Standardised tests are a common, yet contentious, feature of many countries’ schooling systems. In May 2010, over one-quarter of English primary schools boycotted that year’s mandatory age eleven standardised tests (colloquially known as SATs tests). This paper investigates the plausibly causal effect of participation in standardised testing on later end-of-schooling qualification (GCSE) attainment. After controlling for non-random boycott participation, and relying on a selection-on-observables argument, evidence is found of a statistically significant negative effect of boycott participation on various measures of GCSE attainment. Amongst other findings, pupils are estimated to be 0.7 per-cent less likely to achieve five or more GCSEs at grades A* to C due to not sitting their age eleven SATs tests. Thus, evidence is found that individual pupils’ subsequent attainment benefits from their own prior participation in standardised tests.