Vertebrates inhabiting islands are often characterized by fearlessness and tame behavior. While this relaxation of antipredator behavior has been documented in many species endemic to islands, fewer researchers have examined it in species that occur both in insular and mainland habitats. In addition, the mechanisms underlying this shift in behavior are still poorly known. For this work, antipredator behavior of island and mainland populations of the common gartersnake (Thamnophis sirtalis) was surveyed. Observed differences were explained in terms of innate tendencies and/or environmental influences using both neonates and adults in laboratory experiments designed to tease apart these two underlying mechanisms. While antipredator behavior of adults was positively correlated with the predation pressures associated with capture sites, neonate behavior did not fully reflect this pattern. However, some support for hard-wired antipredator behavior at birth was evident. Plasticity experiments also revealed that environmental factors appear to influence antipredator behavior. Thus, the data presented here suggest that predation pressures play a strong role in shaping the antipredator behavior of island animals and that the resulting behaviors are most likely the result of some combination of both nature and nurture.