Twin study

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A twin study is a kind of genetic study done to determine heritability. The premise is that since identical twins have identical genotypes, differences between them are solely due to environmental factors. By examining the degree to which twins (especially twins raised apart) are differentiated, a study may determine the extent to which a particular trait is influenced by genes or the environment.

Modern twin studies have shown personal traits to be partially influenced by one's genes, with some characteristics showing a strong influence (e.g., homosexuality, IQ).

[edit] History

Hippocrates rejected superstition and laid the foundations of medicine as a branch of science.

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. Later, St. Augustine of Hippo (4rth c. CE) pointed to the differences in fraternal twins, such as Jacob and Esau in the Old Testament, to disprove the central tenet of astrology, that time of birth determines personality and fate.[1]

[edit] Outcome measures

There are several ways of using twin studies to determine the extent of the influence of genetic and environmental factors. Particularly when pairwise and probandwise concordance measures are compared, different results will be generated from the same data, which must be borne in mind when interpreting findings from such studies.

[edit] Pairwise concordance

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

For a group of 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%.

[edit] 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%

[edit] Correlational studies

Concordance studies compare traits which are either present or absent in each twin, correlational studies compare the agreement in continuously varying traits across twins.

Fig 2. Heritability for nine psychological traits as estimated from twin studies. All sources are twins raised together (sample size shown inside bars). Identical twins (MZ twins) are twice as genetically similar as fraternal twins (DZ twins) and so heritability (h^2) is approximately twice the difference in correlation between MZ and DZ twins. Unique environmental variance (e^2) 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 (c^2) contributes to similarity in all cases and is approximated by the DZ correlation minus the difference between MZ and DZ correlations.

[edit] Further reading

• Emery, E.M. (1976) Methodology in medical genetics: an introduction to statistical methods. Edinburgh: Churchill Livingstone. [ISBN 0-443-03509-1]
• 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.
• Murray, R. M., Clifford, C. A. And Gurling, H. M. D. (1983) Twin and adoption studies. How good is the evidence for a genetic role? Recent Dev Alcohol. 1:25-48.
• Segal, Nancy L. (1993) Twin, sibling, and adoption methods: Tests of evolutionary hypotheses. American Psychologist. 48(9):943-956.
• Yarbrough, Cathy, "'Jumping genes' contribute to the uniqueness of individual brains". Eureka Alert, June 2005
• Pearson, Helen. (2002). Nature Science Update. Nerve cells mysteriously mislay chromosomes. [2]

[edit] Criticism

• Joseph, J. Twin studies in psychiatry and psychology: Science or pseudoscience? Psychiatric Quarterly, 2002, 73(1), 71-82.
• Joseph, J. (2004). The Gene Illusion: Genetic Research in Psychiatry and Psychology Under the Microscope.New York: Algora. (2003 United Kingdom Edition by PCCS Books)
• Joseph, J. (2006). The Missing Gene: Psychiatry, Heredity, and the Fruitless Search for Genes.New York: Algora.

• 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.

[edit] Twin studies currently recruiting subjects

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

• The Maudsley Bipolar Twin Study, London, UK

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

• University of Pennsylvania Sleep Research Study for Twins

• List of Current Twin Studies

[edit] See also

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