Studies have sought to understand the underrepresentation of women and racial/ethnic minority groups in STEM, but less attention has been paid to primary school students. Using data from a nationally-representative sample, this study identified factors influencing US third-grade children's self-perceived competencies in maths and science, while controlling for their actual abilities. Results indicate that girls had degraded self-perceptions of their maths competencies compared to boys, but similar self-rated science competencies. Black students exhibited buoyed self-perceived maths competencies, while Hispanic and indigenous students underestimated their science competencies. Students who reported being satisfied with parental attention, friends, and neighbourhood had higher self-perceived competencies in both maths and science.