The Functional Neuroanatomy and Psychopharmacology of Predatory and Defensive Aggression
Jordan B. Peterson, Ph. D.
Matthew Shane, M. Sc.
Department of Psychology
University of Toronto
Toronto, Ontario, Canada
Peterson, J.B. & Shane, M. (2004). The functional neuroanatomy and psychopharmacology of predatory and defensive
aggression. In J. McCord (Ed.). Beyond Empiricism: Institutions and Intentions in the Study of Crime. (Advances in
Criminological Theory, Vol. 13) (pp. 107-146). Piscataway, NJ: Transaction Books.
Human neuroscientists are frequently cortex-centric, concentrating on the large prefrontal,
temporal and parietal cortices that distinguish man most clearly from the animals – or, if not that, studying
the structure and function of the underlying limbic system. But the really important circuitry,
phylogenetically ancient and extremely sophisticated, is deep down in the central nervous system, near the
brain stem. When the chips are down, it is the hypothalamus that is in control, not the cortex (and, if not the
hypothalamus, then the periaqueductal gray (PAG), or something else equally demanding, interesting, and
unpleasant). Basic motivation stems from activity in these low level, low resolution, high power,
dominating circuits. In the case of aggression, the hypothalamus and PAG circuits underly negative-affect
potentiated defensive rage, or incentive reward motivated sexual/predation/exporation. Diverse forms of
pathologies or abnormalities, genetic, psychopharmacological, and developmental, likely undermine the
capacity of finely differentiated, phylogenetically newer emotional and cognitive circuits to modulate these
more ancient systems. Poor modulation, regardless of cause, produces chronic, situationally inappropriate,
socially troublesome aggressive behavior, both predatory and defensive.
Peterson & Shane: Defensive and Predatory Aggression 1
Aggression: Basic Behavioral Phylogeny and Bipartite Classification
We know that evolution has in general proceeded from the simple to the more complex.
To find firm ground on which to stand with regards to understanding aggression, it might therefore
be useful to presume a kind of conservation of function, not atypical of evolved systems, to look at
less differentiated organisms, and then to consider the array of aggressive behaviors characteristic
of our species complex variations on a theme. This would allow us to establish the basic
architecture of aggression, before attempting to classify and understand its conceptually confusing
variations. In consequence, we might look first far down the phylogenetic ontogenetic chain, to
find something simple and stable upon which we can build a truly scientific model.
We might begin, even, with animals no more complex than the invertebrate. Crustaceans,
a type of invertebrate whose nervous systems and stable social dominance relationships have been
extensively studied, comprise a surprisingly instructive example. In the analysis of their behaviors,
mediated by a relatively simple nervous system, we can see all the important components of
aggression – and of many closely related and clearly identifiable motivated behaviors. Lobsters,
for example, attempt both to expand and defend their territories. In the former case, we can see
elements of what in more complex creatures becomes instrumental aggression, including
predation, exploratory behavior, and sexuality (as territorial expansion and dominance increases
the probability of successful reproduction). In the latter, we can see elements of defensive
aggression, devoted essentially to the prevention of further failure, and to the maintenance of
territory and position already established. In both cases, we encounter intrinsic aggression, in its
most basic form.
A socially naïve lobster, raised in complete isolation, introduced to a novel environment
where other lobsters are present, will engage in complex, stereotyped, but experientially
modifiable behaviors, many of which are aggressive (Kravitz, 2000). First, if not eating, or
engaging in basic goal-directed consummatory activity, it explores. If it comes into contact with
another animal, it engages in the early stages of its typical agonistic behavioral pattern. The lobster
begins to dance, raising and opening but not closing its claws, moving backwards and forwards,
side to side, mirroring its opponent, and directing a stream of information-rich chemicals towards
its opponent. If a large size discrepancy exists between the two animals, then the smaller will
generally retreat, and refuse to re-engage the larger, upon further contact. Such an interaction
typically lasts about thirty seconds, and often results in the establishment of a stable dominance
In the second stage, assuming no initial decision, one animal moves forward on the other,
claws folded downward, antennae whipping, while its opponent retreats, claws up and opened,
antennae upwards. Then the two animals reverse the display, precisely, the former antagonist
retreating, the defender advancing. In stage three (assuming no retreat, once again), claws extend
and grip, and each animal tries to flip the other over. If one succeeds, the other almost invariably
retreats. If not, the animals advance on one another again, with increasing speed, claws open to
snap shut on unguarded appendages, tails snapping backwards, so that anything gripped can be
damaged. Agonistic encounters of this severity almost inevitably produce a clear winner, and a
clear loser. So far, this is all stereotyped behavior, or at least variations on a stereotyped theme –
highly complex, but evident in the complete absence of social learning. However, this stereotypy
only extends so far.
An animal who has just won a fight, at any of the aggression stages, stands taller, extends
itself, and is statistically more likely to win its next fight. A losing animal, conversely, adopts the
defeated posture of a loser (Kravitz, 2000), and is more likely to lose again when it next fights. In
the immediate aftermath of a losing battle, in fact, a lobster will not fight at all – even against an
opponent it has previously defeated – and appears to remember the outcome for days. The capacity
for aggression is therefore clearly pre-established in the lobster, although context or environment-
dependent learning modulates its expression. Success reinforces confidence and promotes
territorial expansion, while failure potentiates withdrawal, and irritable territorial defense.
Let us move from the lobster’s relatively simple nervous system to that of the cat, a much
closer cousin to the human, and an animal whose brain circuitry and behavior has also been
Peterson & Shane: Defensive and Predatory Aggression 2
extensively studied. Gregg and Siegel (2001) have outlined the basic circuitry underlying two
fundamental forms of aggression in the cat, reminiscent on the one hand of the behavior of a
victorious crustacean, intent on expanding its territory, and on the other of its defeated counterpart.
Gregg and Siegel characterize these two forms of aggression, respectively, as predatory
(instrumental, covert, planned, goal-directed, emotionless) and defensive (reactive, overt,
unplanned, impulsive, negative-affect ridden, characterized by autonomic activation) – two
categories that (1) have recently been deemed useful for the description of childhood aggression
(Vitiello & Stoff, 1997), and (2) that appear analogous to two different categories of trait elements
characteristic of aggressive, antisocial (Davidson, Putnam & Larson, 2000) or even psychopathic
adults (Hare, Clark, Grann & Thornton, 2000; Hare, 2003).
Hare’s well validated and reliable psychometric instrument, the Psychopathy Checklist-
Revised (PCL-R) (Hare, 2003) provides an objective measure of the severity of severely antisocial
behavior. Elevated scores on the PCL-R (>=29) are indicative of trait psychopathy, a constellation
of personality and behavioral features first described in detail by Cleckley (1941/1982), and
characterizing approximately 15% of institutionalized criminals (Hare, 2003). The condition
includes such cardinal features as lack of remorse, lack of empathy, egocentricity, glibness,
superficial charm, flattened affect, and absence of anxiety, as well as parasitical lifestyle and poor
behavioral control. Within applied settings, psychopathy has become an important predictor of
criminal behavior and violence (Salekin, Rogers, Ustad & Sewell, 1998; Salekin, Ziegler, Larrea,
Anthony & Bennett, 2003), and has been associated powerfully (d = . 79) with failure on
conditional release, violent recidivism, and poor treatment response (Hemphill, Hare, & Wong,
1998; Serin, 1996; Skilling, Harris, Rice & Quinsey, 2002; Hart, Kropp, & Hare, 1988). The
psychopathic offender commits more types of crimes than the average offender (Hare, McPherson
& Forth, 1988), and these crimes are generally more violent in nature (Kosson, Smith, &
Newman, 1990). Additionally, psychopathic offenders appear to benefit less from psychiatric
treatment than do nonpsychopathic offenders (Ogloff, Wong, & Greenwood, 1990), possibly due
to their ability to use the knowledge gained in treatment to become more successful offenders.
Psychopathy Checklist scores can be usefully differentiated into Factor 1 and Factor 2
traits, reminiscent, respectively, of predatory and defensive aggression. These two factors are
typically correlated at r~ 0.5 - 0.6, indicating substantive but far from complete overlap (Hare,
2003). Factor 1 includes such features as glibness, lying and manipulativeness, as well as
emotional callousness – traits indicative of an orientation to the social world that appears primarily
incentive-reward mediated. The individual high in Factor 1 appears to exist in an unlimited field of
predatory opportunities. For him, other people appear primarily as cues for consummatory reward,
sexual and dominance related – as tools for personal gain, or objects to dominate and use, instead
of targets of empathy, potential cooperative collaborators, or valid elicitors of grief, guilt, fear or
pain. A variety of forms of psychopath-like predatory aggression appear associated with
personality and behavioral traits that are very unlike those characteristic of defensive aggression.
