Part I describes how much physical aggression occurs between family members. It presents prevalence rates for intra-family physical assaults based on data from nationally representative samples of families in the USA and other countries. This includes separate rates for physical assault in the following types of family relationships: husband-to-wife, wife-to-husband, parent-to-child, child-to-parent, and between siblings. For each of these, there are rates for both minor and severe assaults. For example, the US National Family Violence Surveys have found 94% of parents of children age 4 reported hitting the child in the previous 12 months, and 12% of husbands and 12% of wives reported hitting their partner in the past 12 months. Part II is about the effects corporal punishment by parents on children's physical aggression and antisocial behavior. Corporal punishment is defined as the use of physical force with the intention of causing a child to experience pain, but not injury, for the purpose of correction or control of the child's behavior. The focus is on research to test the hypothesis that corporal punishment by parents increases the probability that a child will subsequently be physically aggressive. It is based on results from several recent prospective and retrospective longitudinal studies. These studies found that, after controlling for many other variables, corporal punishment explains a significant part of the variance in each of the types of family violence described in Part I, and also variance in non-family assault. Part III argues that, because corporal punishment is a risk factor for subsequent physical aggression, and because corporal punishment is among the earliest life experiences of children and is usually a child's first experience of violence (over a third experience it before age 1 in the USA), it can be said that the family is the cradle of violence in the society. Consequently, primary prevention of disapproved forms of family violence such as wife-beating and physical abuse of children, and also primary prevention of physical aggression outside the family, needs to involve ending the socially approved form of family violence in the form of corporal punishment.