There is increasing evidence from field observations that beach cusps are often formed by subharmonic edge waves, edge waves which are generated by an instability in the incoming wind waves. A theoretical analysis suggests that the changing beach topography as the cusps grow provides a negative feedback to the excitation of the subharmonic edge waves. As the cusps grow, the edge waves subside. A maximum cusp amplitude is calculated, based on the assumption that some edge wave activity must persist to maintain the cusps. The theoretical predicition that cusp amplitude will increase with increasing beach slope and increasing incident wave period is in agreement with the trends suggested by some of the more detailed field observations of beach cusps.