Data management and computation of descriptive statistics were conducted using SAS 9.1 (SAS Institute, 2002). Distributions of responses to the maltreatment variables were markedly skewed. Therefore, the maltreatment variables were treated as ordinal variables, and then analyzed assuming that normal continuous liability distributions underlie the ordinal variables. This method retains the statistical advantages conferred by the normality assumptions for the underlying liability, retains an explicit mapping between the underlying liability and observed behavior, and correctly recovers the underlying correlations and parameter estimates (Stallings et al., 2001). Age- and sex-specific thresholds were used. In order to estimate genetic and environmental factors influencing maltreatment in general, a composite maltreatment variable was computed. A z-score was computed for each participant for each form of maltreatment, and then these scores were averaged. This variable was treated as an ordinal variable with five levels.
