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Phylogenetic Regression Theory

Basic Principles

 "The main point here is that in a matter of seconds, a culturally refined, controlled individual can regress to the emotionality characteristic of his evolutionary forbears, and at that moment he is little different from them...under severe stress, threat, provocation or loss of control through alcohol ingestion, drugs and so forth, we temporarily lose our humanity, our culture, and our rationality" Bailey 1978, p. 22).  


    Phylogenetic regression theory is the most unique and distinctive branch of human paleopsychology. My approach to phylogenetic regression was introduced in 1978 and was subsequently addressed in several publications ((see Bailey, 2002).  The regression-progression approach draws heavily from Paul MacLean's triune brain formulation, and my book on the topic (Bailey, 1987) was dedicated to Paul and his work.

     The phylogenetic regression-progression theme is widespread in the literature on human paleopsychology, evolutionary psychopathology, and related areas.  Indeed, most evolution-based models implicitly revolve around the fundamental assumption that all human behavior constantly fluctuates between evolutionarily older and newer levels of functioning where the “primitive” gains ascendance one moment, the phylogenetically “advanced” at another, or the two levels blend or intermix in some way. 

     The phylogenetic progression-regression model addresses the dynamics of movement and inhibition/release between the older (primitive) and newer (advanced) poles of human response.  Most simply, any discernible movement toward the lower “animalistic” pole would constitute phylogenetic regression, whereas analogous movement toward the higher fully “human” pole would constitute phylogenetic progression. 

     From the outset I discerned that “regression down Paul MacLean’s triune brain” was eminently plausible.  In 1983 I attended a Festschrift in honor of Paul MacLean at the University of Chicago where I introduced the phylogenetic progression-regression model.  It was a great pleasure to meet Paul and enjoy a memorable cup of coffee with him the next morning.  He was to later play a significant role in my 1987 book on Human Paleopsychology that was, incidentally, dedicated to him.

     To this day I treasure his handwritten corrections on my section in chapter 2 on the reptilian brain and I also greatly appreciated him sending me a glossy photograph of his famous figure of the triune brain first introduced in 1967.  We corresponded off and on for many years thereafter and his always kind and generous encouragement was a great inspiration to me. 

     One memorable comment stands out in a letter from him on May 3, 1983.  I had previously sent several chapter drafts from my book to Paul for review and, with some trepidation, I asked for his thoughts on the phylogenetic progression-regression model that drew heavily on the dynamics of the triune brain.  In his usual warm and encouraging manner, he said the following: “…it seems to me that the burden of proof is on those who say that the conditions you describe do not represent forms of regression.  What is the stampeding response to being caught in a burning theatre, if it is not a form of regression?  Call it by any other name, it’s a throwback to an unlearned form of responding.”  

     In my 1987 book, several forms of phylogenetic regression were identified including acting “animalistically,” physical atavisms as in Lombroso’s criminogenic stigmata, disintegration of hierarchic systems, and loss of neocortical control over lower centers (neurological regression).

     More recently (Bailey, 2002), numerous other variations on phylogenetic regression were summarized:  the re-activation of older agonic and sexual mechanisms, the de-activation of affectional and kinship systems, a return to earlier ancestral patterns, a breakdown of the thin veneer of culture, “downshifting” from higher to lower brain centers, and so on.

     Most of the above theories emphasize danger, stress or privation as the primary elicitors of the regressive process.  For example, Holloway (1974) says that “We overcome this (our xenophobia or aversion to outsiders) to some extent through cultural learning…But put stress on the system, and age-old dispositions are dominant" (emphasis added). 

     There are several core assumptions that follow from the phylogenetic progression-regression model (see Bailey, 2002):
 First, it is far easier and simpler to regress than to progress phylogenetically- regression merely requires reactivation or passive release of previously evolved motivational, emotional, and behavioral coordinations, whereas progression requires years of cultural programming to effect context-appropriate behavior.

     Second, phylogenetically older tendencies stored lower in the triune brain are more quickly and efficiently activated (e. g., the fast-acting amygdala in the fear response) than are more phylogenetically recent, neocortically mediated outputs involving large amounts of cultural content.  Gilbert (1989) discussed how quickly lower psychoneural functions can be brought into action, and Bailey, Tipton, & Taylor (1977) provided empirical support for the hypothesis that “instinctual stimuli” such as a threatening stare are processed more quickly in profoundly retarded subjects than in presumably more cognitively-oriented mildly retarded subjects.

