An explanation of concordance rate

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).
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). 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), 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) 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).