Hamilton's rule is a well-known concept in evolutionary biology. It states that a social trait is favored by natural selection if BR>C, where B is the benefit for the recipient, C the cost for the donor and R the relatedness between donor and recipient. It is often perceived as a statement that makes predictions about natural selection in situations where interactions occur between genetic relatives. It turns out that this view is incorrect. A simple mathematical analysis reveals that "exact and general'' formulation of Hamilton's rule, which is widely endorsed by its proponents, is not a consequence of natural selection and not even a statement specifically about biology. Instead it is a relationship among slopes of linear regression that holds for any suitable data set. It follows that the general form of Hamilton's rule makes no predictions and cannot be tested empirically.