Private investors and their advisers have an abiding faith in diversification, which, by definition, acknowledges that some money managers will perform better than others at different times and for different reasons.

Yet in an industry where investment talent is paid to perform, this reality doesn’t sit well. It also raises an old dilemma that has gained added significance during this period of heightened market volatility: at what point should you replace an under-performing money manager? And how should you decide?