'Ghost' experiments and the dissection of social learning in humans and animals.

The focus of this review is the experimental techniques used to identify forms of social learning shown by humans and nonhuman animals. Specifically, the 'ghost display' and 'end-state' conditions, which have been used to tease apart imitative and emulative learning are evaluated. In a ghost display, the movements of an apparatus are demonstrated, often through the discrete use of fishing-line or hidden mechanisms, without a live model acting directly upon the apparatus so that the apparatus appears to be operated as if by a 'ghostly' agent. In an end-state condition, an observing individual is shown the initial state of the test apparatus, the apparatus is then manipulated out-of-sight and then represented to the individual in its final state. The aim of the ghost display condition is to determine whether individuals are able to emulate by replicating the movements of an apparatus, or perform a task, without requiring information about the bodily movements required to do so (imitation). The end-state condition is used to identify goal-emulation by assessing whether the observer can replicate the steps required to solve the task without having been shown the required body actions or task movements. The responses of individuals tested with either the ghost display and/or end-state conditions are compared to those of further individuals who have observed a full demonstration by either a human experimenter or a conspecific. The responses of a control group, to whom no information has been provided about the test apparatus or required actions, are also compared and evaluated. The efficacy of these experimental techniques employed with humans, nonhuman primates, dogs, rats and birds are discussed and evaluated. The experiments reviewed herein emphasise the need to provide ghost displays and end-state conditions in combination, along with full live demonstrations and a no-information control. Future research directions are proposed.