Bullies, for example, tend to be larger and more powerful than non-bullies, and show no evidence
whatsoever of higher levels of negative affect or anxiety (Olweus, 1994). Quite the contrary: Such
aggression often appears in concert with higher than normal levels of self-esteem – sometimes
inappropriately high, narcissistic and “brittle” (context-inappropriate and exaggerated,
characterized by sporadic precipitous collapses into states of negative affect (Bushman &
Baumeister, 1998; Baumeister, Smart & Boden, 1996; Baumeister, Heatherton & Tice, 1993) – as
well as with low, rather than high levels of autonomic tone and emotional reactivity (decreased
resting and reactive heart rate and skin conductance, reduced cortisol) (reviewed in Vitiello &
Stoff, 1997). Such individuals, like the prototypical fascist, are perfectly willing to consider their
victims (smaller, weaker and likely to cry) truly deserving of intensive physical punishment and
Factor 2 traits, by contrast, appear associated not so much with predatory aggression, but
with poor modulation and high levels of negative affect – chronic anxiety, anger and depression,
manifesting themselves in heightened irritability – in tandem with an array of cognitive/
neuropsychological deficits that appear to make long-term planning and behavioral regulation
more unlikely and/or difficult (Seguin, Boulerice, Harden, Tremblay & Pihl, 1999). Hare (2003)
Peterson & Shane: Defensive and Predatory Aggression 3
has recently refined his description of the basic psychometric qualities of the PCL-R, identifying a
hierarchical factor structure: Factor 1 contains elements of interpersonal dominance (which appear
much like the classical personality components of extraversion – talkativeness, social dominance,
positive affect) and reduced negative affect (which appear much like low neuroticism or high
emotional stability – low fear, anger, pain, guilt, shame, regret), while Factor 2 contains elements
of parasitical lifestyle (which appears as absence of long-term plans, and willingness to rely on the
earnings of others) and social deviance (impulsive, antisocial behavior). These elements appears
associated with low agreeableness and conscientiousness. It should be noted that these
hypothetical Big Five personality attributes are in keeping with those identified as characteristic of
the psychopath by Loney, Frick, Clements, Ellis & Kerlin (2003) and Lynam (2002; Miller,
Lynam, Widiger & Leukefeld, 2001).
A proposed schema for the classification of aggression is presented in Table 1, building
on the work by Gregg & Siegel (2001), Vitiello & Stoff (1997), Hare (2003), and Davidson,
Putnam and Larson (2000) (as reviewed later in the present manuscript). Analogs of predatory
aggression appear associated with impairments in the experience of negative affect and emotion
related to affiliation, and are characterized by a hyper-dominant, context-independent social
stance. Analogs of defensive rage appear, by contrast, most frequently associated with high or
unstable levels of negative affect and clinical disorders typified by abnormalities in executive
cognitive function, and are characterized by explosive outbursts and physical or verbal aggression
manifested to minimal provocation.
Aggression: Basic Neuroanatomy
Defensive rage in the cat can be produced by electrical stimulation of the ventromedial or
anterior medial hypothalamus, the fundamental controller of motivated behavior (Swanson, 2000)
or the periaqueductal gray (PAG), a brain area whose activation is also associated with a broad
range of motivated states (along with the midline and intralaminar thalamic nuclei and pars of the
prefrontal cortex and anterior cingulate gyrus (Sewards & Sewards, 2003). There are specific
defensive rage neurons in the medial hypothalamus and PAG and (Gregg & Siegel, 2001). When
the hypothalamic defensive rage neurons are stimulated, they excite the PAG defensive rage
neurons which, in turn, activate brainstem and spinal cord systems, responsible for the integrated
behavioral and autonomic manifestations characteristic of defensive rage. This rage response
includes behavioral manifestations such as warning noise (growling, hissing, meowing, yowling),
piloerection, back arching, aggressive paw strikes, pupillary dilation, and increased heart rate and
blood pressure (Gregg & Siegel, 2001). It tends to emerge when the cat, its kittens, its territory or
its position in the social dominance hierarchy is threatened – so, emerges when the physical
integrity of the animal, its offspring, or its ecological niche is threatened or attacked.
Predatory attack, by contrast, can be produced by electrical stimulation of the lateral or
perifornical hypothalamus. The pathways ascending to and descending from this area are more
general than those regulating defensive rage, although the rostro-ventral area of the PAG appears
importantly involved, as a stereotyped form of predatory attack may be elicited by stimulation
from this area (Gregg & Siegel, 2001). Predatory attack postures and behaviors are clearly
dissociable from those associated with defensive rage. Most importantly, perhaps, they completely
lack sympathetic nervous system activation– a marked characteristic of defensive rage (excepting
mild pupillary dilation, more probably associated with heightened interest). Instead, they appear
associated with incentive-reward motivated approach, goal-direction, and stealth (while defensive
rage, in the cat, is associated with hissing, piloerection, yeowling, meowing, etc. ). A perfectly
calm cat – even one previously using an anesthetized rat as a pillow – will change its behavior
markedly when subjected to perifornical hypothalamic stimulation, increasing its degree of
alertness, circling and attacking the rat with bites aimed at its neck, repeatedly, picking up the rat
and shaking it. This attack-oriented behavior is relatively specific, too, indicating the regulation of
visual systems by hypothalamic/PAG predatory motivation. A properly stimulated cat will choose
to prey upon a rat, for example, rather than upon a plastic block, which obviously lacks prey-
specific features (Gregg & Siegel, 2001).
Peterson & Shane: Defensive and Predatory Aggression 4
The hypothalamus is a brain structure system of complex interface between descending
modulatory cortical/subcortical control mechanisms (involving cognition, perception, and
emotion) and direct behavioral output, mediated by a sequence of hierarchically structured motor
neuron systems. Its forward or rostral section appears responsible for basic, goal-directed,
motivated behavior (Swanson, 2000) – regulating eating, drinking and primary social behaviors,
such as reproduction and defense. Its caudal section, by contrast, appears to regulate general
exploratory or foraging behavior, associated with incentive reward activation (Swanson, 2000). An
animal whose brain has been transected just above the hypothalamus – lacking the overwhelming
bulk of its brain – is therefore nonetheless capable of very complex behavior, assuming the
environment remains reasonably constant: it can eat, drink, engage in reproductive activities
(particularly if female), and can mount “very effective and complete defensive responses”
(Swanson, p. 119). Furthermore, it is not hypo-responsive to environmental stimulation, as might
be supposed, but hyper-responsive – a logical consequence of the hypothalamus’ position as the
center of motivation.
The PAG, another relatively low-level subcortical structure, also appears integrally
involved in a number of motivational states, including fear, pain, sexual desire, thirst, hunger and
sleep (Sewards & Sewards, 2003). Electrical stimulation of the dorsal PAG in humans produces
intense anxiety, distress, panic, terror and feelings of imment death (reviewed in Seward &
Seward, 2003) – subjective sensations logically associated with defensive rage. Consideration of
the general function of these brain areas, outlined by Sewards & Sewards (2003), Swanson (2000)
and Gregg & Siegel (2001) make it appear that the hypothalamus and PAG motivate both
fundamental forms of aggression, defensive and instrumental or predatory, and that the expression
of these aggressive forms is then modulated by other, emergent higher-order subcortical and
cortical systems, such as the amygdala, septal-hippocampal system, anterior cingulate gyrus,
dorso-lateral, orbital and ventral-medial prefrontal cortex. Lesions of the PAG can, for example,
completely eradicate the defensive rage response, while damage to the modulatory structures can
not, indicating at a physiological basis a clear distinction between the systems responsible for the
behavioral pattern itself, and those that help differentiate its responses.
The amygdala is a collection of nuclei located deep in the temporal lobe. It receives
multiple cortical, thalamic and brainstem inputs, and projects mainly to the hypothalamus, PAG,
and prefrontal cortex (reviewed in Gregg & Siegel, 2001). It is integrally involved in the learning
of aversive and appetitive associations (Gallagher & Schoenbaum, 1999; Holland & Gallagher,
1999; Ledoux, 1998; Davis and Whalen, 2001; Rolls, 1999), responsible for context-independent
classical conditioning of specific fear responses (Ledoux, 1998; Phelps, LaBar, Anderson,
O’Connor, Fulbright & Spencer, 1998), and provides part of the neuroanatomical substrate for
anxiety, in its extended circuitry (Davis & Whalen, 2001). Stimulation of amygdalic medial and
cortical nuclei, and the medial aspect of the basal nuclear complex, projecting to the anterior
medial hypothalamus and the basal nuclei of the stria terminalis, facilitates defensive rage, while
suppressing predatory attack (at least in the cat). By contrast, stimulation of fibers emerging from
the central and lateral part of the basal nuclear complex suppresses defensive rage and facilitates
predatory attack (reviewed in Gregg & Siegel, 2001), as does stimulation of the medial amygdala
(Stoddard-Apter & MacDonnell, 1980), indicating that these behavioral states, although both
clearly aggressive, are physiologically dissociable and frequently mutually inhibitory.
