Based on the Austrian sample of the Trends in International Mathematics and Science Study (TIMSS) 2011, the effects of number of children and birth order on homework support and mathematics performance test scores were examined. The aim was to find evidence for the confluence model developed by Zajonc and Markus (1975; Zajonc, 2001), and especially for the tutoring effect postulated by this model. The results (based on N = 3,016 cases) showed decreasing homework support by parents and increasing assistance by siblings with a growing number of children in the family. In addition, the pattern of the performance test scores by number of children and birth order followed less the assumptions of a model that took into account the parents’ resources alone than the assumptions of a model that additionally referred to siblings as a resource as well as a learning opportunity (tutoring effect). Strategies for further tests of the tutoring effect are discussed.