     Regressive responses come readily and quickly, we are often told to “count to ten” to delay or arrest these natural and overpowering impulsions and desires.

     Third, regression to or reactivation of “lower” previously evolved behavioral patterns is inherently pleasurable in many instances (Bailey, 1987).  By contrast, activation of “higher” neural functions are more often hedonically neutral, unpleasurable, or even aversive. 

     Bailey, Burns, & Bazan (1982) found that college students reported considerably more subjective pleasure for visceral experiences such as eating, drinking, having sex, receiving a back rub, and so forth, than for “higher” activities such as studying, writing a computer program, attending a lecture, or writing a term-paper. 

     The regression/pleasure link is not a hard and fast rule, however, for a number of ancient adaptations are clearly unpleasurable when activated- e. g., anger, rage, fear, jealousy, envy, submissiveness, and the like.  Thus, when species-typical approach kinds of adaptations are regressively activated the rule seems to hold, but it breaks down where avoidance, loss, and pain-related systems are involved.

     Fourth, individual differences are systematically reduced in regression but may be either reduced (conformity) or augmented in progression (individualism; “free will”).  That is, people tend to become more alike in regression as they move down toward the common species-self and less alike as they move up toward their typical level of race, ethnicity, and culture. 
 
     Generally, individual differences (and “free will”) are smoothly and progressively reduced with regression down the triune hierarchy, but gender differences represent a special case.  Whereas males and females show typical patterns of reduced individual differences for regression within their respective genders, patterns are more complex between the genders. 

     Human gender differences tend to be augmented in regression to limbic or mammalian levels of the MacLean hierarchy where natural and species levels of “femaleness” and “maleness” are recovered.  However, such differences tend to disappear at the lowest levels of the reptilian brain where all humans react basically the same. 

     For example, men and women tend to easily fall into their ancestral gender patterns when under stress, high sexual arousal, or in producing and parenting offspring (Bailey, 1987), but gender differences may be negligible or absent in reptilian rage reactions, in screams of anguish during serious injury, or in the emotional anguish following loss of a cherished loved one.

      Fifth, any stimulus condition that compromises or nullifies neocortical control over lower systems may incite regression, but stress- especially stress related to survival of self or kin- is fundamental to the process.  In my approach, the stress-regression coordination is viewed as an evolved adaptation that assures that ancestrally proven coping mechanisms are readily accessible when needed. 

     Under stressful conditions, the human brain not only relies upon learned habits and skills for resolution, its calls upon any and all ancient adaptations that have proved themselves in the evolutionary past. 

     I argue that the brain actually scans these ancient adaptations before considering more recently learned and cultural adaptations!  There is no political correctness at the lower levels of the brain-  when survival is at stake, fighting, fleeing, begging, manipulating, stealing, and favoring self and kin over others tend to rule over our “let’s just get along” protestations.
  
      Natural selection is both efficient and economical, and it makes sense that frequent re-activation of proven species-typical adaptations and even pre-human adaptive patterns would be generally preferable to the creation of novel solutions for each challenge encountered.
    
      Sixth, phylogenetic regressions may be either passive or active.  Passive regression “refers to loss of neocortical ability to inhibit archaic impulses and imperatives.  Here one effortlessly slips back, often unconsciously, into species…patterns of behavior” (Bailey, 1987, p. 99).  For example, one often sees cognitively challenged women in senior facilities gently cradling their “baby” dolls and some even try to “nurse” them.  This reflects not only an ontogenetic return to an earlier phase of her life, but also a “regression” to the mammalian, primate, and hominid roots of the species.  
 
     The passive type is the most common form of phylogenetic regression and may be set into motion by any internal condition or external circumstance that weakens, compromises, or immobilizes rational, conscious, and culturally-targeted controls over our ancient and powerful species selves.

     External elicitors include any and all stimulus conditions outside the physical body that encourage or induce a passive regressive response.  Almost anything external can set off regression depending the individual’s age, ethnicity, culture, state of physical and mental health, and the situational context. 

     When comforting survivors at the funeral home as they stand at the casket, note how meticulously careful one must be to avoid setting off a tearful or even angry response.  Loved ones are already in states of regression due to their agonizing loss, and even the slightest provocation will regress them further.

     Internal elicitors for passive regression include the individual’s genetic background, brain damage/dysfunction, physical disease and illness, alcohol and drug intake, fatigue, activation of internal stress mechanisms and so forth.