Dysfunctions in amygdala circuitry have often been associated with inability to regulate
emotion (Davidson et al., 2000) and to learn from punishment, and with more specific impairment
in the capacity to recognize facial and emotional signs of sadness and pain (Blair, Sellars,
Strickland & Clark, 1995; Blair, Colledge, Murry & Mitchell, 2001; Blair, Jones, Clark & Smith,
1997; Blair, 2003a; Blair, 2003b). Individuals with amygdala dysfunction, impaired in the ability
to experience and recognize fear and anxiety, may therefore be less likely to recognize the social
inappropriateness of their motivated aggressive actions, or to experience the negative emotion
necessary to give meaning to consideration of the potentially detrimental long-term consequences
of such action. A large body of research has demonstrated, for example, that psychopaths manifest
deficits, cognitive or affective, that reduce their ability to learn from threat (Newman & Kosson,
1986; Newman, Patterson, Howland & Nichols, 1990).
Peterson & Shane: Defensive and Predatory Aggression 5
Kiehl (in press) has summarized an impressive and consistent body of work suggesting
that psychopaths manifest particular dysfunction in the paralimbic system, a brain area underlying
the cortical mantle, differentiated cytoarchitectonically by Brodmann, and including the amygdala,
anterior superior temporal gyrus, rostral and caudal anterior cingulate, posterior cingulate,
orbitofrontal cortex, and parahippocampal regions (Kiehl, Smith, Hare, Mendrek, Forster, Brink &
Liddle, 2001; Mesulam, 2000). Kiehl suggests that this dysfunction is associated specifically with
reduced reactivity to negative-affect relevant stimuli (including decreased sympathetic activation
and autonomic reactivity, as well as reductions in fear-potentiated startle, abnormalities of the
orienting response, and attenuation of the P300 component of the cortical evoked response
potential) (Kiehl, Smith, Hare & Liddle, 2000; Kiehl, Hare, McDonald & Brink, 1999; Kiehl,
Hare, Liddle & McDonald, 1999). This hypothesis subsumes that offered previously by Blair,
described previously, who has produced an impressive body of work associating psychopathic-like
traits directly with amygdalic dysfunction. Kiehl has also demonstrated that psychopaths show an
elevation of late stage ERP negativity, also observed in patients with temporal lobe damage
(particularly if involved the amygdala and anterior superior temporal gyrus), which he attributes to
late-stage prefrontal cortical attempts to process information that should be processed earlier by
circuitry more directly involved in affect.
The septal-hippocampal formation, prefrontal cortex and anterior cingulate gyrus also
clearly modulate aggression. The hippocampal formation, responsible primarily for the context-
specific modulation of behavior (logically associated with the important role it plays in fear
learning, spatial learning and general, declarative memory (Gray, 1982; Gray & McNaughton,
1996), projects via the precomissural fornix to the septal area and then to the medial and
perifornical hypothalamus (reviewed in Gregg & Siegel, 2001). The prefrontal cortex and anterior
cingulate gyrus, by contrast, may influence the hypothalamus more indirectly, projecting to the
mediodorsal thalamic nuclei, through the midline thalamus, to the nucleus reunions, and then to
the perifornical hypothalamus (regulating predatory aggression). In addition to such amygdala-
hypothalamic and frontal-hypothalamic circuits, there also appear to be inhibitory connections
between the prefrontal cortex and the amygdala, capable of secondarily modulating the more basic
hypothalamic circuitry (Davidson, Putnam & Larson, 2000; Bouton, Mineka and Barlow, 2001).
Pare, Royer, Smith & Lang (2003) have noted, for example, that firing rates in the basolateral
amygdala are among the lowest anywhere in the brain, suggesting considerable tonic inhibition of
these circuits. Furthermore, Fanselow and Gale (2003) have demonstrated that fear conditioning
lasts the entire lifetime of an organism, and suggest that there must be continuous inhibition of
amygdala circuitry to keep anxiety responses from being constantly manifested.
The prefrontal cortex of higher-order primates, modulating amygdalic, hippocampal, and
hypothalamic functioning (in addition to its diverse additional tasks) is very highly developed,
particularly in human beings, both in terms of absolute size and the specifics of its neural
representations. It appears critically involved in cognitive operations undertaken over broad spans
of space and time, and manages the so-called “executive processes” of complex organisms (Luria,
1980: Shallice and Burgess, 1991) – learning new information (particularly if it involves
motivational or affective reversal), planning ahead, regulating actions according to environmental
stimuli, responding to novelty, monitoring error, and shifting behavioral sets (Duffy & Campbell,
1994) and perceptual frames (Peterson & Flanders, 2002), in concert with the septal-hippocampal
system (Gray & McNaughton, 1996). Five frontal-subcortical circuits have so far been identified
(Tekin & Cummings, 2002). The motor and oculomotor circuits are involved in the regulation of
motor functions, per se, while the dorsolateral prefrontal, orbitofrontal, and anterior cingulate
circuits are involved in executive functioning, regulation of social behavior and monitoring and
modulation of motivated states, respectively. Dysfunction of the dorsolateral, orbitofrontal (and
associated ventromedial) prefrontal circuits have therefore each logically been associated with a
variety of neuropsychiatric syndromes, as have irregularities in the function of the anterior
A large literature on aggression in animals suggests that lesions to the prefrontal cortex
generally increase aggressive behavior (Egger & Flynn, 1967; Mirsky & Seigel, 1994), while
hypothalamically-elicited aggression can be inhibited by the stimulation of the ipsilateral
Peterson & Shane: Defensive and Predatory Aggression 6
prefrontal cortex (Siegel, Edinger & Dotto, 1975). Research on both normal and abnormal human
populations clearly, beginning with Luria (1980), supports this general notion. The Vietnam Head
Injury Study found, for example, that subjects with lesions limited to the frontal lobes tended to
show more aggressive and violent behavior compared with patients with non-frontal head injury
and control participants (Grafman, Vance, Weingartner, Salazar & Amin, 1986). Similarly,
patients with frontotemporal dementia have commonly been found to have higher rates of
antisocial behavior, even when compared with equally cognitively impaired control groups (Stip,
1995; Miller, Darby, Benson, et al., 1997).
Goyer, Andreason, Semple, Clayton et al. (1994) used PET to show that an increased
number of aggressive acts was associated with reduced glucose metabolism in the frontal cortex of
17 personality-disordered patients. Raine, Meloy, Birhle, Stoddard, LaCasse & Buchsbaum (1998)
reanalyzed their earlier data suggesting that prefrontal glucose metabolism was reduced in
murderers, compared to normal controls, particularly in the medial and lateral zones of the PFC (in
association with hyperactivation of the right, but not left amygdala). They further separated the
murderers into predatory and impulsive-aggressive groups, and revealed that only members of the
latter group were characterized by lateral PFC metabolic abnormalities. In addition, they
demonstrated that “limbic” metabolic activity (an average of hippocampus, amygdala, thalamus
and midbrain) was also significantly increased in the impulsive, aggressive group.
Numerous studies have also reported decreased prefrontal brain volume (Raine, Lencz,
Bihrle, LaCass & Coletti, 2000), abnormal frontal EEG activity and diminished frontal event
related potentials (O’Connor, Bauer, Tasman, et al., 1994; Finn, Ramsey, & Earleywine, 2000;
Fishbein, Hearning, Pickworth, et al., 1989) in non-brain damaged individuals with antisocial
personality disorder or histories of aggression. Raine (2003) has also demonstrated an association
between aggression and decreased glucose metabolism but increased overall size and
abnormalities in shape in the corpus callosum and associated white matter, indicative of an
unspecified dysfunction in interhemispheric communication. It also appears possible that this
increased CC size is a late stage adaptation to decreased signal emanating from lower level brain
systems – systems which should be producing negative-affect or affiliative cues to regulate
behavior, but which are not (in keeping, perhaps, with Kiehl’s observation of elevated late-stage
ERP processing among psychopaths).
Finally, Davidson, Putnam and Larson (2000) have suggested that displays of anger are
associated with activation of the orbitofrontal cortex and the anterior cingulate gyrus. These
authors presume, however, that this activation is associated with the regulation of anger or
aggression, rather than with its production (something presumably attributable to the
hypothalamus and associated circuitry). They also detail a study demonstrating wide individual
variation in the ability to modulate negative affect, voluntarily – associated with activation of the
left prefrontal cortex (and, more hypothetically and more specifically, the left orbitofrontal cortex)
(Jackson, Malmstadt, Larson & Davidson, 2000).
