistical dispersion, indicating how its possible values are spread around the expected value. ...

Like all behavior genetic research, the classic twin study begins from assessing the variance of a behavior (called a phenotype by geneticists) in a large group, and attempts to estimate how much of this is due to genetic effects (heritability), how much appears to be due to shared environmental effects, and how much is due to unique environmental effects - events occurring to one twin but not another. Individuals in the mollusk species Donax variabilis show diverse coloration and patterning in their phenotypes. ... In genetics, heritability is the proportion of phenotypic variation in a population that is attributable to genetic variation among individuals. ...

Typically these three components are called A (additive genetics) C (common environment) and E (unique environment). the so-called ACE Model. It is also possible to examine non-additive genetics effects (often denoted D for dominance (see below for more complex twin designs).

Given the ACE model, researchers can determine what proportion of variance in a trait is heritable, versus the proportions which are due to shared environment or unshared environment. While nearly all research is carried out using SEM programs such as the freeware Mx, the essential logic of the twin design is as follows: Sem may refer to: Sem or Shem; (Hebrew) One of the sons of Noah in the Bible. ...

MZ twins raised in a family share both 100% of their genes, and all of the shared environment. All differences between them in this framework are unique. The correlation we observe between MZ twins provides an estimate of A+C. DZ twins have a common shared environment, and share 50% of their genes: so the correlation between DZ twins is a direct estimate of 1/2A + C.

r_MZ = A+C

r_DZ = .5*A+C

These two equations allow us to derive A C and E:

A = 2*(r_mz- r_dz)

C = r_mz-A

E= 1-r_mz

Where r_mz and r_dz are simply the correlations of the trait in MZ and DZ twins respectively. Twice difference between MZ and DZ twins gives us A: the additive genetic effect. C is simply the MZ correlation - our estimate of A, and E is estimated directly by how much the MZ twin correlation deviates from 1. (Jinks & Fulker, 1970; Plomin, DeFries , McClearn, & McGuffin, 2001).

Modern Modeling

Beginning in the 1970s, research transitioned to explicitly modeling the values of A, C, and E within a maximum likelihood framework (Martin & Eaves, 1977). While computationally much more complex, benefits of this approach are manifold, and modeling tools such as Mx (Neale, Boker, Xie, & Maes, 2002) have made the new techniques relatively accessible.

Assumptions

Equal environments. It can be seen from the modelling above, that the main assumption of the twin study is that of equal environments. At an intuitive level, this seems reasonable - why would parents note that two children shared their hair and eye color, and then contrive to make their IQs identical? Indeed, how could they? This assumption, however, have been directly tested. An interesting case occurs where parents believe their twins to be non-identical when in fact they are genetically MZ. Studies of a range of psychological traits indicate that these children remain as concordant as MZs raised by parents who “treated them as identical” (Kendler, Neale, Kessler, Heath, & Eaves, 1993).

A particularly powerful technique for testing the twin method has recently been 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 in the domains of cognition, personality, and psychopathology.

Pairwise concordance

Fig 1. Twin concordances for seven psychological traits (sample size shown inside bars).

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. Image File history File links No higher resolution available. ... Image File history File links No higher resolution available. ...

For example, a group of 10 twins have been pre-selected to have one affected member. During the course of the study four other previously non-affected members become affected. This gives 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, 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. This gives a probandwise concordance of 8/(8+6) or 8 / 14 or 57%

Continuous variable or Correlational studies

While concordance studies compare traits which are either present or absent in each twin, correlational studies compare the agreement in continuously varying traits across twins. Positive linear correlations between 1000 pairs of numbers. ...

Fig 2. Heritability for nine psychological traits as estimated from twin studies. All sources are twins raised together (sample size shown inside bars). As outlined above, identical twins (MZ twins) are twice as genetically similar as fraternal twins (DZ twins) and so heritability (h²) is approximately twice the difference in correlation between MZ and DZ twins. Unique environmental variance (e²) is reflected by the degree to which identical twins raised together are dissimilar, and is approximated by 1-MZ correlation. The effect of shared environment (c²) contributes to similarity in all cases and is approximated by the DZ correlation minus the difference between MZ and DZ correlations.

