instinct, term used generally to indicate an innate tendency to action, or pattern of behavior, elicited by specific stimuli and fulfilling vital needs of an organism. Examples of almost purely instinctive behavior are found in the behavior of many lower animals, in which activity (often quite complex) is performed that is not based upon past experience, e.g., reproductive and food-gathering activity in insects. Instinctive behavior generally acts as an initiator or triggering mechanism to arouse the organism, and it is modified by learned behavior as well as innate regulatory mechanisms. For example, nest-building by birds is a complex activity triggered by instinctive drives and modified by environmental conditions, such as the availability of materials and sites. Among animals, fixed patterns of instinctive behavior include fighting, courtship behavior, and escape; even these can usually be shown to be modified by experience (see ethology). Freud used the term instinct when referring to human motivational forces, such as sex and aggression. Sociobiologists and ethologists such as Konrad Lorenz have sought to understand social behaviors in terms of instincts, among humans as well as other animals. The usage of the term among psychologists has largely died out; today, motivational forces among humans are generally referred to as instinctual drives.