Adult psychopathology is often rooted early in life and first emerges during childhood and adolescence. However, as most imaging genetic research to date has involved adult participants, little is known about how risk genes affect brain function to influence biological vulnerability in childhood. We examined the impact of neuregulin1 (NRG1), a probable susceptibility gene for schizophrenia and bipolar disorder, on brain function in a sample of 102 healthy 10-12 year old boys including 18 pairs of monozygotic twins, 24 pairs of dizygotic twins and 18 singletons. Each participant performed a perceptual matching task, while brain responses were measured using functional magnetic resonance imaging. Response accuracy and reaction times did not differ as a function of NRG1 genotype; however, individuals with two high-risk alleles showed relatively increased brain activation in a distributed network comprising the precuneus bilaterally, and the left cuneus, middle occipital gyrus, angular gyrus and caudate nucleus. These results indicate that genetic variation in NRG1 significantly affects cortical function during perceptual and monitoring processes in healthy children as young as 10-12 years of age.