     Certain of these internal conditions appear to encourage regression to the lower centers by occupying neocortical resources that would be otherwise used for inhibition (as when coping with serious disease), others directly interfere with or compromise neocortical inhibitory processes (e. g., brain damage or alcohol intake), and sometimes ancient systems of response just simply overwhelm the neocortex as when your ankle is in the jaws of a pit bull! 
      
    Whereas passive regression represents a kind of yielding or surrender to lower and more powerful systems of the brain, there are times when consciousness, rationality, culture, and self-perceived morality side with ancient imperatives and even aggravate and amplify them.  In active regression, the culturally-programmed neocortex fans the flames of regression by providing dissonance-reducing rationalizations and justifications for primitive eruptions and outpourings of ancient adaptations. 

     The more pernicious forms of active regression include ideology-based warfare, terrorism, ethnocentrism, racial prejudice, political jingoism, religious cultism, and fear and hatred of any and all things different.  However, mild and even neutral forms may be seen in advertising, the media, entertainment, education, and even science when neocortical propaganda and high-sounding self-delusion serve to amplify and mask “true” underlying concerns with sex, dominance, rank, and general selfishness regarding the individual and his or her inclusive fitness system.

     Indeed, the history of science is rife with dramatic and “regressive” stories of warfare, spite, revenge, and even physical violence between scientists over matters of priority, rank, recognition, money, and sometimes minute differences in theory or ideology.  Regression “excused” is still regression.

Regression as a deviation from “normality” or natural balance
    
     Phylogenetic regression theory assumes that archaic humans (e. g., hunter-gatherers) operated within a species-typical “zones of normality” in the course of day.  However, sudden, stressful and evocative circumstances could cause a deviation from “normality” and induce a regressive state.  The individual might freeze, fight, flee, seek help, or otherwise employ phylogenetically older modes of response in hopes of solving the immediate problem.  Even archaic man would exhibit time-limited regressions under fairly specific evocative conditions.  I suspect that he stayed in the regressive state only briefly before returning to his typical zone of everyday normality.  

     Theoretically, any individual who is basically “normal” physically and mentally, and is enjoying a “normal” day, would exhibit minimal regressive activity.  This would be true for both archaic and modern human beings.  We see, then, that phylogenetic regression is, among other things, a deviation from the normal course of activity.  These deviations are a natural, instinctive response, and may occasionally benefit the individual, but, as adaptations from an earlier time, they may also cause a whole new set of problems.  For example, flight is the oldest and most urgent behavioral system in nature, but we cannot just flee from every problem.   

Regressive episodes versus regressive states

     The prototypical example of phylogenetic regression is a brief episode where, due to stress or provocation, the individual reverts subconsciously from modern modes of problem-solving to ancient ones.  A man might note, for example, that one of his poker pals is cheating- and in a fit of primeval anger, he pulls out a knife and gravely injures the card cheat.  The individual has no violent past and is known by all as a really nice, peaceable guy.  But in that regressive episode he has deviated from his normal behavioral zone in a deadly, violent way.  This one, momentary regressive episode has had a ruinous effect on the individual, his victim, and all the families involved.

     The regressive episode is straightforward theoretically, but the regressive state is another matter.  Whereas the regressive episode comes and goes like a summer thunder storm, the regressive state comes and may stay an indefinite period of time.  For example, a wife might catch her husband in adultery and immediately descend into a regressive episode and explode with a fury.  Yet, months later she may still be highly stressed, fuming, and in a dire state of both physical and mental health.  For months or perhaps years she may live in a chronic and unhealthy regressive state where is “not herself” and cannot seem to dig herself out of the mire.

     I believe that ancient humans experienced a smattering of regressive episodes, but they rarely wallowed in regressive states over significant periods of time.  Such wallowing is the reigning disease of modern human beings who can cling to their resentments, their prejudices, and their hurt feelings sometimes for a lifetime.

     The phylogenetic regressed state compromises the physical and psychological health of the individual and his or her family and friends, and it may end in tragedy for all concerned.  Just think of all of those divorcing couples whose dysfunctional regressed states lead to misery for everyone around them for months and even years-  and especially so for the children.
     Talk to any policeman who has had to intervene in so-called “domestic disputes.”  He may not know the terminology, but he knows how dangerous and deadly it can be to enter the home of family members who are in deep and dysfunctional regressive states.  He knows that that life and death regressive episodes are likely to follow.

     A major assumption of phylogenetic regression theory is that people in smoldering regressive states- even relatively mild ones- are especially prone to dangerous and often violent regressive episodes that can lead to much violence and mayhem.    

