Correlation of Intergenerational Family Sizes Suggests a Genetic Component of Reproductive Fitness

Reproductive fitness is a complex phenotype that is a direct measure of Darwinian selection. Estimation of the genetic contribution to this phenotype in human populations is confounded by within-family correlations of sociocultural, economic, and other nongenetic factors that influence family sizes. Here, we report an intergenerational correlation in reproductive success in the Hutterites, a human population that is relatively homogeneous with respect to sociocultural factors that influence fertility. We introduce an estimator of this correlation that takes into account the presence of multiple parent-offspring pairs from the same nuclear family. Statistical significance of the estimated correlation is assessed by a permutation test that maintains the overall structure of the pedigree. Further, temporal trends in fertility within this population are accounted for. Applying these methods to the S-Leut Hutterites yields a correlation in effective family size of 0.29 between couples and their sons and 0.18 between couples and their daughters, with empirical P<1×10−6 and P=.0041, respectively. Similar results were obtained for completed families (0.31 between couples and their sons and 0.23 between couples and their daughters; empirical P<1×10−6 and P=.00059, respectively). We interpret these results as indicating a significant genetic component to reproductive fitness in the Hutterites.