Imprinting is a term used in ethology and psychology, referring to a form of learning that takes place during early postnatal life, and which is highly resistant to extinction. Imprinting was first identified by biologist Douglas Spalding in the 1870s, and studied extensively by the Austrian zoologist Konrad Lorenz. Lorenz demonstrated that newly hatched greylag geese would become attached to any moving stimulus presented to them within a given time period, which he called a critical period.