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?

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