“According to neuroscientist Paul MacLean’s venerable Triune Brain Theory, the human brain is composed of a primeval reptilian segment, a later mammalian segment, and a relatively recent neocortical segment. These three levels correspond roughly to instincts (reptilian), feelings (mammalian), and thoughts (neocortex). In 1983, I asked professor MacLean if it made sense to speak of “regressing down the triune brain” or “progressing up the triune brain”? He averred that it made perfect sense. My 1987 book, Human Paleopsychology: Applications to Aggression and Pathological Processes (Erlbaum) was dedicated to MacLean and his work.

Human beings are literally designed to “regress” down the triune brain with ease, but “progressing” up is unnatural, difficult, and requires years of cultural shaping and formal education in industrialized societies. Simply speaking, when regressive processes are set against progressive ones, regression tends to win. Partying tends to win out over studying, impulsivity over self-control, amorality over morality, and disorder over order. Human Paleopsychogy focuses on individual and social breakdowns of cultural, moral, religious, and economic systems that have taken thousands of years to reach their present form. Yet, with the slightest provocation in the form of social malaise, insult, drug or alcohol intake, exposure to pornography, or even sudden changes in the stock market, we see that good will, manners and civility, social order, and concern with “higher things” can disappear in an instant.

The process whereby this occurs is termed phylogenetic regression and it refers to the sudden stripping away of the thin veneer of culture and the complementary re-activation of ancient evolved programs of selfishness, tribality and xenophobia, aggression, sexuality, and the like. In other words, when highly stressed and/or provoked, a person easily slips back into earlier evolutionarily adaptive programs that may have served our ancestors well in precultural times but may be amoral/immoral, socially chaotic, illegal, and even pathological today. For example, sexual promiscuity and male gang behavior in hunting contexts may have served young men well 30,000 years ago, but activation of these tendencies today in the absence of moral, religious, legal, or other constraints can easily lead to rape, gang warfare, or even worst case scenarios like the “inexplicable” murderous actions of the two young men in Littleton, Colorado on April 20, 1999. Human paleopsychology tries to make sense of these “inexplicable” events and others including serial murder involving cannibalism, body mutilation, and storage of body parts, mothers brutally killing their infants and young children, and even phenemona such as rage killing, road rage, and the brutal initiation ritual of the Glenbrook North High School sorority girls who literally outdid their chimpanzee cousins in chaotic violence.” — Kent Bailey (professor emeritus of clinical psychology at Virginia Commonwealth University in Richmond, Virginia)