Types of phylogenetic regression

1.   The most general type of phylogenetic regression occurs when the higher, “more human” neocortical brain centers lose some of their inhibitory control over lower “more animal” functions.  The brain, as a whole, is primarily an organ of inhibition (Bailey, 2002), and any insult in the form of injury, drugs, or alcohol can compromise its control over less human and less presentable outputs.  Various external stresses and provocations also compromise inhibitory control over less human lower functions. 

2.   The most common form of coordinated regression is to the fight or flight system under threat to life or limb.  A peaceful neighborhood walk can turn into “fight or flight” regressive episode when faced with an angry dog, the sight of a bad car accident, or any perceived threat that pulls one out of his or her zone of normality.  For a brief moment, the individual shares an archaic adaptation with not only archaic man but with the rest of the animal world- especially where flight is involved.


3.  As we have seen, regression to the hunting and gathering modality is a major avenue for “re-experiencing” a few glimmerings of the ancient way of life.   Of course, even in the most rigorous safaris or hunting expeditions we do not fully become our archaic hunting and gathering ancestors; nevertheless, we can indulge ourselves in bits and pieces of their world and enjoy some of its pleasures and salutary outcomes. Note how many of our vacation venues revolve around exploration of new lands, hunting, fishing, and especially intense and close relationships with our families and friends during our time away from modernity.

 
4.  Regression by gender is an especially interesting variation.  “Like most of our other human characters, sexuality reflects our evolutionary heritage, especially that in the hunting and gathering phase” (Bailey, 1987, p. 184).  It follows then, that archetypal “maleness” and “femaleness” are likely to re-emerge under evocative conditions- especially survival stress.

     The male archetype is more simple and straightforward and much less burdensome than that of the female.  The archaic male had to secure a mate or mates; he had to protect himself, his mate and offspring; he had to hunt and provision his mate and group; and, poor fellow, he was biologically charged with spreading his sperm in any willing receptacle, mate or otherwise.  He was, loosely speaking, a gender-based, and dedicated reproductive unit.

     By contrast, the female archetype was a much more tightly bound and dedicated reproductive unit, and, once pregnant, she was a virtual prisoner of her hormones, bodily changes, and the thrust of new life.  Both pregnancy and childbirth were very demanding, energy-depleting and dangerous, and her survival required the constant support of others (viz., her mate and other females). 

     In archaic contexts, the sexual division of labor ruled decisively and there was virtually no opportunity to alter or escape the naturally assigned sex and gender roles. 

     In the modern context, gender and sex roles are all over the place, pregnant mothers are protected, provisioned, and supported by professional “strangers,” and males are often peripheral to the whole process of pregnancy and childbirth. 

     In the modern insemination/pregnancy/childbirth sequence, the female “matches” or is more similar to the ancient hunting and gathering patterns than is the male.  Indeed, many inner city males abandon the responsibilities of fatherhood entirely, whereas all females who produce offspring must contend with the “regressive” inconveniences of morning sickness, limitations on freedom, weight gain, and stretch marks.  The generation of new life comes with many costs- most of which are paid by the female.

     We see, therefore that the modern females are required to endure rather massive phylogenetic regressions in the course of mating, pregnancy, and childbirth.  By contrast, the male, if of deficient character, can opt to by-pass everything but the insemination process.

     Along with “pregnancy as regression” for the female, there is another gender-related form of phylogenetic regression- males tend to regressively favor other males in gender disputes, and females similarly favor other females.  Recall the infamous Battle of the Sexes tennis match between Billie Jean King and Bobby Riggs in 1973.  Virtually every male in America was desperately rooting for Riggs and the women were similarly behind King.  Gender phylogenetic regression ruled on that day!

5.  Regression to the inner chimpanzee is one of the most interesting and theoretically intriguing variations.  Given that human beings share over 98% of their DNA with chimpanzees, we should not be surprised that “we go there” behaviorally and psychologically with considerable regularity. 

     In my view, this simple fact is one of the most important and most overlooked scientific findings of the twentieth and current century.    If even true to the slightest, how could a finding of such psychological import be basically ignored by the social sciences?  Most, if not all, social scientists are aware of the vast DNA overlap between humans and chimps, but they seem to ignore or deny the equally vast theoretical import of these findings

    Put bluntly, there is considerable apeness in our humanness and to ignore this glaring fact is to greatly handicap our understanding about who we are as a species and as individual human beings.  I believe that a great amount of “inexplicable” and uncivilized human silliness, mayhem, and senseless violence can be attributed- at least, in part- to a return or re-activation of the mischievous and amoral “inner chimp.”  

