The main purpose of the current study was to provide empirical evidence to support or refute assumptions of phenotypic deficits in motivation for children with Down syndrome (DS). Children with moderate intellectual disability (MID) associated with etiologies other than DS were recruited in an extension of a previous study that involved children with DS and typically developing children. The participants were 29 children with MID and 33 children with DS who were matched on mental age to 33 typically developing children, aged 3-8 years. Mastery motivation was assessed on task measures of curiosity, preference for challenge and persistence, as well as parental reports. There were no significant group differences on the mastery motivation tasks. Parental ratings of mastery motivation differed, with typically developing children generally being rated more highly than each of the disability groups. The view that motivational deficits are part of the DS behavioural phenotype was not supported.