As a predator approaches, prey base decisions about when to flee on a balance between degree of predation risk and costs of escaping. Lost opportunities to perform activities that may increase fitness are major escape costs. Paramount among these are chances to increase fitness by courting and mating and by driving away sexual rivals. Because sexual selection imposes different social demands on the sexes, social opportunities can have different consequences for males and females, but effects of sex differences in social opportunity costs on escape behavior are unknown. We conducted a field experiment showing that male striped plateau lizards (Sceloporus virgatus) given the opportunity to court or perform aggressive behavior permit closer approach before fleeing, but females do not. Males allowed a simulated predator to approach closer before initiating escape if a tethered male or female rather than a control stimulus was introduced to them, but females initiated escape at similar distances in all conditions. For males, a trade-off between the greater predation risk accepted before fleeing due to the likelihood of enhancing fitness by sexual or aggressive behavior accounts for closer approach allowed in the presence of conspecifics. Mating opportunities are not limiting for females in most species and females often have little to gain by interacting aggressively with other females. Therefore, presence of a conspecific male or female may not justify taking greater risk. Results confirm the prediction of optimal escape theory that flight initiation decreases as cost of escaping increases. The sex difference in effect of presence of conspecifics on flight initiation distance is a consequence of the sex difference in costs of escaping. Copyright 2007, Oxford University Press.