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A twin study is a kind of genetic study done to determine heritability. The premise is that since identical twins (especially identical twins raised apart) have identical genotypes, differences between them are solely due to environmental factors. By examining the degree to which twins are differentiated, a study may determine the extent to which a particular trait is influenced by genes or the environment. Genetics (from the Greek genno γεννώ= give birth) is the science of genes, heredity, and the variation of organisms. ... Heritability, as used professionally in genetics, has a very precise definition. ... 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 are usually, but not necessarily, born on the same day. ... 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. ...

Twin studies have been called into question because of the difficulty of isolating environmental factors from genetic factors. For example, twins that are "raised apart" are still often raised in similar environments, even by different parts of the same family. Inadequately controlling for such similarities may bias studies towards overestimating the influence of genetic factors (e.g., see Joseph, 2002).
Most importantly, it is currently becoming clear that "innate" is not the same as "inherited". Many, especially psychological, traits (like eating habits) seem to result from a combination of genetic factors and the prenatal imprinting by environment experienced during embryo/fetus development (like hormone or blood sugar levels of the mother, or even, at late fetus stage, ambient noise like that produced by music). Thus, similarities between identical twins raised apart may not be due to genetic factors alone; in many cases, the question of heritability may be entirely moot since both factors, genetic and environmental, are required in combination to yield a phenotype. Research into this phenomenon is ongoing, but still at an early stage (imprinting processes are generally poorly understood). Embryos (and one tadpole) of the wrinkled frog (Rana rugosa you beezie). ... Fetus at eight weeks A fetus (alternatively foetus or fœtus) is an unborn human offspring from the end of the 8th week of pregnancy (when the major structures have formed) until birth. ...

Also, there are different ways of calculating concordance (the presence of a similar disease phenotype in twins) which can give markedly different results. The phenotype of an individual organism is either its total physical appearance and constitution, or a specific manifestation of a trait, such as size or eye color, that varies between individuals. ...

Pairwise concordance

For a group of twins in which at least one member of each pair is affected, pairwise concordance is a measure of how many of each pair will have both members affected. It can be calcuated with a formula of C/C+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 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%.

Further Reading

• Emery, E.M. (1976) Methodology in medical genetics: an introduction to statistical methods. Edinburgh: Churchill Livingstone. [ISBN 0443035091]
• Joseph, J. Twin studies in psychiatry and psychology: Science or pseudoscience? Psychiatric Quarterly,2002, 73(1), 71-82.
• 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):111-29.