THE HFG REVIEW OF RESEARCH (Vol. 3, No. 1, Spring 1999)
THE BIOLOGY OF AGGRESSION
Continuity vs. (Political) Correctness: Animal Models and Human Aggression
D. Caroline Blanchard, Mark Hebert, and Robert J. Blanchard
One of the sadly enduring themes in aggression research is the inability of scientists researching humans and animals to make meaningful impressions on each other's understanding of the behaviors on which they focus. This problem is not restricted to aggression research, but it appears to be more strongly expressed in this area than in the study of most other behavioral phenomena. Researchers on animal or human aggression have tended to remain separate from and disdainful of each other, even while attending and participating in the meetings of a single dedicated society, the International Society for Research on Aggression. Needless to say, this situation has impeded the development of any sort of unified view of aggression that applies to both humans and nonhuman animals. It has thus been one of several major problems that have hindered progress in aggression research during a period in which many other somewhat similar areas, for example the neurobiology of defense and its relationship to human emotionality, have made considerable advances.
Aggression research does have some distinctive problems in bridging the gap between animal and human behavior. One is that the concept of aggression as a phenomenon with substantial biological underpinnings is widely perceived as running counter to many worthy social and political views. The assumption underlying the distancing of many of the proponents of such views from too close contact with animal aggression research is correct: Animal work often suggests that aggressive behavior has extensive roots in biology, and it is quite likely that such research may raise questions about the modulation of human aggression that are difficult to answer from the perspective that there is no direct biological influence on this behavior.
These antibiological views encompass a range of concepts, from the position that war, specifically, has no biological underpinnings, through denial of any direct involvement of biology in the substantial interindividual (socioeconomic, ethnic, subcultural, gender, age) variation in violence and violent crime. But since individual differences in human as well as animal aggression are undeniable on a phenomenological level, and since these so often seem to be associated with factors that clearly relate to biology, such as gender and age, the possibility of interactions between biology and experience may be admitted. Gender differences may be interpreted as reflecting greater opportunity for aggression or greater reinforcement of aggressive behaviors for boys/men as opposed to girls/women. Eiither source, in turn, may derive from cultural norms (a relatively "biology-free" explanation) or from differences in gender-typical group composition or social activities (an account that suggests, but does not dwell on, a biological origin for these differences). Similarly, while accounts focusing on self-esteem and social skills as modulators of aggression may acknowledge that these reflect an interaction of personality factors such as behavioral inhibition with experience, the existence of biological differences directly relating to aggression is often denied, minimized, or ignored.
A second problem in acknowledging a relationship between human and animal aggression involves the cognitive distance between humans and other animals. There appears to be a relatively well-developed consensus that no known nonhuman species has cognitive or linguistic capabilities that are close to those of humans. The difference is highly relevant to the study of aggression because many instances of human aggression are clearly accompanied by complex cognitions, or expressed in terms of mechanisms that rely on cognitive and technological achievements, which may have no direct parallels in nonhuman species' behavior. Also, the use of technology can ensure that a human act of aggression causes an immense amount of damage impossible for animals to achieve. Both factors may be involved in some discrepancies between human and animal findings. For example, in nonhuman mammals, alcohol sometimes increases aggression at low to moderate dose levels but almost always reduces it at higher doses (see Berry and Smoothy, 1986 for review). The human literature on alcohol and aggression provides little evidence of such a nonparallel relationship. A strongly alcohol-impaired human can inflict damage both verbally and with a weapon, whereas an equally impaired animal cannot, no matter how high its motivation to attack may be.
Perhaps the most important consequence of the cognitive and technological gap between man and nonhuman animals relates to war. War is defined both in terms of aggressive action and in terms of the social organization and tactical capabilities of the opponent groups involved in this action. If these organizational and tactical capabilities are inadequate, then the aggressive behavior does not represent "war," no matter how focused or how damaging, or how clearly it involves groups rather than individuals. Such a conceptualization enables a statement that war is a uniquely human phenomenon, with no direct parallels in nonhuman animals, but it does so only on the basis of cognitive/technological differences.
