By determining what are called "concordance" rates for a disease or trait among identical and fraternal twin pairs, researchers can estimate whether contributing factors for that disease or trait are more likely to be hereditary, environmental, or some combination of these.
A concordance rate is a statistical measure of probability - if one twin has a specific trait or disease, what is the probability that the other twin has (or will develop) that same trait or disease.
In addition, with structural equation modeling and multivariate analyses of twin data, researchers can offer estimates of the extent to which allelic variants and environment may influence phenotypic traits.
[1] Some twin registries seek to cover all twins in an entire country,[1] including Sweden,[2] Denmark,[3] Norway,[4] Finland, Australia,[5] Sri Lanka[6] and the United Kingdom.
[4] By comparison, enlisting with the Australian Twin Registry is voluntary.