1. A system for monitoring the activity of objects comprising: a detector arranged to receive radiation from a scene; readout apparatus for monitoring and processing signals from the detector to identify objects meeting predetermined criteria appearing in the scene and to monitor the activity of identified objects within the scene; and data processing and storage apparatus configured to compile statistical data over time indicating the occurrences of two or more object behaviours and to determine the distributions of the two or more object behaviours over respective contiguous areas defining all or part of the scene, wherein the system is configured to adjust a parameter against which one of the object behaviours is judged based on a contiguous area of another one of the object behaviours.