Likelihood ratio tests were used to test for the significance of each of the variance components. Importantly, because twin studies focus on variance, which must be a positive quantity, the likelihood ratio tests for significance is not distributed as a χ2 with, for example, 1 df. Instead, the null distribution is a 50:50 mixture of 0 and χ2 with 1 df (Wu & Neale, 2012). The presented hypothesis tests take this finding into account.
