A BASIC PRINCIPLE GOVERNING behaviour is that people tend to do things associated with positively valued outcomes for them. In contrast, they usually do not persist with alternative actions that from past experience have produced little of consequence, or even unwanted effects. Positively valued outcomes can of course take many forms. Some (e.g. obtaining food, water, and shelter) are necessary for physical survival, while others (e.g. attractive company) are less basic but still important. Events that are even less tangible, yet highly valued just the same, include positive features of interpersonal contact mentioned as examples in Box 4.1. A friendly smile, a word of praise, warm congratulations, generous applause or enthusiastic response from an attentive listener are all reactions that we, from time to time, find appealing. Not only do we find them appealing, we tend to act in ways that bring them about. The fact that such reactions can influence what we do in this manner, by making it more likely that we will engage in certain behaviours in preference to others, is central to the concept of reinforcement as an interpersonal skill.