Twin study

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.

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).

Also, there are different ways of calculating concordance (the presence of a similar disease phenotype in twins) which can give markedly different results.

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 (http://en.wikipedia.org/w/index.php?title=Special:Booksources&isbn=0443035091)
• Joseph, J. Twin studies in psychiatry and psychology: Science or pseudoscience?  (http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=11780600) 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. (http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=12866384) J Health Soc Behav. 2003 Jun;44(2):111-29.