In ETHOLOGY, a form of learning in very young animals at certain CRITICAL PERIODS. The learning is rapid and usually irreversible. For instance, some species of baby ducks will follow the first moving object they encounter after being hatched. This object, on which they are imprinted, is usually their mother, but, asKONRAD LORENZ showed, it could just as easily be an ethologist. Imprinting is thus a useful biological mechanism, in an inflexible and limited kind of way, which includes most of the BEHAVIOUR usually termed as INSTINCTIVE.