Damage to orbitofrontal circuitry, per se – particularly if right-sided (Miller, Chang,
Mena, Boone, & Lesser, 1993) – appears to present the greatest risk of increased emotional
dysregulation, and has been associated with increased hostility, impulsivity and aggression
(Damasio, 1995; Grafman, Schwab, Warden, Pridgen, Brown & Salazar, 1996). Individuals with
lesions of the orbitofrontal cortex manifest signs of what has been termed acquired sociopathy
(Damasio, 1995), although their aggression is clearly defensive and reactive in nature, rather than
predatory (Blair, Colledge & Mitchell, 2001), and are characterized by marked personality change,
including behavioral disinhibition, emotional lability, lack of empathy and explosive aggressive
outbursts (Stip, 1995; Miller, Darby, Benson, Cummings & Miller, 1997). They tend to neglect the
long-term consequences of their behavior, although they may be able to verbalize them, and they
manifest manifest abnormal (low) galvanic skin responses to threat or punishment (Bechara,
Tranel, Damasio & Damasio, 1997).
The anterior cingulate gyrus (ACC) also appears importantly involved in the regulation of
hypothalamic function (Tekin & Cummings, 2002). Lesions in this area cause decreased
motivation, indifference to pain, thirst or hunger, and impaired response inhibition (Mesulam,
Peterson & Shane: Defensive and Predatory Aggression 7
2000). Recent EEG research has focused on the error-related negativity – a steep negative evoked-
response potential, emanating from the ACC, and occurring immediately after errors on simple
flanker or go/nogo tasks (Holroyd & Coles, 2002; Pailing, Segalowitz, Dywan & Davies, 2002).
The error-related negativity indexes attention to committed error. Its magnitude, inversely
associated with number of errors committed on a given task, appears reduced among antisocial
Antisocial and aggressive individuals also appear characterized by functional
impairments, suggestive of prefrontal deficits. Evidence of purely cognitive deficits in individuals
characterized by a preponderance of Factor 1 traits has tended to be mixed, at best. Gorenstein
(1982) demonstrated that psychopaths showed greater dorsolateral dysfunction (as measured by
performance on the Wisconsin Card Sort Task) than did nonpsychopaths in an inpatient substance
abuse program, and Schalling & Rosen (1968) showed similar dysfunction in a population of
imprisoned psychopaths. Kandel and Freed (1989) found only weak evidence for an association
between dorsolateral dysfunction and specific violent criminal behavior, and the symptoms
demonstrated by individuals with dorsolateral lesions – apathy, inaction, reduced intelligence – do
not generally appear consistent with the description of Factor 1 psychopathic traits. However, a
more recent meta-analysis by Morgan and Lilienfeld (2000) determined that there was in fact a
small, but significant, deficit in executive functioning among antisocial individuals (d = . 29), who
also consistently score approximately 8 points lower on standard intelligence tests (Heilbrun,
1979; Heilbrun & Heilbrun, 1985; Henry, Caspi, Moffitt, Harrington & Silva, 1999). Furthermore,
more recent work, using better neuropsychological tests, has pointed to executive dysfunction, at
least among populations characterized by defensive aggression (Grafman et al, 1996; Seguin et al.,
Abnormalities in other prefrontal control systems, more recently identified, might also
underly the disinhibition of predatory and/or defensive aggression. A class of prefrontal
visuomotor neurons, comprising the “mirror neuron system” (recently discovered by Rizzolatti,
Fogassi and Gallese, 2001) constitutes one reasonable candidate for further investigation.
Rizzolatti et al. (2001) describe a neurophysiological system that allows an individual to “mirror”
the motivated state of another, using the full apparatus of his or her own body. Mirror neurons are
located in Brodmann’s area F5, which contains neurons that code both observed and individual
enacted “goal-related” motor acts, such as grasping (desired objects). These neurons are
remarkable, in a number of ways. For example, they neither respond to the presence of a
motivationally significant object in isolation, like a piece of fruit, nor to the sight of a conspecific
engaged in a particular action, such as grasping. Instead, they respond to the sight of a conspecific
making a grasping action in the presence of a motivationally significant object (so, to an observed
conspecific grasping a fruit). More importantly, their pattern of action when an animal is
observing precisely matches that engaged when such a sequence is actually undertaken by the
observer. This means that the observing animal, using the mirror system, can use its own body to
represent the state of an observed body – in principle, down to the emotional and perhaps even
autonomic and hormonal state of that observed body. This means, in principle, that an individual
observing the acts of a motivated other can adopt the motivational frame of that other, and come to
“see and feel” in the world made visible through another’s eyes.
Is it possible that the primary deficit of the aggressive individual (particularly one
characterized by a preponderance of Factor 1 traits) is an abnormality at some important level of
this newly described system? There is a relatively non-trivial body of evidence suggesting that
something like this might be the case. It seems very likely, first, that the mirror neuron system
underlies narrative cognition. Oatley and Johnson-Laird (1996), as well as Peterson (1999;
Peterson and Flanders, 2002), have proposed that the frames through which individuals see the
world (and which therefore govern their thoughts, their patterns of behavior, and their autonomic
and endocrine reactivity) are essentially narrative in structure: when an observer understands the
goal of a perceived, motivated actor, then the observer’s frame shifts to match that of the
perceiver, and his emotional systems and body attune themselves to the actor (hence the shared
experience characteristic of immersion in a narrative). Now it turns out that the mirror neuron
systems in monkeys are in a location analogous to that of Broca’s area in humans which, of
Peterson & Shane: Defensive and Predatory Aggression 8
course, underlies the capacity for voluntary speech. What this implies, essentially, is that the
function of language is the communication of motivated action frames, and their associated
cognitive and emotional states, and motor output patterns (as a means of motivated mimicry, and
the transfer of adaptive behavior).
Psychopaths “know the words, but not the music” (Johns & Quay, 1962). Cleckley noted
that the purposiveness and significance of semantic communication seemed lost on psychopaths,
although they could technically understand the words. They could not get the meaning, which is
the significance of the word for action, or for the structure of the frames that govern action
(Peterson, 1999) – in fact, Brinkley, Bernstein & Newman (1999) noted directly that psychopaths
did not understand the relationship between spoken narrative and action as well as controls. They
showed abnormalities in hemispheric lateralization, associated with language processing
(reviewed in Kiehl, in press), particularly with regard to left hemisphere function, and were
arguably characterized by left hemisphere dominance (Kosson, 2003), something which appears
akin to Ramachandran’s (1996) self-deceptive anosognosic neglect. Psychopaths do not process
abstract words well (Hare & Jutai, 1988), and they use more beats – hand gestures incongruent
with speech content – while speaking (Gillstrom & Hare, 1988), a finding that theoretically
indicates less coherence of speech over more extended period of time (and, therefore, choppier and
less coherent narratives). Many observers have noted that the psychopath typically contradicts him
or herself repeatedly, in a lengthy conversation, and can not really string together a coherent story.
Psychopaths also do not process abstract words well (reviewed in Kiehl, in press), do not
show normal ERP differentiation between abstract and concrete words, and do not show the
expected pattern of hemodynamic differentation between abstract and concrete words in the right
anterior temporal lobe. Furthermore, they show no right anterior superior temporal gyrus
activation during process of abstract words, relative to baseline (Kiehl, Liddle, Smith, Mendrik,
Forster & Hare, 1999). These findings seem to indicate that the more generalized significance of
abstract communication for alteration of behavior and structures of meaning is likely lost on them.
This is of interest with regards to the mirror neuron systems, as well, because Rizzolatti et al. have
also demonstrated “higher-order” mirror neuron systems, which appear to generalize across
instances of motor behavior, and facilitate the mimicry of abstracted action (whose “meaning” is
inductively derived from repeated presentations). Day and Wong (1996) have demonstrated that
psychopaths are characterized by abnormal hemispheric laterality while processing negatively
valenced word stimuli, and have suggested that psychopathic individuals cannot make use of
emotional processes based in the right hemisphere. This finding and hypothesis clearly hearken
back to Kosson’s (2003) work, suggesting abnormal left-hemisphere dominance in psychopaths,
and Raine’s work demonstrating abnormal corpus callosum development, as described previously.
