From birth to death, every one of us is affected in our attitudes, beliefs and behaviors by the attitudes, beliefs, and behaviors of those with whom we come into contact. We exist in a web of social relations, and you can think of that web as resembling a force-field in physics. All else being equal, the effect of a stronger force is greater than the effect of a weaker force. Also, the effect of multiple forces pulling or pushing in the same direction is greater than the effect of any individual force. To understand and predict the social impact that people have on one another, we need to explain how one individual affects another, and how the effects of multiple individuals can add up.