This report looks at the progress made during secondary school of young people from different backgrounds who were in the top third of attainers at the end of primary school. High attaining pupils from disadvantaged backgrounds have the best chance of becoming socially mobile, and "Social Mobility: The Next Generation" will explore, through a series of reports, what barriers there are to that group fulfilling their potential, and what are the circumstances and policies associated with educational and labour market success. This first report, "Lost Potential at Age 16," authored by the Trust's Erica Holt-White and Carl Cullinane, uses data from the National Pupil Database and the COSMO longitudinal study to explore the progress during secondary school of pupils with the same grades on entry, but different socio-economic backgrounds, and shows the extent to which pathways diverge at this age. [The inclusion of the Sutton Trust Opportunity Cohort in the COSMO Study has been made possible by funding provided by leading algorithmic trading firm, XTX Markets. This report includes data from the COSMO Longitudinal Study.]