Preliminary analyses were performed using SPSS (version 15.0 for Windows, 2006). Quantity × Frequency and Quantity were log transformed (log10(x + 1) and Frequency was square root transformed (Tabachnick and Fidell, 1989) to correct for positive skew. The effects of sex, age, age2, sex × age, and sex × age2 were examined using stepwise linear regression, with significant terms (p < .05) regressed out and residuals used in the linkage analyses (for significant effects, see supplementary material, Table 1).