      In order to fully understand human psychology, we need to know as much as possible about chimpanzee psychology.  There is a vast literature on the higher apes, but let me recommend the following: Bailey (1987); Jane Goodall’s The Chimpanzees of Gombe (1986); Franz de Waal’s Peacemaking Among Primates (1989); Jared Diamond’s The Third Chimpanzee (1992); and Wrangham and Peterson’s Demonic Males: Apes and the origins of human violence (1996).  For a well-received and fascinating semi-popular approach to the chimp-human overlap, see Frans de Waal’s Our inner ape: A leading primatologist explains why are who we are (2005).  

     According to Diamond (1992), there is not just one chimpanzee but three; the common chimp Pan troglodyte, the smaller more gracile version bonobos or Pan paniscus, and finally, the human chimp, Homo sapiens.  The genetic overlap between the common and bonobos forms is around 99.5 percent, and that between humans and both other chimps is around 98.5 percent.  Thus, both chimps are only slightly more related to each other than they are to human beings.  In fact, humans and chimps are more closely related than are chimps to the other apes like the gorilla!  The deep kinship of the three “chimpanzees” is both undeniable and a very large piece of the paleopsychology of the human species.

     Politically liberal academic primatologists, such as de Waal in the Inner Chimpanzee, strain mightily to make the sexy, matriarchal, hyper-feminine, and more gentle and peaceful bonobos the “ideal” progenitor of human beings.  For decades they have struggled with the idea that we might be, in essence, a little too much like the less physically attractive, hyper-masculine, literally “big-balled”, rank-conscious, angry, and often violent common chimpanzee.  Certainly, the bonobos would fit much more nicely into the modern Edens where academic types dwell- the college campus.

    A paleoconservative like myself sees things quite differently.  I believe there are vestiges of both subspecies coloring human emotions and behavior, but common sense and the bloody history of our species reveals a much greater kinship with the common chimpanzee.  In fact, one could argue that the numerical rarity of bonobos and their confinement to a small, river-protected geographic region in the Democratic Republic of Congo relegates them to an evolutionary afterthought.  Just imagine what would happen to them if they were attacked en masse by an ill-tempered army of common chimpanzees-  immediate extinction! 

      What are some of the motivational, emotional, and behavioral aspects of chimps that may contribute, in some degree, to our own personal “inner chimpanzees?” 

     First, let’s look at sexual and reproductive behavior.  Both chimp subspecies are “sexy” by human standards, but the “anything goes” and “polymorphous perverse” bonobos set an incomparably new standard for sex for its own sake.  In fact, sex- both heterosexual and homosexual- is so frequent and rampant in the bonobos community that “who is daddy” is not an issue.   By contrast, the common chimp mates both promiscuously and competitively, but only when females are in estrus.  The male’s extremely large testicles come into play as multiple males engage with a single female. 

     Second, there is the matter of chimpanzee aggression, violence, and a proclivity for predatory hunting.  It has been known for decades that common chimps engage in coordinated predatory hunting in all male groups, but recent research indicates that bonobos occasionally hunt as well.  The male common chimp- but not the male bonobos- is very rank conscious and he engages in frequent episodes of bluffing and fighting.  He may also intimidate and physically abuse both females and juveniles.  The common chimp not the “killer ape” of Robert Ardrey, but he a pretty tough guy, ready to fight, defend territory, raid other groups and go on a testosterone-driven hunting jag with his buddies.  

 
     Third, recent research indicates that chimpanzees have elaborate, well-formed and distinctive personalities, and many of their traits are shared with humans.  Georgia State University researchers Robert Latzman and William Hopkins concluded that chimps and humans have almost the same personality traits and they are structured almost identically.  Their analysis found that the most fundamental personality trait for chimps was dominance; that is, whether a given individual is more toward the “alpha” pole of rank-conciousness and disinhibition, of the “beta” pole of playfulness and sociability. 


     Fourth, intelligence is an important trait that assumes a central role in the personalities and social lives of both chimps and human beings.  Chimps exhibit a wide range of intellectual skills in both the wild and domestic settings.  Dr.  Willian Hopkins and colleagues at Georgia State University studied the cognitive abilities of chimps on a kind of IQ test and concluded that level of intelligence is almost entirely genetic, it runs in families, and “cultural” and environmental factors play inconsequential roles.  And chimps have their “geniuses” too!  After extensive testing, the female chimp Natasha at Ngamba Island Chimp Sanctuary in Uganda was found to be the smartest of the 100 chimps tested.  Her caretakers already knew how smart she was following her teasing, frequent escapes, and ability to communicate and understand communication.


