The impact of socio-economic background on mathematics performance in England can be seen from the most to least disadvantaged. As socio-economic background of pupils increases, so does average mathematics performance; the gap between the most and least disadvantaged is equivalent to over three years' of schooling. However, many factors other than socio-economic background also affect performance, and these other factors are relatively more important to pupil performance in England than in other countries. Furthermore, the impact of socio-economic background is lower among the most disadvantaged half of the population. Examining other countries and the lack of change over time suggests that it is very difficult for countries to reduce the impact of socio-economic background on performance. Disadvantaged pupils who perform better than average, given their socio-economic background, tend to be autumn-born, are more confident in their abilities, and are less likely to truant. Our analysis shows that pupils would benefit from a more granular measure of deprivation; that summer-born pupils need a strategy to ensure they are not left behind; that further research would be beneficial to understand better the characteristics of pupils that perform better than their background would predict; and that schools should be further supported in tackling underperformance of disadvantaged pupils.