The wide generality of the feature-positive effect (FPE) has caused speculation that the FPE may represent innate biases in the stimulus control of discriminative responding. There is little experimental evidence to date ragarding this possibility. In the present study, 1-or 4-day-old chicks were trained on a feature-positive (FP) or feature-negative (FN) discrimination with heat reinforcement. After the acquisition phase, these subjects received extinction training followed by a reacquisition phase. The FP performance was superior to the FN performance in both age groups. Extinction resulted in improved discrimination performance in both the FP and FN conditions. Unmasking ofFN learning by the extinction treatment suggests that the FPE represents a deficit in performance, rather than an inability to learn the FN task. These data demonstrate that adult-like performance on feature discriminations is evident as early as the first day post-hatch.Jenkins and Sainsbury (1969, 1970) found that the ability of pigeons to learn a visual discrimination between two stimulus displays was dependent on whether the distinguishing feature appeared on reinforced (S+ ) or nonreinforced (S-) trials. When the feature appeared on S+ trials (the feature-positive, or FP, condition), the discrimination was learned readily. However, when the feature appeared on S-trials (the feature-negative, or FN, condition), the discrimination was learned with difficulty, if at all. This superiority of FP performance relative to FN performance is referred to as the feature-positive effect (FPE). A more detailed analysis of response patterns during S+ trials indicates that FP subjects respond directly at the feature, whereas FN subjects direct response toward background cues present on both S+ and S-trials (referred to as the "common" element). Thus, this tendency to respond to or "track" the best predictor of reinforcement results in excellent FP discrimination performance and marginal FN performance.Recent evidence (Hearst, 1984(Hearst, , 1987 indicates that subjects manifesting poor FN performance may have actually learned the discrimination. Following initial FN discrimination training, subjects were given sessions of extinction in which responding to S+ or S-was not reinforced. Surprisingly, the subjects demonstrated a dramatic