Computerised biometric systems are automated methods of verifying or recognising the identity of a user on the basis of some physiological characteristic, like a fingerprint or some aspects of behaviour such as keystroke patterns. Behaviourally based biometric systems include signature, speaker and keystroke verification. The investigation of psychological factors which might impact on the efficiency of a behavioural biometric computer security monitoring system has to our knowledge not been conducted. Of particular concern in the present paper are the potential effects of state anxiety on individual's physiological and performance responses. It is suggested that in a behaviourally based biometric computer security monitoring system, state anxiety may have sufficient effects to alter typical physiological and performance responses, resulting in an increased risk of security challenges, interruption of work-flow and resultant poor performance. It is also proposed that behaviourally based biometric systems may have the potential to be used as electronic performance monitoring systems, and typical responses to such systems need to be examined when developing and implementing any behaviourally based biometric security system.