Psychopaths also have difficulty identifying the guilty party in story passages (Blair et
al., 1995), cannot easily recognize fearful vocal affect (Stevens, Charman & Blair, 2001), do not
differentiate between affective and neutral words in their own speech (Louth, Williamson, Alpert,
Pouget et al., 1998), and have difficulty processing linguistic information relating to affect
(Williamson, Harpur & Hare, 1991). Intrator, Hare, Stritzek, Brichtswein, Dorfman, Harpur,
Bernstein, Handelsman, Schaefer, Rosen and Machac’s (1997) finding that psychopathic
individuals show greater activation for neutral than affective stimuli, bilaterally, in temporo-
frontal cortex may be an indication (1) that processing of such words is more difficult for them or
(2), as Kiehl (in press) points out – and this is something in keeping with his general hypothesis –
that they have to recruit different, and probably non-emotional – circuits to process emotional
Newman’s influential suggestion that psychopaths are characterized by impaired
response modulation (Newman, Schmitt & Voss, 1997)(something he associates with dysfunction
of the septal-hippocampal circuit, as described by Gray, 1982; Gray & McNaughton, 1996)
appears in keeping with this line of reasoning. The hippocampus appears integrally involved in
memory processing, spatial orientation, identification of novelty, and orienting – functions which
are frequently reported as impaired among aggressive and/or psychopathic individuals (reviewed
in Kiehl, in press). Although these processes are often treated as independent – indeed, even as
mutually exclusive—they are not. First, all analysis of novelty has to occur within a perceptual
Peterson & Shane: Defensive and Predatory Aggression 9
frame (Peterson, 1999; Peterson & Flanders, 2002). Something is only novel in comparison to
what is expected. So, the septal-hippocampal comparator described by Gray (1982) – capable of
disinhibiting, if not producing, anxiety (perhaps as a consequence of its interactions with the
amygdala) – has to use its access to long-term memory storages to build the frame within which
novelty-analysis (and, therefore, fear disinhibition, as well as orienting) takes place. It is perfectly
reasonable to presume that such a frame has a narrative structure (I am here, and I am going there,
and some things are going to happen on the way), for reasons that are too complex to describe here
(Peterson, 1999; Peterson & Flanders, 2002). Newman has suggested, essentially, that
psychopaths do not use contextual information to modulate their emotional responses –and, in an
associated theoretical conjecture, supported by his data – that they do not change their tack, once it
is established (regardless of the emergence of experience suggesting that they are wrong or in
danger). This seems very much like (1) a consequence of failure to experience or process emotions
associated with threat, failure, frustration, etc., or with affiliation, (2) impaired inter-hemispheric
communication (in that the right-hemisphere mediated threat response systems are either not
functioning or are not getting their message across), and (3) impaired understanding of context
(something logically associated with “not getting the story right”).
The Basic Psychopharmacology of Aggression
It appears likely that both the medial hypothalamus and the basal amgydala release an
excitatory amino acid (most probably glutamate) onto NMDA receptors in the PAG, to activate
the PAG neurons responsible for defensive aggression, in the cat. The neurokinin cholecystokinin
(CCK), which plays an important role in regulating anxiety, also appears to alter the expression of
aggression, at least according to Gregg & Siegel (2001), who demonstrated that microinjections of
the CCK-B receptor agonist pentagastrin into the PAG facilitated defensive rage elicited by
electrical stimulation of the medial hypothalamus. The neurokinin Substance P (SP), involved in
nociception (and, thus, linked logically to aggression, as a response to pain), has receptors
distributed widely in the amgydala, hypothalamus, and PAG. Agonists effecting NK-1 receptors,
mediating SP functions, produce spontaneous hissing in the cat, in addition to facilitating
defensive rage elicited from the medial hypothalamus (Gregg & Siegel, 2001). This is all very
interesting, given the accruing evidence that all negative affective states overlap, from the
perspective of trait personality (linked through neuroticism), neuropsychology, and behavioral
Dopamine, which is the major neurotransmitter involved in incentive reward and hope or
approach-motivated positive affect, also plays an important role in the modulation of aggression.
This relationship appears particularly true, however, with regards to D2 but not D1, dopamine
receptor activation (Gregg & Siegel, 2001). Microinjections of D2 agonists into the anterior
medial hypothalamus, for example, potentiate defensive rage, while microinjections of D1
agonists do not (reviewed in Gregg & Siegel, 2001). Anger, unlike other forms of negative affect,
also has pronounced incentive reward aspects (activation of approach circuitry) (Fox & Davidson,
1988), which are undoubtedly mediated by dopamine, as well. This means that dopamine is
probably involved in the target-locked and approach-oriented incentive-reward motivated behavior
associated with predatory aggression, male-male aggression and dominance hierarchy ascension.
Abnormally high levels of homovanillic acid, a dopamine breakdown byproduct, have in fact been
reported as characteristic of violent offendors, in conjunction with decreased 5-HIAAA, a
serotonin breakdown byproduct (Soderstrom, Blennow, Manhem & Forsmann, 2001) – which
indicates that they are both highly motivated by incentive reward and poorly regulated with
regards to that motivation.
The often overlooked but probably critical role of opiate peptides is also well worth
considering (reviewed in Pihl & Peterson, 1995). Opiates are potent psychomotor stimulants
(Wise & Bozarth, 1987) and their stimulant properties, like those of cocaine or amphetamines, are
capable of inducing analgesia, in and of themselves (Franklin, 1989). However, opiates have
additional analgesic properties, perhaps due to their operation in areas of the brain such as the
periaqueductal gray (Panksepp, Siviy and Normansell, 1985; Wise and Bozarth, 1987). Tactile or
sensory system stimulation, above a certain variable threshold, evidently produces pain, and elicits
defensive aggression or escape behaviour (Gray, 1982; 1987) – typical reactions to punishment.
Peterson & Shane: Defensive and Predatory Aggression 10
There appear, in addition, to be other less obvious forms of pain- or punishment-like response,
mediated by the same or similar neurochemical systems, operative in the absence of aversive
tactile stimulation. Gray (1982; 1987) has, for example, described the absence of an expected
reward, disappointment or frustration, as equivalent to pain, based on his analysis of behavior and
opiate agonist reduction/opiate antagonist potentiation. The absence of social interaction also
produces a state that appears very similar to pain, in terms of its effects upon spontaneously
generated and manipulated behaviour (Panksepp et al., 1985). This means that the endogenous
opiate system has apparently come to govern more than reaction to aversive sensory stimulus in
the course of mammalian evolution.
It appears possible that the opiate system mediates interpersonal attachment, or love
(Herman and Panksepp, 1978), as well as responding to punishment and the absence of expected
reward (Panksepp et al., 1985), and that destruction of close social bonds might result initially in
the development of a pain-like condition (including loneliness, grief and despair). The social
affiliative process, after all (like pain), has as one of its primary goals protection against physical
destruction, particularly in infancy. Furthermore, this circuitry is phylogenetically ancient, and
mature at birth (Valzelli, 1981), as opposed, say, to the circuitry that mediates fear and anxiety,
which matures much later (Gray, 1982; 1987). Finally, the interpersonal bonding that underlies
love is mediated in large part through stimulation of the tactile system, which is also integrally
involved in the experience of pain. Such bonding/social support/tactile stimulation, tremendously
important for regulating stress responses in primates (Virgin & Sapolosky, 1997), is a likely
consequence of opiate system functioning (Drolet, Dumont, Gosselin, Kinkead, Laforest &
Trottier, 2001), facilitated by maternal, affiliative, or sexual touch, and by more abstract cues, such
as adult gaze and infantile facial configuration, as well as olfactory and gustatory stimulation and
consummatory satisfaction (particularly with regards to sugar and fat, both potent analgesics) (see
Pihl & Peterson, 1995, for a review). Behaviors and emotional states exhibited as a consequence
of pain, withdrawal of love, and opiate withdrawal share many important core features in
common, including increased levels of lacrimation (crying) and distress vocalization, withdrawal,
aggression and depression (Herman and Panksepp, 1978). Some of these behaviours are common
across many species.
A complex interplay between affiliation-induced de-activation of pain circuitry and
isolation-induced pain appears to be critically important to further development (Najam and
Panksepp, 1989). What is proper interplay depends on the maturity of the animal, possibility for
social interaction with other conspecifics, the duration of isolation (reviewed in McKinney, 1985),
and on the voluntary or involuntary initation of the separation (Robertson and Robertson, 1971). It
is possible that a necessary balance exists between endogeneous opiate production, in the presence
of mother, and opiate withdrawal. Administration of the opiate antagonist naloxone, which blocks
the reinforcing effects of opiates, can increase development, but increases social deprivation-
induced distress behaviour and can induce distress in non-deprived animals (Knowles, Conner and
Panksepp, 1989). Morphine administration, by contrast, can reduce social distress (Knowles et al.,
1989), but slow and retard development (Najam and Panksepp, 1989).Rat pups chronically treated
with morphine, an opiate agonist, develop slower than normal rats, for example, in terms of
physical development and motor coordination (Najam and Panksepp, 1989). Withdrawal from
morphine – a state associated by Panksepp with social-isolation, grief and loneliness (Pihl &
Peterson, 1995) – produces increased aggression, as does administration of the opiate antagonist
naloxone (which also produces increased sensitivity to pain-like stimulation, including frustration,
disappointment and grief). This increased aggression tends to take the form of defensive rage, like
that precipitated by fear and/or amygdalic stimulation. Opiate agonist miocroinjection into the
PAG, BNST and nucleus accumbens – in principle tantamount, pharmacologically, to the
consequences of tactile stimulation, maternal care and social grooming (Pihl & Peterson, 1995) –
appears to suppress defensive rage, decrease pain, and decrease autonomic reactivity, in logical
keeping with such argumentation.
