Maternal and paternal reports of twin CP were available for 996 and 862 twins, respectively. Child interviews were available for 996 twins. As expected based on prior meta-analyses of informant effects (Achenbach et al. 1987), the various informant reports of CP were moderately intercorrelated (maternal- and paternal-informant reports were correlated 0.43; child interviews were correlated 0.32 and 0.27 with maternal- and paternal-informant reports, respectively; all p < 0.01). CP data were averaged across informants to create a CP composite. When only one informant report was available (n = 3 twins), that report was used for analyses. The use of this combined informant approach is thought to allow for a more complete assessment of twin symptomatology than would the use of any one informant alone (Achenbach et al. 1987). Consistent with manual recommendations (Achenbach & Rescorla, 2001), analyses were conducted on the raw CP composite. Data were log-transformed prior to analysis to adjust for positive skew (skews before and after transformation were 2.28 and 0.54, respectively).
