Social facilitation is said to occur when one animal increases or decreases its behaviour in the presence of another animal which does not otherwise interact with it. Typically, a chicken might be found to eat more when another chicken is present, even if this other chicken does not reinforce, communicate, exhibit eating behaviour, or compete for food. Likewise, social facilitation is said to occur when humans run faster, read less, type quicker, or do fewer arithmetic problems in the presence of another person, but only if the other person does not reinforce the behaviour, show how it is done, set a performance standard, or compete.

These changes in behaviour were first studied as a phenomenon in 1898, and have since become known as social facilitation, whether the changes in behaviour are an increase or a decrease. In research, a human subject will perform a task alone and in the presence of another person, and the two types of conditions are compared.

It can be seen that social facilitation is defined through exclusion: it is said to occur when no other explanation (competition, reinforcement, cueing, cooperation) is possible. This makes it difficult to say exactly what social facilitation is, except by demonstrating that a behaviour has increased or decreased in the presence of another animal and that other explanations are not possible.

Put in these terms, it might be wondered why anyone would bother studying such finicky and elusive effects. The fact is, however, that social facilitation is one of the oldest topics in social psychology, and lays claim to being the first topic studied in experimental social psychology.