Recent experiments have demonstrated that enhanced interpersonal tactile contact
substantially increases thriving (weight gain and neurological development) among premature
babies (Field, Schanberg, Scafidi, Bauer, Vega-Lahr, Garcia, Nystrom & Kuhn, 1986), but also
Peterson & Shane: Defensive and Predatory Aggression 11
that the detrimental developmental effects of maternal deprivation in rats can be mimicked by
artificial administration of beta-endorphin (Greer, Bartolome and Schanberg, 1991). This latter
effect seems to be similar to the consequences of long-term, despair stage deprivation. There is
evidence, as well, that exposure to interpersonal loss in childhood, and later in life, predisposes to
development of depression (McKinney, 1985). It is also perfectly reasonable to note in this regard
that socially isolated animals, more susceptible to grief, pain, frustration and disappointment, are
less confident, more likely to retreat, less likely to engage in predatory/dominance related behavior
– more depressed, in a word – because of alterations in their opiodergic systems, but also, on the
sociological plane of analysis, because their isolation actually renders them weak. First, isolated
animals are less likely to have coalition or kin members who will take their side in a battle or
dispute, or who will punish transgressions against them (a very important consideration with
regards to complex primate behavior)(Abbott, Kaverne, Bercovitch et al., 2003). Second, isolated
animals are much less likely to access to beneficial, direct, physical social contact, including
tactilely mediated grooming and consolation, absolutely vital to regulation of the long-term
detrimental effects of aggression-circuit activation (including excess production of cortisol). This
means they are much more sorely affected, physiologically as well as psychologically, by any
agonistic encounter – particularly in the case of a defeat (Abbott et al., 2003; Virgin & Sapolosky,
1997). This is all very interesting and relevant, particularly given that social bonding mechanisms
obviously play an important role in establishing a vital and effective maternal-child relationship in
infancy (Kraemer, 1985), that early disruptions in maternal care (teenage pregnancy, low SES
mother, low education certainly elevate risk for aggressive behavior (Vitaro, Brendgen &
Tremblay, 2002), that Hare’s Factor 1 and 2 Psychopathy Checklist (revised) traits appears
associated with dysregulated affiliation and negative affect, and that endogeneous opiate systems
play a critical role in the maintenance of homeostasis during stressful conditions (Drolet et al.
Developmentally, it appears that initial, voluntary social isolation produces a state similar
to opiate withdrawal, eventually culminating in the need for renewed social contact, but that
prolonged, involuntary social isolation perhaps reverses this process, flooding the organism with
endogeneous opiates, eliminating need for social contact. Infants deprived of close contact first
protest, and then give up (Bowlby, 1969; Robertson and Robertson, 1971). Once they give up, it is
difficult for them to re-establish bond, and death is a common consequence. In the despair stage,
they stop seeking conspecific contact. This pattern of reaction to separation is not necessarily
limited to infancy, and such separation often precedes development of nonbipolar depression in
humans (McKinney, 1985). It may be that a certain amount of controlled, voluntary separation,
whose effects are then modulated by a return to maternal care, promotes proper physical and
psychosocial development, but that too much or too little severely retards or impedes maturation.
Something similar appears to occur with regards to agonistic encounter among chimps, regulated
in its detrimental effects by post-encounter grooming and mutual reconciliation (De Waal, 2000;
De Waal & De Waal, 1990).
It also appears that GABA plays an important role in the regulation of aggression –
particularly with regards to the mutually inhibitory relationship between defensive rage and
predatory aggression. The medial amygdala circuit, utilizing Substance P to activate medial
hypothalamic cells, activates an inhibitory circuit, utilizing GABA, to suppress predatory attack
(originating in the lateral hypothalamus), at the same time it activates an excitatory circuit,
utilizing glutamate, to facilitate defensive rage (controlled by the PAG). Gregg & Siegel (2001)
point out the various logical reasons why defensive and predatory aggression should be
discriminable: an affectively activated, angry, afraid, pain-or-frustration ridden animal is
attempting to defend itself, or its territory, and manifests various perceptible displays of its
discomfort and easily-precipitated attack to whatever is threatening it, while an animal attempting
to move up the dominance hierarchy, or to engage in predatory behavior, must plan carefully and
act quietly in order to maximize its chances of success.
The regulation of aggression by serotonin (the last major neurotransmitter whose activity
we will cover) can be profitably considered from such a perspective (that is, one dealing with the
interaction between neurophysiology, affective state, and dominance hierarchy position and
Peterson & Shane: Defensive and Predatory Aggression 12
direction of movement). In the lobster – as mentioned previously – defeat induces a state of
behavioral withdrawal, such that (1) the recently defeated animal will fight no one, including an
animal it has previously defeated, for some time after its loss (Kravitz, 2000), and is more likely to
lose its next fight, while the winner is more likely to win, even against a different animal, the next
time. Similar if not identical responses characterize the members of many animal species. Edwards
and Kravitz (1997) point out, interestingly, that the “loser” mentality of a defeated animal can be
reversed, for short periods of time, by injections of serotonin (which appear to specifically
increase their willingness to stand their ground and fight, rather than withdraw). Now it is also the
case that dominant lobsters, and those treated with serotonin, “stand tall,” looking confident and
proud, so to speak, as A1 ganglion 5HT cells enhance motor output from circuits that generate
postural flexion (the postural stance characteristic of dominant animals). Injections of the
serotonergic antagonist octopamine, with the opposite effect, produce animals that appear
subordinate – extensors flexed, flexors relaxed, ready to withdraw immediately from provocation
(is this relevant to the defeated stance and gaze aversion characteristic of depression, chronic
anxiety, and borderline personality disorder?).
Since lobsters appear to “size each other up” upon first encounter, with the smaller
lobster almost always admitting defeat, the enhanced postural tone characteristic of the winning
lobster clearly marks the crustacean who has made the most of his size. 5HT injections into
subordinate lobsters (and crayfish) produces animals who increase the duration and maximum
intensity of subsequent encounters and who, furthermore, will advance on former dominants
(Kravitz, 2000). Edwards and Kravitz (1997) describe this “aggression-enhancing” effect of 5HT
on lobsters as “directly opposite” to that seen in most vertebrates (where increased serotonergic
function appears to ameliorate aggression) but this appears to be a consequence of confusion about
aggression subtype: the dominant vertebrate is less likely to manifest defensive aggression, but is
certainly more confident and, if less willing to fight, definitely more willing to hold its ground, to
withstand challenge, not to retreat, and to moderate its own internal negative affect (which is far
more to the point). So it appears reasonable to suppose that the vertebrate characterized by reduced
5HT functioning is, like its crustacean counterpart, more susceptible to withdrawal (the major
behavioral substrate of negative affect) and less capable of holding its ground (manifested in a
general tendency towards negative affect and corticosteroid overproduction, ameliorated by SSRI
treatment), while the dominant vertebrate, like its crustacean counterpart, acts confidently, looks
its opponent straight in the eyes and forthrightly withstands challenge.
In complex creatures, like primates, the situation is more complex, of course: animals
with high serotonergic tone do not necessarily use aggression to maintain their social status.
Instead, they are more affiliative, and more socially skilled – and use the second-order
consequences of that affiliation and skill to bolster their social positions (de Waal, 2000). But it is
still easy to read this as associated with better regulation of negative affect, less defensive
reactivity, less chronic stress, lower glucocorticoid production, and more confidence (something
logically associated with the behavior of the confident lobster) – as well, perhaps, with unimpaired
maternal relationships, or at least close kin relationships, early in life. It is extremely interesting,
as an elaboration of this observation, to consider the findings that stress suppresses testosterone
production, across a wide variety of species (Sapolsky, 1990), given Abbott et al.’s (2003)
observation that withdrawal of kin or social support produces more stress in primates than the
objectively measurable level of unpredictable danger that characterizes a given environment
(correlations above -. 85 (!)). The resentful and sexual-predator fantasies that obsess the most
violent human beings can profitably be considered in such a light: isolation, lack of love, rejection
produces chemical castration, inducing resentful counter-fantasies, aimed at the presumed
eradication of the source of such subordination (female or otherwise).
