# 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 ([APA Dictionary](https://dictionary.apa.org/concordance-rate)).

**What does this mean for a physical trait?**

For example, the concordance rate for eye colour in identical twins (monozygotic - MZ - twins) is 98% ([Nature article, 2021](https://www.nature.com/articles/s41433-021-01749-x#Sec10)). That means that out of 100 identical twin pairs, 98 pairs will have identical eye colour

**What does this mean for a disorder like MDD?**

The concordance rate for mental health disorders, except for genetic disorders like Huntington’s disease (98.7%, [Plos Genetics article, 2018](https://journals.plos.org/plosgenetics/article?id=10.1371/journal.pgen.1007274)), 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.

**A relevant twin study**

[Kendler et al. (2006)](https://guide.fariaedu.com/ib-psychology-guide/chapter-4-biological-approaches-to-understanding-behaviour#5.2-genetic-similarities) 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.

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**References**

American Psychological Association. *APA Dictionary of Psychology,* accessed 30 December 2021, at <https://dictionary.apa.org/concordance-rate>

Chao, M.J. et al. (2018). Population-specific genetic modification of Huntington’s Disease in Venezuela, *PLoS Genetics, 14*(5): e1007274. <https://doi.org/10.1371/journal.pgen.1007274>

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. <https://doi.org/10.1176/appi.ajp.163.1.109>

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). <https://doi.org/10.1038/s41433-021-01749-x>