Regardless of what intellectual or emotional comforts may ensue from treating animal and human aggression as fundamentally different and unrelated phenomena, unless they genuinely have no significant connections, the tactic is scientifically counterproductive. It removes the possibility of comparative analysis and cross-fertilization of hypotheses between the two realms. It deprives human-focused researchers of an extensive literature using experimental methodologies to investigate aggression phenomena, while those animal researchers who ignore the human literature inevitably fail to learn of findings that could open up new and fruitful areas of investigation using nonhuman animals. Is there any way that this segregation can be bridged and common ground found for serious consideration of phenomena with which both human and animal researchers are deeply involved?
Varieties of Animal Aggression
While the term "aggression" can often be used without misunderstanding in ordinary conversation, it has proved to have so many different meanings, and to be so difficult to conceptualize, that Benjamin (1985) selected it (along with "intelligence" and "self-esteem") as a particularly egregious example with which to demonstrate the difficulties of concept analysis. Analyses from the animal literature suggest one important reason that a single, and universally accepted, scientific definition of aggression has been difficult: Several different phenomena are encompassed by the term.
The distinction between "offensive" and "defensive" forms of adult aggression has received a good deal of systematic attention (e.g. Blanchard and Blanchard, 1977; Blanchard et al., 1984), with both of these typically being differentiated from play fighting (Pellis, 1988) and from predation. Offensive aggression occurs in the context of a resource (including territory) or dominance dispute and its successful outcome is the termination of the dispute through "victory," manifested as flight or defeat-related behaviors by the opponent. Dominants (consistent victors) gain resources, including access to females (a particularly important "resource" from an evolutionary perspective) and food (Flannelly and Lore, 1977; Blanchard et al., 1984). Offensive attack involves a set of species-typical behaviors that enable the aggressive animal to contact particular body sites on the opponent where bites or blows are delivered. Offensive attack can thus be differentiated from other forms of aggression on the basis of the specific target sites for attack, as well as the behaviors by which these sites are reached.
Defensive attack is seen only when the subject is defending its own body, not when it is attacking another animal to "defend" a disputed resource. It includes a salient threat component not seen in offensive attack, with loud vocalizations and display of weapons such as teeth or claws. The bites or blows delivered tend to be made on different body sites on the opponent than those contacted in offensive aggression. The successful outcome of defensive aggression is discouragement of the body-threatening conspecific or predator and discontinuation of its attack. This can occur prior to the defensive attack, as the result of defensive threat, or following the delivery of a bite or blow, particularly to the sensitive eye/snout sites that are the targets of defensive attack. While relatively little field research has been done on the effectiveness of defensive threat and attack, the strong inhibitory effect of fear on predation suggests that defensive threat may serve as a considerable deterrent (Pellis et al., 1988).
Play fighting is common among the young of many mammal species, dropping off in frequency after sexual maturity is attained (Pellis and Pellis, 1991b). The behaviors involved in play fighting have considerable structural similarity to those of adult attack and defense, in that a species-typical attack pattern is used to approach and make contact with a specific site on the body of the opponent, while the defender utilizes species-typical behaviors to make that body site unavailable to the attacker. In addition, across-species studies suggest that, as in adult fighting, attack and defense in play fighting are motivationally distinct behavior patterns (Pellis and Pellis, 1991a). However, the transition from juvenile play fights to adult fighting does not appear to involve a continuity in individual levels of attack tendency from one to the other: Males that show the highest attack rates during play fighting tend to become subordinates rather than dominants (Pellis and Pellis, 1992a; Smith et al., 1996). Also, play fighting, at least for males, may be more linked to adult sexual behavior than to adult fighting, as the play fighting attack target in a spar between males may correspond to an important contact target on the female, utilized in adult male sexual behaviora view that is supported by the finding that deprivation of play fighting has more of a deleterious effect on sexual behavior than on fighting skill (Pellis et al., 1992). In addition, play fighting defenses in juvenile females may involve some of the responses that later become useful in fending off the sexual advances of males.
