Gender stereotypes associate mathematics and sciences with boys, whereas reading and writing are typically associated with girls. This study investigated sex differences in primary school children's motivation (confidence and value) across four academic subjects (maths, science, reading and writing) and examined how their identification with stereotypical masculine and feminine traits related to their motivation in these subjects. Five hundred and thirty-two children (aged 9-11, 52% boys), from five UK schools participated. Sex differences in reading and writing motivation were wider than sex differences found in maths and science motivation. Interestingly, the extent to which children identified with feminine traits was a stronger predictor of their reading and writing motivation than their sex. Gender identity provides an innovative approach to the study of sex differences; it challenges the dichotomy inherent within sex differences research and can lead to a more critical and nuanced understanding of sex differences in education.