Not only which behaviors are emitted but also their magnitude is largely determined by the social context. In general, the presence of other people tends to increase the expression of simple behaviors, such as well-learned motor responses, and it tends to interfere with complex behaviors, such as learning new material or responses (Zajonc, 1980). This effect of social context on the magnitude of behavior is generally referred to as social facilitation or inhibition. Social facilitation of behavior has been defined as "increments in the frequency or intensity of responses already learnt by the individual, shown in the presence of others usually engaged in the same behavior" (Crawford, 1939, p. 432).

In order for social effects to occur, the other individuals must in some way be involved with the subject. This involvement could simply mean that the other individual provides an audience, or the other individual could be a co-actor, actively engaged in a behavior along with the subject. Alternatively, the other individual may be behaving while the subject is the audience. This may evoke modeling or imitation by the subject, or it could provide vicarious learning. The mere presence of another is not enough. If the other person is blindfolded and the subject knows there will be no interaction, then social facilitation does not occur (Cottrell et al., 1968).

Simple behaviors tend to be increased in magnitude by social influences. Because eating is a very simple behavior, it would be expected that more would be eaten when dining occurs with others present. Such an effect has long been known to occur with animals, who eat more in the presence of other animals than when they eat alone. Bayer (1929) demonstrated this phenomenon when he allowed a chicken to completely satiate by eating as much wheat as it wanted. he then introduced a hungry chicken who began to eat. The first chicken, although just satiated, immediately began to eat again. The same phenomenon has been replicated in pigs (Hsia and Wood-Gush, 1984).

The general phenomenon of social facilitation of eating has subsequently been demonstrated in a large variety of species, including chickens (Rajecki et al., 1975; Tolman and Wilson, 1965), fish (Welty, 1934), rats (Harlow, 1932; Hoyenga and Aeschleman, 1969), gerbils (Forkman, 1991), puppies (James, 1960), and primates (Harlow and Yudin, 1933). As an example, the results of the Harlow (1932) study with rats are summarized in Figure 20-1. Harlow fed rats either in pairs or alone on alternate days. As shown in Figure 20-1, the animals always ate more in pairs than when alone.

Even though the animal research clearly demonstrated that social facilitation was a robust phenomenon, it was still believed that somehow people were different. Indeed Harlow (1932, p. 12) wrote "…in the presence of individuals like ourselves…eating is influenced, probably not so much as