Phenomena related to predation are also often subsumed under the rubric of aggression. In the laboratory, many rats kill and eat mice, a behavior that, due in part to the similarities of rats and mice, suggests conspecific aggression. In fact, predatory (as opposed to conspecific) attack can occur in species even more closely related and more similar than rats and mice: Grasshopper mice kill and eat laboratory mice. However, both the target sites for attack and the behaviors typical of the attack pattern are different from conspecific attacks by grasshopper mice (Pellis and Pellis, 1992b). These findings indicate that predatory attack can and should be differentiated from conspecific attack, even when the combatants involved are closely related animals. In addition to differences in stimuli, response patterns, target sites for contact, and outcome of these various behavior patterns, recent work on the anatomic and neurochemical systems associated with some of these strongly suggests that the physiology of these systems is also different (e.g. Bandler and Shipley, 1994). Most of the work on the neurobiology of "aggressive" behaviors has actually involved defensive threat and attack, with a relatively substantial literature also on "quiet biting attack," which likely corresponds to predation. The pharmacology of offensive and defensive aggression appear to be different, with the former (Olivier, et al., 1991) but not the latter (Blanchard et al., 1985) responding dramatically to a class of "serenics" with effects at various serotonin receptor subtypes. Motivational variables also produce different effects on these behaviors, with fear reducing offensive attack (Blanchard et al., 1988) and predation (Pellis et al., 1988) but not altering defensive attack (Blanchard et al., 1980).
Varieties of Human Aggression
If the function of offensive aggression in animals is to obtain and hold some sequestrable resource, how does this relate to more complicated human emotions and impulses? We have argued (Blanchard and Blanchard 1984) that "anger" is the emotional response to a challenge to some resource for which the angry individual has a claim. For example, data on violence resulting from "love triangles," (Wilson and Daly, 1992) suggest that these reflect attempts to discourage challenges to the perpetrator's relationship to and control over the love-object, regardless of whether the latter, or a third-party challenger, serves as victim. Similarly, Katz's (1988) descriptions of the subjective rewards of criminal and violent behavior suggest that the "resources" gained by violence often involve status and access to particularly valued women and other status symbols rather than money or goods. For many habitually violent individuals, a challenge successfully overcome is associated with strongly positive emotions; the more able the challenger, the sweeter the victory. This analysis is couched in terms that may seem inappropriate when applied to animals, but it might be considered that for violent offenders and, for example, male rats responding to an intruding challenger, the stimuli and situations eliciting aggression, and the consequences of successful and unsuccessful aggression, are very similar.
These considerations also suggest why a "challenge" is so frequent and potent a stimulus to elicit human aggression. As Daly and Wilson (1988) have pointed out, in populations of young men (who notoriously account for a disproportionate share of crimes of violence, e.g. Blanchard and Blanchard, 1983; Campbell, 1995), failures to respond to challenge jeopardize the status of the individual in the group and limit his ability to command important resources. A simple paradigm"challenge elicits aggressive response"emerges as a common feature of mammals, particularly young postpubertal male mammals. The specific challenge involved is typically either to the status of the individual within the group, to his access to females or other important resources, or to both. Higher primates complicate this paradigm in that other group members may get involved, either as seconds (Pereira, 1989; Silk, 1992) or in an attempt to control and defuse the situation (Reinhardt et al., 1986). Humans have contributed the factor that challenges may be purely verbal (and often quite inventive). They have also created a very encompassing form of sequestrable resourcemoney. The paradigm, however, remains, and it is a mammalian pattern, not one found only in dysfunctional human social groups.
