Uncovering factors that shape variation in brain morphology remains a major challenge in evolutionary biology. Recently, it has been shown that brain size is positively associated with level of parental care behavior in various taxa. One explanation for this pattern, the 'parental brain hypothesis is' that the cognitive demands of performing complex parental care may require increased brain size. We tested this prediction by examining populations of stickleback that vary in their degree of parental care. We found that sexual dimorphism in brain size is reversed in non-parenting stickleback populations: males have smaller brains than females. Thus, while several alternatives need to be ruled out, the parental brain hypothesis appears to be a viable explanation for sexual dimorphism in brain size in threespine sticklebacks.