These files contain data supporting all results reported in Zipple et al. Competitive social feedback amplifies the role of early life contingency in male mice. Contingency (or ‘luck’) in early life plays an important role in shaping individuals’ development. Here we show that competition magnifies early contingency by comparing the developmental trajectories of functionally genetically identical, free-living mice who either experienced high levels of resource competition (males) or did not (females). Male resource competition results in a feedback loop that magnifies the importance of early contingency and pushes individuals onto divergent, self-reinforcing life trajectories, while the same process appears absent in females. Our results indicate that the strength of sexual selection may be self-limiting and highlight the potential for contingency to lead to differences in life outcomes, even in the absence of any underlying differences in ability (‘merit’).