Background & Aims: We tested the Diagnostic and Statistical Manual of Mental Disorders (DSM) hypothesis that so-called specific developmental disorders are marked by a pattern of specific discrepant achievement, and an alternative hypothesis that children with these disorders show a pattern of relatively pervasive low achievement. Methods & Procedures: Children with a diagnosis of Mixed Receptive Expressive Language Disorder (RELD; n = 21) were compared with children with no previously suspected disorder but low standard language scores (less than 80; n = 22) selected from a representative sample, and children with Developmental Coordination Disorder (DCD; n = 20) were compared with children with no previously suspected disorder but low standard motor skills scores (n = 28) selected from a representative sample. Outcomes & Results: Children with diagnosed disorders were more pervasive underachievers. The RELD group obtained lower scores on measures of verbal comprehension, emotion understanding, theory of mind, working memory and response inhibition; the DCD group obtained lower scores on measures of perceptual organization, verbal comprehension, receptive and expressive language, and visual inspection time. Conclusions & Implications: We conclude that relatively pervasive underachievement distinguishes disordered from normal low achievers. (Contains 1 table.)