This paper investigates the phenomena of overclaiming -- the propensity for individuals to claim more knowledge about an issue or topic than they really (or could possibly) do. Using Programme for International Student Assessment (PISA) data from nine Anglophone countries and over 40,000 young people, we examine teenagers' propensity to claim knowledge of three mathematics constructs that do not really exist. We find substantial differences in young people's tendency to overclaim across countries, genders, and socio-economic groups. Those who are most likely to overclaim are also found to exhibit high levels of overconfidence and believe they work hard, persevere at tasks, and are popular amongst their peers. Together this provides important new insight into overclaiming, how this differs across groups, and how it relates to other psychological constructs.