A computer implemented technique for developing a behaviour tool to estimate a behaviour of a particular person in response to a given situation, comprises maintaining, in a storage device, digital memories and a record of associations between the digital memories, where a given digital memory is generated in response to a given event associated with the particular person and is determined from analysis of multiple items of data associated with the given event, including at least items of personal data derived from signals gathered from a plurality of sensors used to monitor the particular person. Processing circuitry is then employed to analyse the digital memories and the record of associations between the digital memories, in order to generate a digital twin of the particular person comprising one or more personalised cognitive models, each personalised cognitive model being arranged to emulate an associated cognitive skill of the particular person. At least one of the one or more personalised cognitive models forming the digital twin is then output for incorporation within the behaviour tool so as to cause the estimated behaviour of the particular person in response to the given situation to be influenced, at least in part, by model output data generated by the at least one of the one or more personalised cognitive models.