It is very interesting to consider the broader-term implications of defensive rage states
versus predatory attack states with regards to the potential interaction between structural and
functional modulatory system abnormalities and psychopharmacological function. Defensive rage,
the likely substrate of negative-affect associated threat response, is also part of the classic
sequence of psychological and physiological transformations generally classified as the stress
response (Abbott et al., 2003). This response involves (1) the release of catecholamines
Peterson & Shane: Defensive and Predatory Aggression 13
(epinephrine and norepinephrine) from the sympathetic nervous system and adrenal medulla, (2)
release of glucocorticoids and (3) suppression of growth, digestion, reproduction, tissue repair,
and inflammatory and immunological function, logically related to long-term rather than short-
term survival (Abbott et al., 2003). The stress response stimulates hepatic glucose release and
visceral lipolysis, enhancing delivery of glucose, fatty acids and triglycerides to skeletal muscle
and brain. So it can easily be stated that a stressed animal (one stressed by, for example, lack of
kin, lack of social support, and/or unpredictably dangerous environmental circumstances) (Virgin
& Sapolsky, 1997) activates physiological systems concerned with the here and now, and
withdraws resources from systems concerned with longer spans of time. This means enhanced
fight or flight reactivity, beneficial for moment-to-moment combat or escape, but increased
susceptibility to corticosteroid-overproduction-induced chronic disease states: hypertension, type
II diabetes, gastrointestinal ulceration, anovulation, impotence (associated with chronic
testosterone suppression (Sapolsky, 1990)), osteoporesis, psychogenic dwarfism, etc. (Sapolsky,
This switch to short-term concern appears particularly relevant, with regards to the
association between prefrontal executive dysfunction and defensive aggression: the prefrontal
cortical circuits, orbitofrontal, medial and dorsolateral, are primarily concerned with long-term
issues. Although the classic causal inference has been “prefrontal decrement = impulsive,
aggressive behavior,” as a consequence of lack of inhibition (Seguin et al., 1995), it could easily
be that the chronic stress experienced by an isolated, low-status animal or human, in their
subordinate niche, shuts down higher-order circuitry concerned with long-term survival, because
of the increased irrelevance of that circuitry, in an environment rendered more unpredictable and
uncertain by objective circumstances (and, therefore, less amenable to successful long-term
planning). The elevated rates of future discounting characteristic of antisocial or drug-abusing
individuals provides evidence in support of such a causal argument (Petry, 2002).
Consideration of the important role serotonin plays in the modulation of violence helps
shed additional light on abnormal function of the prefrontal cortex. The prefrontal area is a major
target of serotonergic (5-HIAA) projections and is characterized by a high density of serotonin
type 2 receptors (Biver, Lotstra, Monclus et al., 1996). Abnormalities in serotonergic function in
humans – particularly those associated with decreased central serotonergic production – have been
consistently associated with aggressive, impulsive behavior. Reduced levels of 5-HIAA have, for
example, been found in aggressive psychiatric patients, suicide victims, conduct-disordered boys,
adult criminals characterized by recidivism, and impulsive violent offendors and fire setters (Roy
& Linnoila, 1988; Linnoila & Virkkunen, 1992; Coccaro, 1989; Maris, 2002; Kruesi, Rapoport,
Hamburger, Hibbs, Potter, Lenane & Brown, 1990; Rawlings & Linnoila, 1996; Virkkunen,
Eggert, Rawlings & Linnoila, 1996). Furthermore, aggression can be potentiated as a consequence
of dietary tryptophan depletion (Chamberlain, Ervin, Pihl & Young, 1987), and is characteristic of
a group of individuals typified by a polymorphism in the gene that codes for tryptophan
hydroxylase (TPH), the rate-limiting enzyme in serotonin biosynthesis. Furthermore, aggressive
individuals do not show the expected increased PFC/ACC glucose metabolism in response to
challenge with fenfluramine, a potent 5HT agonist (reviewed in Davidson et al., 2000).
We reviewed evidence earlier suggesting (1) that the amygdala is associated with
responsiveness to the facial displays of negative affect on the part of others, and (2) that the
circuitry used by normal individuals to understand the distress of others is damaged in the case of
the psychopath. It may be profitable at this point to return to description of the potential analogs
between dominant animals and humans, characterized by high serotonin levels, with regards
specifically to the psychopath: both share erect, dominant posture, forthright and unwavering gaze,
well-inhibited negative affect and, arguably, charm. It is not unreasonable to suppose that the high
tonic 5-HT levels characteristic of the dominant primate do dampen the baseline levels of activity
as well as the stimulus-related reactivity of the negative affect systems, since this is precisely the
mechanism of action desired for the “antidepressant” serotonergic reuptake inhibitors, which have
a broader effect than merely the antidepressive. However, the genuinely (deservedly?) dominant
individual, social animal or human, also seems genuinely prosocial, as noted previously. Even
among higher-order primates, such as chimps, the road to long-lasting social dominance appears
Peterson & Shane: Defensive and Predatory Aggression 14
most reliable if it is associated with such behaviors as positive attention to non-estrous females, as
well as to infants, and with mutual grooming and giving of aid. This is where genuinely dominant
individuals and psychopaths, incapable of seeing beyond the boundaries of their own being,
appear to truly diverge. Very little that psychopaths do is for the benefit of others. Something
important seems to be lacking from their psychological structure. Not only do they not fear, but
they can not really love.
It is very useful in this regard to note the implications of recent work completed by
Fowles & Kochanska (2000), who demonstrated two divergent paths to socialization of children:
high N (or high negative-affect) children were amenable to regulation of their behavior through
threat, as well as through modulation of affiliation. Low N children, by contrast – emotionally
stable, from one perspective, callous from another – were only amenable to socialization through
affiliation. So the presence of three fundamental regulatory systems, modulating predatory
aggression (including incentive reward seeking and sexual behavior) might be hypothesized: one
cognitive, associated with executive function, one negative affect related, associated with the
paralimbic areas (as well, at higher levels, with the septal-hippocampal system), and one
associated with affiliation (and underlying, at least in part, Kochanska’s “committed compliance”
– her well-elaborated affiliative alternative to fear-based socialization) (Kochanska, 2002;
Kochanska, Tjebkes & Forman, 1998).
Normal individuals modulate their reward-seeking behavior, with regards to other people,
by consideration of the potentially negative impact such behavior might have on those people (as
well as on any of their own plans or their current status). Psychopaths, by contrast, do not seem as
affected by the possibility of negative outcomes (a form of inattention or blindness or neglect that
even seems to extend to themselves). Normal individuals also like other people – that is, they feel
an affect-based affiliative bond, which is probably associated with trait Agreeableness and opiate
functioning, and which can be seriously disturbed, at least in principle, by early neglect and/or
abuse (as reviewed previously). Now low fear in and of itself is not enough to produce Factor 1
psychopathy traits, and is probably not characteristic of Factor 2 at all, because low N individuals
are, arguably, healthier than those characterized by high N, who are prone to depresssion, anxiety,
phobia and cortisol hyper-production. However, the negative affect systems do constitute one part
of the higher-order modulation of incentive reward seeking and dominance striving, and
abnormalities or even extreme but still normative reduced functioning of such systems might
constitute one necessary but not sufficient condition for the emergence of psychopathy. So it is
probably not reasonable to identify psychopathy with one, or even two abnormalities, personality,
cognitive or otherwise, but with a constellation of features: high extraversion, low neuroticism,
low agreeableness, low conscientiousness, for Factor 1; cognitive dysfunction, particularly in
executive control systems, high neuroticism, low agreeableness, and low conscientiousness, for
Basic motor schemas are organized at the level of spinal cord, upwards, towards the
hypothalamus and PAG (Swanson, 2000; Gregg & Siegel, 2001). The world of experience
essentially emerges at somewhere near this uppermost pinnacle of the strictly motor system,
concerned with the establishment of fundamentally motivated goals, including those constituting
ingestive and defensive behavior (as well as sleep/wake cycling, temperature regulation, etc. ).
