PLF was considered to have partially mediated the relationship between EVK and externalizing if shrinkage in the value of c to c’ was significantly different than zero. The difference between c and c’ can be approximated by calculating the product of paths a and b. Therefore, the size of this reduction was evaluated with a bootstrapping approach (for review, see Shrout and Bolger, 2002) where 5,000 indirect effects (ab, equivalent to the change in c to c’) were simulated from the sample with replacement, keeping the proportion of matched- to unmatched-twin-pairs constant. Confidence intervals were estimated from this distribution of simulated ab effects to evaluate statistical significance.
