This study researched the impact of anthropometrics and size-of-family of orientation on women's fertility by using path analysis. The data were collected as part of the anthropological study conducted in Ireland by Harvard University personnel before the Second World War. The women included in this analysis were all over age 49 and were either married or widowed at the time of the survey. Our results indicate that the heritability of fertility is moderate in this sample and that there is a tendency for heavy women to have a higher fertility. However, when anthropometrics and size-of-family of orientation were entered as independent variables in a path diagram, an insignificant portion of the variation of fertility was explained. In this Irish population, the main cause of differential fertility was cultural rather than biological. A large portion of women never married and no unmarried woman reported producing a child. Am. J. Hum. Biol. 15:16–22, 2003. © 2002 Wiley-Liss, Inc.