Exceptions, qualifications, etc., but the link is to an academic website whose thesis is that deprivation of physical affection is the root of violence.
The site's anchor for me is an article from a 1975 Bulletin of the Atomic Scientists, "Body Pleasure and the Origins of Violence," by one James Prescott. Prescott collated the data from studies of cultures around the world, and found that violence was proportional to corporal punishment and to repression of adolescent sexual exploration.