Furthermore, and equally importantly, the hypothalamus and PAG regulates sexual behavior,
predatory aggression and exploration. These are very basic motivational states, understood by
everyone (maybe even the psychopath): “I am hungry, thirsty, hot, cold, tired, in need of sexual
release, bored, angry, curious, dangerous, cruel. ” There are things that need to be done. Doing
those things is valuable, and if they are interfered with, there must be repercussions. Those
repercussions can involve aggression (to fend off an attack or eradicate a competitor) or
exploration (to gather more information and determine what has gone wrong or what else might be
The hypothalamus and PAG are regulated by other, increasingly complex circuits,
including those governing affiliative regulation of lower level stress-response (likely opiate-
Peterson & Shane: Defensive and Predatory Aggression 15
mediated). The addition of the amygdala allows threat and uncertainty to potentiate and modulate
its activity. The septal hippocampal system, by contrast, allows context to play its role (so that the
accidental aggressive act on the part of a juvenile, for example, does not elicit a full-blown
defensive response). The orbitofrontal and medial cortex allows stimuli that once had one valence
to obtain another, adding another level of motivational flexibility, and bring a sense of extended
time and place into modulatory service. The anterior cingulate gyrus allows more high-resolution
analysis of aggression displays on the part of others, among its other fnctions. Finally, the
dorsolateral prefrontal cortex allows abstract thought to alter the interpretive framework from
within which judgements of motivational relevance are made (Peterson, 1999). These circuits
mediate gradations of negative affect (so, defensive rage/pain/anger is modulated by and further
differentiated into fear, shame, guilt, anxiety, etc.), further discrimination of context (septal-
hippocampal) and the emergence of higher order cognitive control and long-term planning – more
awareness of time, more differentiation of cues of consummatory reward, more sophisticated and
complex behavior, still goal-directed but subtle.
As maturation occurs, these higher order regulatory circuits allow for ever more
discriminated modulation of basic motivated behavior. These are the more complex motivational
states, understood by most individuals – but not by all: “I am lonesome, I am afraid or anxious, I
am guilty and regretful, I am ashamed and disgusted, I care for someone, I trust someone, I
understand your perspective, I have plans for the future, I will voluntarily cooperate.” These more
complex motivational states do not eliminate those that are more basic, lower-resolution, but they
modulate them, and add shades of color to a world that would otherwise be shades of gray, if not
absolutely black and white. An animal, operating at the level of the hypothalamus, views the
world in a very simplified but still functional manner. Bereft of subcortex and cortex, it still
perceives and orients itself towards the world actively, but in a very low resolution, manner. This
means that such an animal inhabits a world where groups of stimuli that more complex animals
would see as separable – and behave towards as separable – appear instead as unified (so there is
no capacity to distinguish, for example, between an aggressive facial display from a conspecific
made in play, rather than in earnest – a disability that clearly characterizes aggressive children.
The unconstrained hypothalamic circuitry lumps, by default, whole classes of potentially
objectively differentiable environmental events together in basic, motivationally-relevant,
pragmatic categories: things to eat, things to drink, things to prey on, things to mate with, things to
explore and advance towards (Swanson, 2000) – and, perhaps, even fails to discriminate that
much, so that things to prey on, mate with, and explore all remain in one category.
If maturation fails, and sophisticated modulation does not develop, then two forms of
pathology appear to emerge. In the first case, hypothetically, a child enters the world with a very
high threshold for negative affect activation (a form of immunity that could under beneficial
circumstances produce a very emotionally stable and hardy individual). Imagine, however, that his
environment is also harsh, from the affiliative viewpoint – characterized by rejection and
emotional abandonment, even hatred. Affiliation is not possible, or is even counterproductive.
Instead of death, or a chronic defensive rage response – the reaction of a hurt and isolated animal –
imagine the low neuroticism child shuts down his affiliation systems, perhaps as a consequence of
endogeneous opiate over-production – or fails to devote resources to their development. No fear
and no attachment: all that is left is a low-level, hypothalamically-mediated predatory stance – a
few basic motivational states (dominate, explore, copulate) – given additional fluency and
flexibility by whatever cognitive systems still work well, in the absence of motivation for long-
term planning and the avoidance of failure. Under such conditions, only the incentive reward
affective valence of the environment remains, motivating chronic, undifferentiated predatory,
sexual, exploratory, dominance-system activation, associated with a context-independent
orientation towards territorial expansion. In such a manner, a damaged or rejected child adapts to a
niche, a harsh niche, characterized by absolute isolation. He or she embodies a very low-
resolution, black and white, world view, although one different from that inhabited by the more
purely defensive aggressor: In the case of the predator, everything and everyone is a target for the
derivation of incentive reward, with no context-dependent modulation. Could this not be viewed
as one form of adaptation to an extremely specialized niche? Female marmosets, sufficiently
stressed, will shut down their reproductive systems entirely (a major Darwinian sacrifice) to pull
Peterson & Shane: Defensive and Predatory Aggression 16
themselves out of social situations where they cannot win. This removes them from status-oriented
competition, and radically decreases their steroid production. A stressed animal will sacrifice
whatever it has to, in order to live. It will chew off its leg, if trapped. It will desperately attack a
large predator, if cornered. Might a child not be willing to alter its neurodevelopmental pathway, if
that was the only way out? And – so this theory does not become too dependent on environmental
pathology for its explanatory and heuristic utility – could not something analogous to this not also
be the end point of diverse forms of neurodevelopmental failure, among humans – even many not
directly associated with environmental deprivation?
In the second case, a child enters the world with normal or high levels of capacity for
negative affect. He or she is then further stressed, rendered subordinate, subjected to arbitrary and
unpredictable environmental events, left bereft of kin and social support. The low resolution
viewpoint of the hypothalamus and PAG not only suffices, under such conditions, but actually
provides an accurate representation of the world and life: nasty, brutish and short. Social
relationships are unreliable, if present at all, and there are many things to be genuinely afraid of.
There is not much point in long-term planning, either, because the environmental niche inhabited
has neither the stability nor the opportunity to make diverting resources from the concerns of
short-term survival to such longer term calculating worthwhile. An individual in such
circumstances is continually concerned not with territorial expansion, but with its preservation,
and is charcterized by chronic disinhibition of the defensive rage system. An individual
characterized by such activation is low status, in a steep dominance hierarchy (following Wilson
& Daly, 1977 and Abbott et al., 2003), and so lives in an environment that is both unstable, and
characterized by reduced opportunity for kin and social support. He or she is dominated by
negative affect (anxiety, fear, anger, most primarily) and is irritable, as the activation of these
higher order negative affect systems disinhibits defensive rage. Furthermore, this individual is
characterized by reduced executive function, potentially exacerbated by environmental deprivation
and abuse, but also equally likely a secondary consequence of chronic, demanding,
hypothalamically-mediated defensive posturing – part of the stress response, which diverts
metabolic resources away from systems associated with long-term survival. He follows a chronic
pattern of slavish adherence to more phylogenetically primitive perspectives: for the individual
unable to differentiate the environment in a sophisticated manner, anything poorly understood or
poorly mastered causes pain and fear, justifying aggression.
The long-term consequences of defensive rage disinhibition, associated with uncertainty,
low status, and withdrawal or absence of social support, compromise neurological functioning,
specifically, as the hippocampus suffers cortisol damage, and health, more generally, as
immunological functioning is impaired and necessary long-term repair processes shut down. Such
a person is not very social – not, perhaps, because he can’t be, but because he lacks the resources,
both objective and intrapsych, for situational and perhaps, over the long term, for constitutional
reasons. Such an analysis suggests that the individual characterized by chronic defensive
aggression is in fact depressed, trapped in a subordinate niche, unable to rise or adapt, and might
benefit most from increased access to genuine opportunities for the achievement of status
(following Wilson & Daly, 1997, who demonstrated that murder is powerfully precipitated by
social inequality), or from treatment with theoretically “antidepressive” but actually much more
broadly operative SSRIs. This does not mean, however, that every aggressive individual is
reacting to perceived threat, or would respond positively to treatment with serotonergic agonists or
reuptake inhibitors. The predatory aggressor – hyperconfident and calm – appears to be a different
kind of animal altogether.
Peterson & Shane: Defensive and Predatory Aggression 17
TYPE FEATURES SUBTYPE
Inhibited negative affect (decreased anger, except when status
challenged, fear, pain, grief, guilt), low trait neuroticism, high
extraversion, low agreeableness, increased positive affect,
increased approach behavior, high but sometimes brittle and
inappropriate self-esteem, bully, glibness, superficial charm,
predatory stare (psychopathic eye contact), decreased sympathetic
activation, narcissism, invulnerability to threat, normal prefrontal
function, high 5HT
Negative emotion (anger, fear, pain, grief, guilt), high trait
neuroticism, inhibited extraversion, inhibited agreeableness,
decreased positive affect, decreased approach behavior, low self-
esteem, bully victim, sympathetic activation, corticosteroid
production, prefrontal inhibition, future discounting,
catecholamine production, suppression of growth, digestion,
reproduction (including testosterone production), tissue repair, and
inflammatory and immunological function; stimulation of hepatic
glucose release and visceral lipolysis, social isolation, lack of kin
support, unpredictable danger in environment; chronic disease
(hypertension, type II diabetes, gastrointestinal ulceration,
anovulation, impotence (associated with chronic testosterone
suppression), low 5HT, dysregulated opiate function
Peterson & Shane: Defensive and Predatory Aggression 18
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