Governments around the world collect huge amounts of personal data from their citizens for counterterrorist purposes. While mining this data has arguably increased the security of populations, the practices through which these data are currently collected in many countries have been criticised for violating individuals’ rights to privacy. Yet it is not clear what a permissible data collection regime would look like and thus also how we could reform existing regimes to make them morally acceptable. This article explores a number (...) of ways in which we might justify a data collection regime to those affected in spite of the setbacks to their privacy. In contrast to existing justifications, I argue that individuals can be asked to surrender their personal data as a requirement of reciprocity in a cooperative system in which they gain security from others doing likewise. Relying on this justification, though, has significant implications for how we should reform existing data collection regimes. In particular, more stringent limits will need to be placed on the forms which these regimes can legitimately take. (shrink)