Second, this meta-analysis is somewhat unique in the sense that rather than obtaining reported effect sizes for an outcome of interest, we obtained summary data of different group correlation matrices. This level of data extraction allowed us to fit a standard model to the data derived from all studies such that effect sizes could be meaningfully aggregated. However, this approach has some drawbacks. For example, reporting of full cross-sibling cross-time correlation matrices was rather rare. Instead we were required to derive these matrices from a variety of different behavioral genetic models reported in the original articles. When fully saturated models were reported, the parameters allow for a direct estimation of the underlying data. When only reduced or trimmed models were reported, the group correlations are imperfectly estimated. In practice, the majority of studies reported models that had trimmed the shared environmental parameters. Short of having access to all of the raw data from the studies used, we have provided the most accurate estimates possible from published data. A further issue associated with extracting correlation matrices relates to differences in variance