If offensive aggression has some close animal-human parallels, what about defensive attack? Although defensive attack is easy to define, it is hard to observe in people, for both ethical and practical reasons. Polarized hypothetical scenarios, however, elicit consistent differences for defensive vs. offensive attack situations. Fukunaga-Stinson (reported in Blanchard and Blanchard, 1983) asked male and female students to respond to scenarios involving either a physical threat (attack by a stranger in an isolated spot) or a resource dispute by indicating the likelihood of specific actions or feelings. With reference to emotional response, fear dominated in the former, and anger in the latter. Physical and physiological responses to the two situations also differed, with "freezing" and becoming "stiff" or showing "nervous breath" describing the fear situation, while becoming "hot" or "burning" was associated with the resource dispute situation, as were "clinching fist," "staring at," and "adrenalin surge."The first choice action for the fear situation was to leave as soon as possible, followed by (for women) looking around for something to hit the attacker with, and "hit to harm" among the first five choices for both sexes. Neither of these hitting-related choices was among those selected as likely in the resource dispute situation, although a strong desire to attack the challenger was often cited. What is important is not just that these two scenarios elicited a variety of strongly differentiated physiological responses and subjective feelings, but that they were both associated with a perceived tendency to either attack, or to want to attack, the opponent.
Play fighting obviously occurs in children as well as in the young of most other mammal species. Prepubertal boys (Boulton, 1993; Honig et al., 1992; Maccoby and Jacklin, 1980), like prepubertal rats (Pellis and Pellis, 1990), participate more often in fights than do comparable females. Also as in other mammals, play fighting and serious fighting in children are different, and this difference reflects the actions involved as well as factors such as facial expression and the apparent intent of the participants (Boulton, 1991a). However, the literature does contain suggestions of potential differences between the play fighting of prepubertal children and that of rats. In rats, the animal that proves to be subordinate in adulthood initiates most of the play attacks (vide Pellis and Pellis, 1992a; Smith et al., 1996), whereas play fighting in middle school children tends to involve partners that like each other and are closely matched in strength, with both weaker and stronger children initiating bouts (Boulton, 1991a,b). Another difference is that serious fighting is common enough in prepubertal children to be of concern to parents (Boulton, 1996), whereas serious fighting is seldom observed in prepubertal rats (Smith et al., 1996). Some component of this difference may reflect specific learning: In a study of middle school English children, many of the behaviors (e.g., karate chop, back kick, scissor kick) seen in play fighting but not in serious fighting represent actions that are likely to have been learned through observation/imitation, perhaps of television programs. The latter two did not occur in play fighting among Zapotec children, who did, however, show some distinctive behaviors of their own, such as burro kick and knuckle rap (Fry, 1987), suggesting that the form of playfighting in 8-12 year olds already may have been greatly altered by culture-differentiated practices.
The view that human predation has a biological link to that of closely related mammals is supported by findings that a variety of primates predate other vertebrates, including mammals (e.g. Anderson, 1986; Kudo and Mitani, 1985). In addition, primates appear to be among the few mammals that also seek out and kill animals of species that predate them, or that serve as major competitors for prey (Hiraiwa Hasegawa et al., 1986), suggesting that primate hunting is by no means limited to animals that are to be consumed. Thus the hunting of large and dangerous animals not meant for food, a feature of virtually every society that has lived in proximity to such animals, also has a clear parallel in nonhuman mammalian behavior.
Animal and Human Aggressions: Parallel Neurobehavioral Systems?
We are suggesting that, rather than try to establish a basic parallel between animal and human "aggression," it might be advantageous to look at the concept of aggression as consisting of a number of different neurobehavioral systems, at least some of which show considerable evidence of continuity between nonhuman mammals and people.
