Theory of mind requires an understanding of both desires and beliefs. Moreover, children understand desires before beliefs. Little is known about the mechanisms underlying this developmental lag. Additionally, previous neuroimaging and neurophysiological studies have neglected the direct comparison of these developmentally critical mental-state concepts. Event-related brain potentials were recorded as participants (N = 24; mean age = 22 years) reasoned about diverse-desires, diverse-beliefs, and parallel physical situations. A mid-frontal late slow wave (LSW) was associated with desire and belief judgments. A right-posterior LSW was only associated with belief judgments. These findings demonstrate neural overlap and critical differences in reasoning explicitly about desires and beliefs, and they suggest children recruit additional neural processes for belief judgments beyond a common, more general, mentalizing neural system.