Nearly all traits showed highly significant evidence (p < 0.0005) for differences in trait means between the sexes (75 of 98 traits) and across age bands (91 of 98 traits). The evidence that sex and age play a key role in determining quantitative trait variation motivated us to investigate whether estimates of variance components (and therefore heritabilities) differed between the sexes, or between young and old. We found significant differences in variance components by sex for 40 traits (consistent with a recently published study of 17 quantitative traits in the Hutterites [46]). Not surprisingly, evidence for heterogeneity was found for the five anthropometric traits (height, weight, BMI, waist, and hip circumference) for which sexual dimorphism is obvious. More interestingly, we observed that, when there were differences in heritability by sex (21 traits), heritability was generally larger among females (in 16 traits). The remaining 19 traits showed heterogeneity in the total variance, but similar ratios of genetic and environmental variances within each sex. The differences were sometimes dramatic: weight had heritability of approximately 50% among females, but only approximately 35% among