Image File history File links No higher resolution available. ... Image File history File links No higher resolution available. ...

Further reading

• Textbook, software, and example scripts for twin research
• Jang, K.L., McCrae, R.R., Angleitner, A. Riemann, R. & Livesley, W.J. (1998). Heritability of facet-level traits in a cross-cultural twin sample: support for a hierarchical model of personality. Journal of Personality and Social Psychology 74:1556-1565.
• Plomin, DeFries, McClearn & McGuffin (2000). Behavioral Genetics: A Primer 4th edition. W.H.Freeman & Co Ltd.
• Nancy L. Segal (2005) Indivisible by Two: Lives of Extraordinary Twins. New York, Harvard University Press.

Critical Accounts

• Peter Schonemann (1997). Models and muddles of heritability. Genetica, 99, 97-108: [1]
• Peter Schonemann and Roberta D. Schonemann (1994). Environmental versus genetic models for Osborne’s personality data on identical and fraternal twins. CPC, 1994, 13 (2), 141-167 [2]
• Kamin, L. J. (1974). The Science and Politics of I.Q. Potomac, MD: Lawrence Erlbaum Associates.
• Kempthorne O. (1997). Heritability: uses and abuses. Genetica, Volume 99, Numbers 2-3, 1997 , pp. 109-112(4)
• Joseph, J. (2006). The Missing Gene: Psychiatry, Heredity, and the Fruitless Search for Genes.New York: Algora.

This book has been critically reviewed for the 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' [Electronic Version]. [PsycCritiques], 50, e14. Peter H. Schönemann is a German born psychometrician and statistical expert. ... Peter H. Schönemann is a German born psychometrician and statistical expert. ...

• Christiane Capron, Adrian R. Vetta, Michel Duyme and Atam Vetta (1999). Misconceptions of biometrical IQists. Cahiers de Psychologie Cognitive/Current Psychology of Cognition 1999, 18 (2), 115-160
• Horwitz AV, Videon TM, Schmitz MF, Davis D. Rethinking twins and environments: possible social sources for assumed genetic influences in twin research. J Health Soc Behav. 2003 Jun;44(2):111-129.

And in reply to this article see:

• Freese J and Powell B Tilting at Twindmills: rethinking sociological responses to behavioral genetics. J Health Soc Behav. 2003 Jun;44(2):130-135.

See also

• Gene-environment interaction
• Heritability
• Nature versus nurture
• Behavioral genetics
• human nature
• The Gene Illusion

Gene-environment interaction is a term used to describe any phenotypic effects that are due to interactions between the environment and genes. ... In genetics, heritability is the proportion of phenotypic variation in a population that is attributable to genetic variation among individuals. ... The nature versus nurture debates concern the relative importance of an individuals innate qualities (nature) versus personal experiences (nurture) in determining or causing individual differences in physical and behavioral traits. ... Behavioural genetics (behavioral genetics) is the field of biology that studies the role of genetics in animal behaviour. ... For other uses, see Human nature (disambiguation). ... The Gene Illusion [1] is a book by clinical psychologist Jay Joseph[2] which challenges the evidence underlying genetic theories in psychiatry and psychology. ...

External links

Several academic bodies exist to support behavior genetic research, including the Behavior Genetics Association [3], the International Society for Twin Studies, and the International Behavioural and Neural Genetics Society [4]. Behavior genetic work features prominently in several more general societies, for instance the International Society of Psychiatric Genetics. [5] The Behavior Genetics Association is a group that promotes research into the connection between heredity and behavior. ... IBANGS, the International Behavioural and Neural Genetics Society, was founded in 1996 as the European Behavioural and Neural Genetics Society. ...

The following Twin Studies are ongoing studies that are recruiting subjects:

• Maudsley Bipolar Twin Study, London, UK (Home page)

• The Twin Research Unit (King's College London) UK

• University of Pennsylvania Sleep Research Study for Twins

• List of Current Twin Studies

It is proposed that this article be deleted, because of the following concern: non notable study If you can address this concern by improving, copyediting, sourcing, renaming or merging the page, please edit this page and do so. ...

References