     Fifth, chimpanzees are intensely social creatures, and most of their day is spent in intense social interactions with other members of the group.  Most of the interactions are peaceful and positive as in mother-infant interactions, grooming activities, bonobos sexual encounters, female-to-female support systems, various alliances between the weak to counter the strong, in food sharing, and so on.  At other times, aggression and violence may enter the picture in rank and dominance contests, explosive outbursts in chimp males, political plotting between factions, territorial defense against outsiders, coordinated raids on other groups, and the occasional “predatory hunt-kill-and-share the meat” extravaganza.  All of these interactions are colored by the particular personalities and levels of intelligence in play at the moment.
   
Concluding Comment
    
     We see that the hierarchical and triune structure of the human brain and our extremely close evolutionary ties with the animal world give rise to a biologically adaptive but culturally problematic process called phylogenetic regression.  This refers to the instant, automatic, and “instinctive” re-activation or return- often under stress- of motivations, feelings and action patterns that served our ancestors in the evolutionary past.  Many times- as with regression to the flight or fight system under stress- the ancient outpourings may be socially acceptable in moderation.  However, regression to the polymorphous sexuality of bonobos or the raucous bad temper and violence of the common chimp is behaviorally, socially, and morally problematic and perhaps even criminal.

    We also see why phylogenetic regression theory plays such a fundamental and central role in human paleopsychology.  I argue that the daily activity of any and all modern human beings is a constant and unremitting process of regression down toward and progression up away from the animal world from whence we come.  Moreover, regression down is easy, quick and efficient, and occurs in fractions of a second. 

     In modern meritocracies and technocracies, progression upward is slow, incremental, uncertain, weakly represented in the brain, and literally requires decades of formal education and moral shaping to meet minimal standards of social acceptance.  It took thousands of years for your first-world culture to develop and you spent 25 years of your life becoming the “perfect person”- only to have all disappear in the few seconds it took to pull the trigger and send you wife’s lover to glory.

     Such is the power of phylogenetic regression in the lives of modern human beings.  We ignore, deny, and flee from this fact at our peril.  Indeed, the test of our morality and our humanity is how well we (more or less subconsciously) manage and orchestrate our personal processes of regression-progression and acquit ourselves proudly as the “third chimpanzee.”
  


Bailey, K. G. (1978).  The concept of phylogenetic regression.  Journal of American Academy of Psychoanalysis, 6, 5-35.

Bailey, K. G. (1983).  Psychobiological regression and aggression.  Invited address at symposium in tribute Paul D. MacLean at Chicago Academy of Sciences, May 1983.

Bailey, K. G. (1987).  Human Paleopsychology: Application to aggression and pathological processes. Hillsdale, NJ: Erlbaum

Bailey, K. G. (2002).  Upshifting and downshifting the triune brain: Roles in individual and social pathology.  In Cory, G. A. and Gardner, R. Jr., The evolutionary neuroethology of Paul MacLean: Convergences and Frontiers, pp. 317-344.  London: Praeger.

Bailey, K. G., Burns, D. S., and Bazan, L. C. (1982).  A method for measuring “primitive” and “advanced” elements of pleasures and aversions.  Journal of Personality Assessment, 46, 639-646.  

 Bailey, K. G., Tipton, R. M., and Taylor, P. F. (1977).  The threatening stare: Differential response latencies in mild and profoundly retarded adults.  American Journal of Mental Deficiency, 31, 599-602.

de Waal, F. (1989).  Peacemaking among primates.  Cambridge Mass.: Harvard University Press.

de Waal, F. (2005).  Our Inner Ape: A leading primatologist explains why we are who we are.  New York: Riverhead (Penguin).

Diamond, J.  (1992).  The Third Chimpanzee.  New York: HarperCollins.

Gilbert, P.  Human nature and suffering.  Hillsdale. NJ: Erlbaum.

Goodall, J. (1986).  The chimpanzees of Gombe: Patterns of Behavior.  Cambridge, Mass.: Harvard University Press.
  
Holloway, R. L. (Ed.).  Primate aggression, territoriality, and xenophobia: A comparative perspective.  New York:
Academic Press.

Wrangham, R. and Peterson, D.  (1996).  Demonic males: Apes and the origins of human violence.  Boston: Houghton Mifflen.
   
           

    

  
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Kent G. Bailey kgbailey1@verizon.net