Anger or wrath is an intense emotional response usually involving agitation, malice, or retribution. It is an emotion that involves a strong uncomfortable and hostile response to a perceived provocation, hurt or threat. Anger can occur when a person feels their personal boundaries are being or are going to be violated. Some have a learned tendency to react to anger through retaliation as a way of coping. Raymond Novaco of University of California Irvine, who since 1975 has published a plethora of literature on the subject, stratified anger into three modalities: cognitive (appraisals), somatic-affective (tension and agitations), and behavioral (withdrawal and antagonism). William DeFoore, an anger management writer, described anger as a pressure cooker: we can only apply pressure against our anger for a certain amount of time until it explodes. Anger is an emotional reaction that impacts the body. A person experiencing anger will also experience physical conditions, such as increased heart rate, elevated blood pressure, and increased levels of adrenaline and noradrenaline. Some view anger as an emotion which triggers part of the fight or flight brain response. Anger is used as a protective mechanism to cover up fear, hurt or sadness.