Wondrously elaborate weapons and displays that appear to be counter to ecological optima are widespread features of male contests for mates across the animal kingdom. To understand how such diverse traits evolve, here we develop a quantitative genetic model of sexual selection for a male signaling trait that mediates aggression in male‐male contests and show that an honest indicator of aggression can generate selection on itself by altering the social environment. This can cause selection to accelerate as the trait is elaborated, leading to runaway evolution. Thus, an evolving source of selection provided by the social environment is the fundamental unifying feature of runaway sexual selection driven by either male‐male competition or female mate choice. However, a key difference is that runaway driven by male‐male competition requires signal honesty. Our model identifies simple conditions that provide clear, testable predictions for empirical studies using standard quantitative genetic methods. Biologists and the public are captivated by extravagant traits used during male‐male contests for mates. Elaborations such as weapons, showy signals, and complex courtship are curious because they don't obviously contribute to survival, and in many instances would seem to work to attract predation. We provide a quantitative genetic model that shows how such elaboration can evolve, and when populations will shoot past an optimum leading to runaway evolution.