An explanation of concordance rate
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Concordance rate
The concordance rate is the percentage of pairs of twins or other blood relatives who exhibit a particular trait or disorder ().
What does this mean for a physical trait?
What does this mean for a disorder like MDD?
A relevant twin study
References
For example, the concordance rate for eye colour in identical twins (monozygotic - MZ - twins) is 98% (). That means that out of 100 identical twin pairs, 98 pairs will have identical eye colour
The concordance rate for mental health disorders, except for genetic disorders like Huntington’s disease (98.7%, ), is much lower than for physical traits. This shows that environmental factors have a part to play in the development of the disorder. This is why identical twins who have been raised apart from a very young age are ideal research participants when looking at the concordance rate for disorders, because this minimises the interference given by being raised in the same environment.
found an average concordance rate for MDD across all twin pairs (identical and non-identical) was 38%. This does not mean that 38% of the twins had MDD. It means that out of 100 twins that had MDD, 38 of the 100 will also have a twin with MDD. When the researchers looked at identical female twins they found the concordance rate to be 44%, compared with male-male MZ twins (31%). This means that for every 100 females who are one of an identical twin pair and have MDD, 44 of their twins will also have a lifetime occurrence of MDD. This is nearly half and suggests a strong genetic component.
American Psychological Association. APA Dictionary of Psychology, accessed 30 December 2021, at
Chao, M.J. et al. (2018). Population-specific genetic modification of Huntington’s Disease in Venezuela, PLoS Genetics, 14(5): e1007274.
Kendler, K. S., Gatz, M., Gardner, C. O., & Pedersen, N. L. (2006). A Swedish national twin study of lifetime major depression. The American journal of psychiatry, 163 (1), 109–114.
Mackey, D.A. (2021). What colour are your eyes? Teaching the genetics of eye colour & colour vision. Edridge Green Lecture RCOphth Annual Congress Glasgow May 2019. Eye (2021).
