Correlation but with categories rather than continuous variables, for example as used in https://en.wikipedia.org/wiki/Kendall_rank_correlation_coefficient http

Correlation but with categories rather than continuous variables, for example as used in https://en.wikipedia.org/wiki/Kendall_rank_correlation_coefficient https://en.wikipedia.org/wiki/Kendall_rank_correlation_coeff... -- so "has the same sexuality as their twin" would be concordant, or I think what the parent comment is referring to is more specifically "is homosexual and their twin is too". In the general population, you'd expect a concordance of at most 10% (a high-ish estimate of the prevalence of homosexuality in the population), though of course that's conditional on the first person in the comparison being homosexual, otherwise it'd be much higher.