# An explanation of concordance rate ![](/files/oV1q3xtilrgA5Ugzawdn) **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. ![](/files/KkpSzk3aOeImx52gUEIb) **References** 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). --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter: ``` GET https://guide.fariaedu.com/psychology-teacher-articles/biological-approach-1/genetics-and-behaviour/an-explanation-of-concordance-rate.md?ask= ``` The question should be specific, self-contained, and written in natural language. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.