This is not to say that everything included under the human aggression rubric will have a direct counterpart in other mammals. Obviously, the "aggressive" investor or lawyer or businessman has no direct correspondent in infrahuman mammals, but this may be because the nouns are inappropriate, not the adjective (if the noun were "politician" the phrase might have an enhanced correspondence). All of these designations reflect a common theme, of actions and attitudes that seek to expand claims to resources, rights, or influence, in a variety of relevant arenas. When particular aggression paradigms are individually examined there may be either parallels that might be overlooked when an undifferentiated "aggression" concept is employed or transformations in the organization of that specific aggression paradigm as larger-brained mammals with more complex social organizations and technical capabilities are examined. This offers the possibility of real breakthroughs in relating specific human aggression phenomena to their nonhuman mammalian parallels.
What is likely to continue to be a stumbling block is the one that has been there all along, that "aggression" is one of the most value-laden terms in any language, and probably will continue to be. Thus, while aggression can be a "good" thing in the context of pursuit of a valued goal, even here it carries the baggage of an implication of activity encroaching on the rights of others; the tendency is for the protagonist to label precisely the same actions as "defense" of the desired goal, making the other guy the aggressor. As the old saw goes, virtually every country in the world has a ministry, bureau, or department of "Defense" while not one has a bureau of "Aggression."
The problem, in short, is that many aggression researchers are afraid of the term and of the concepts that it represents. In particular, students of human aggression are afraid of some of the implications of the view that human aggression is essentially similar to aggression in other mammals. Among these implications is that some instances of aggression, and the situations and stimuli that elicit them, are embedded in normal circumstances of life for people as well as for most nonhuman mammals. In this sense, some aggressive behaviors may be legitimate and normal. Another such implication is that instances of or tendencies toward aggression may be evolutionarily adaptive for the individual. If this is true, then these individuals, aggressive under particular circumstances, may leave more descendants, and these descendants would be expected to express, to varying degrees, that tendency. From the perspective of animal research, these implications seem to be undeniable, although the various mechanisms involved are in need of a great deal more analysis, as is the equally undeniable relationship of aggression to experience and to the interaction of experience with a host of genetic, gender, hormonal, and neurological factors. But, although some aggressive tendencies, in some situations, may be normal and adaptive, an emphasis on different types of aggression and on their relationship to specific eliciting circumstances makes the point that violent acts not in agreement with these guidelines may be neither normal nor adaptive. Clearly maladaptive aggressive behaviors, such as those of the occasional male rat that kills females and even related young, do sometimes appear in nonhuman mammals as well, to the detriment of both the individual and its companions.
Set against some of the past and contemporary horrors of human history, acceptance of the view that aggression is basically an adaptive behavior pattern involving a variety of complex biological systems interacting with experience, even if correct, may be interpreted as political naivete. We take this point. In opposition, however, is another point. The truth may not set you free, but it does make you somewhat better equipped to cope with reality. Acceptance of important continuities in stimulus, organism, response, and outcome components of particular aggressive behaviors between nonhuman mammals and people promotes a deeper and more comprehensive understanding of human aggression. The goal of science is to understand, predict, and control phenomena. In order to improve the latter two of these, the first is required.
Adapted from D. Caroline Blanchard, Mark Hebert, and Robert J. Blanchard. In press. Continuity versus (political) correctness: animal models and human aggression. In Marc Haug and Richard E. Whalen (Eds.), Animal Models of Human Emotion and Cognition, Washington, D.C.: American Psychological Association. Adapted with permission.
Preparation of this article was supported in part by NSFIBN95-11349.
D. Caroline Blanchard is Research Professor at the Pacific Biomedical Research Center and Professor of Genetics and Molecular Biology at the John A. Burns School of Medicine of the University of Hawaii. Mark Hebert is Assistant Professor of Psychology at the University of Hawaii. Robert J. Blanchard is Professor of Psychology and Neurosciences, University of Hawaii. With HFG support, D. Caroline Blanchard and Robert Blanchard are writing a book on the biology of aggression.
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