The Price Equation describes how a genetic trait transmit from one generation to the next. By expanding this equation, we can explore the conditions under which altruistic behavior evolves. This evolution is usually explained by kin selection and reciprocal altruism theories, which can be unified through the Price Equation. However, the Price Equation and its implications appear to be little known, especially among evolutionary psychologists. So I will clarify the Price Equation as simply as possible. For now, I have made a PDF version available for download, but I am considering posting it in a blog-like format in the future.