Modelling agency in economics rests primarily on the assumption of instrumental rationality. Managerial agency is more often analysed with a more complex ‘behavioural’ approach. This has led for years to a sterile debate about the usefulness of the abstract rationality postulate between those who think that it is all but sufficient and those who doubt if it is even necessary. This paper argues that positing an abstract (but real) rational core to managerial agency that is then ‘concretised’ towards actual managerial (...) behaviour is the way forward. Rationality is a necessary but not sufficient characterisation of managerial agency. The theoretical argument is supported by reference to the case of the ill-fated Long Term Capital Management built around the Black, Merton and Scholes Nobel Prize-winning derivatives pricing model. It is argued that the 1998 failure of the portfolio was not the result of a failure of rational agency but of the complexities of its implementation. (shrink)