The perceptual validity of synesthesia has been established by behavioral and neuroimaging evidence; however, its developmental origins remain unclear. Here we tested the hypothesis that synesthesia arises when there is less experience-dependent pruning during development. We did so by comparing adults with and without synesthesia on another phenomenon that results from experience-dependent pruning: perceptual narrowing of discrimination to native categories (native phonemes and upright human faces) to the detriment of non-native categories (non-native phonemes, chimp faces, and inverted human faces). Synesthetes were better than controls at all of the non-native discriminations, with no difference for the native categories. The results support the hypothesis that in synesthesia, there is decreased experience-dependent modification of the cortical hyperconnectivity present in early development.