Inclusive fitness theory predicts that altruism should often be directed towards reproductive relatives, but it is unclear whether individuals that are most likely to help or harm relatives are also most likely to identify kin in the first place. Here I show that species and sibships of spadefoot toad tadpoles (Spea bombifrons and S. multiplicata) that were most likely to produce an environmentally induced cannibalistic morph were also most likely to avoid eating kin. Moreover, tadpoles avoided eating kin when they expressed the cannibal phenotype, but not when these same individuals reverted to the non–cannibalistic morph. Thus, individual tadpoles facultatively adjust their level of discrimination according to how likely they are to harm kin. In general, sensory systems and/or decision rules enabling recognition may be especially likely to evolve among those individuals that are most often faced with the problem of discrimination.