A system and method is provided for adjusting natural language conversations between a human user and a computer based on the human user's cognitive state and/or situational state, particularly when the user is operating a vehicle. The system may disengage in conversation with the user (e.g., the driver) or take other actions based on various situational and/or user states. For example, the system may disengage conversation when the system detects that the driving situation is complex (e.g., car merging onto a highway, turning right with multiple pedestrians trying to cross, etc.). The system may (in addition or instead) sense the user's cognitive load and disengage conversation based on the cognitive load. The system may alter its personality (e.g. by engaging in mentally non-taxing conversations such as telling jokes based on situational and/or user states.