This article looks at handedness as an influence on child cognitive development and the consequent impact of child development on future educational and labour market outcomes. The issue is important as about 10% of the world's population is left handed. The research used children drawn from the first wave of the Longitudinal Study of Australian Children (LSAC). It was found that left handed or mixed handed children perform significantly worse in all measures of child cognitive development and that the disadvantage is larger for boys than girls. Mixed handed children performed worst of all - approximately double the disadvantage between left and right handed children. These differences cannot be explained by socioeconomic characteristics of parents, social environment or brain pathology.