In early attachment theory, behavioural drive reduction was proposed by Dollard and Miller (1950) as an explanation of the mechanisms behind early attachment in infants. Behavioural drive reduction theory suggests that infants are born with innate drives, such as hunger and thirst, which only the caregiver, usually the mother, can reduce. Through a process of classical conditioning, the infant learns to associate the mother with the satisfaction of reduced drive and is thus able to form a key attachment bond. However, this theory is challenged by the work done by Harry Harlow, particularly the experiments involving the maternal separation of rhesus monkeys, which indicate that comfort possesses greater motivational value than hunger. In social psychology, drive theory was used by Robert Zajonc in 1965 as an explanation of the phenomenon of social facilitation. The audience effect notes that, in some cases, the presence of a passive audience will facilitate the better performance of a task, while in other cases the presence of an audience will inhibit the performance of a task. Zajonc's drive theory suggests that the variable determining direction of performance is whether the task is composed of a correct dominant response (that is, the task is perceived as being subjectively easy to the individual) or an incorrect dominant response (perceived as being subjectively difficult). In the presence of a passive audience, an individual is in a heightened state of arousal. Increased arousal, or stress, causes the individual to enact behaviours that form dominant responses, since an individual's dominant response is the most likely response, given the skills which are available. If the dominant response is correct, then social presence enhances performance of the task. However, if the dominant response is incorrect, social presence produces an impaired performance. Increasing performance of well learned tasks and impairing performance on poorly learned tasks. Such behaviour was first noticed by Triplett (1898) while observing the cyclists who were racing together versus cyclists who were racing alone. It was found that the mere presence of other cyclists produced greater performance. A similar effect was observed by Chen (1937) in ants building colonies. However, it was not until Zajonc investigated this behaviour in the 1960s that any empirical explanation for the audience effect was pursued. Zajonc's drive theory is based on an experiment involving the investigation of the effect of social facilitation in cockroaches. Zajonc devised a study in which individual cockroaches were released into a tube, at the end of which there was a light. In the presence of other cockroaches as spectators, cockroaches were observed to achieve a significantly faster time in reaching the light than those in the control, no-spectator group.