Lessons we learned from the Lothian Birth Cohorts of 1921 and 1936.

The authors are, respectively, the founding and current Directors of the Lothian Birth Cohorts of 1921 and 1936. In this invited and, admittedly, self-regarding and necessarily self-citing piece, we enumerate and explicate some things we learned from working with the cohorts and their data. Some of the lessons are scientific results, some are to do with scientific practice, and some are more general reflections. We hope the paper provides a useful summary of some of the main findings from these too-many-papers-to-read cohorts and an enjoyable account of our building a research team and a network of collaborators. The original aim of assembling the cohorts was to fashion a tool to discover why some people's thinking skills aged better than others'. That tool, we discovered, had many additional uses.