The ventromedial hypothalamus and aggressive behaviour in rats

Male rats were given bilateral lesions in either the anterior or posterior ventromedial hypothalamus (VMH). The intermale aggressive behaviour of these animals within their own territory was observed before and after the surgical procedure and compared with the behaviour of sham-operated animals. The effects of anterior VMH lesions include an increased tendency to respond with frontal threatening upon approach of a conspecific male. This behaviour closely resembles the aggressive responses described in “shock-induced aggression” tests. Posterior VMH lesions facilitate territorial aggressive behaviour characterized by approaching the opponent followed by lateral threatening and fighting. It is suggested that 2 distinct neural substrates exist, which serve to inhibit defensive (anterior-VMH) and offensive (posterior-VMH) intermale aggression, respectively.     

Agonistic behavior between wild and genetically sterile male Norway rats (rattus norvegicus)

Genetically sterile male Norway rats, Rattus norvegicus, were tested in the laboratory to determine both 1) behavioral characteristics, and 2) the ability of sterile males to compete aggressively and sexually with wild Norway rat males. Sterile males were larger in weight, more frequently dominant, won as many fighting encounters, were as aggressive as wild males, and mounted females more frequently. Behavioral activities were similar for both strains when compared under laboratory conditions with no apparent abnormal behavior exhibited by the sterile males. Use of sterile males in biological control programs is discussed.     

Evolution of Destructive Aggression

Aggression is defined as a mechanism of spacing by means of force or displays. It has evolved independently in different animal groups. The mechanisms underlying it are therefore not homologous throughout the animal kingdom. The phenomenon of aggression is so widespread, however, that strong selection pressures must be responsible for its development along analogous lines. Its most obvious functions are in competition for mates, natural resources, and territories, and in the preservation of group identity in many gregarious species. Aggression is often ritualized so that no damage is done to conspecifics. This ritualization may appear as modification of fighting into a tournament, or as the development of submissive postures which block further aggression in the opponent shortly after the onset of a potentially damaging fight. Animal aggression is preprogrammed by phylogenetic adaptation in well-defined ways, but can be modified by experience. The inborn programs involve motor patterns, innate releasing mechanisms, releasers, motivating mechanisms, and learning dispositions specific for the species. Aggression on this biological level can be observed in humans as intragroup aggression. Certain motor patterns and signals which lead to the release of aggression are universal. Some can even be found in deaf- and blind-born people, proving their innateness. A number of patterns of aggression in man are highly ritualized and - in a way analogous to that found in many animals - mechanisms of control have evolved inhibiting the killing of a conspecific. There are strong indications of the existence of motivating mechanisms within the brain, e.g., in the form of neuronal circuits, that show a degree of spontaneity. The type of destructive aggression which we call war, is a product of cultural evolution. War takes advantage of the given motivational structure of man, including his fear of strangers, which develops in every baby independently of experience and makes men inclined to form closed groups and causes them to be wary of or hostile to strangers. Based on these tendencies, man underwent a process of cultural subspeciation. Groups demarcated themselves from others by custom, erecting communication barriers. The development of languages demonstrates how fast and efficient this process is. Members of the same group, during this process, were defined as the “real man,” outsiders often were to be valued less -or even considered nonhuman. On the basis of this self-indoctrination, cultural codes of conduct developed, which allowed members of other groups to be killed when groups competed for resources. A cultural fiiter of norms was established which demanded killing under defined conditions, and was superimposed upon the biological filter of norms which inhibits the killing of a human being. This results in a conflict of norms, which is universally felt as guilt, since the biological filter of norms, though superimposed, is nonetheless working, particularly in the circumstance of a personal encounter. The more advanced the technique of armament, which allows fast and distant killing, the less the inhibitions are activated. Nonetheless, ritualizations occur on the cultural level. Warfare is sometimes ritualized and conventions are developed to prevent escalation into massacres, or the wholesale destruction of the subjugated enemy. To a great extent, this is certainly a result of our inborn moral code, If nothing like this were given to man our situation would be disastrous indeed. Whether cultural evolution will, in the future, be guided by moral maxims in accord with our human nature is a deeision men must make rationally. Although a ruthless ethnocentrism may bring advantage to a warring group, this may eventually prove fatal to mankind as a whole. In the escalating competition mankind runs the danger not only of exhausting its resources, but of destroying itself with its new weapons. If the outcome were not selfdestruction but domination by one group it would impoverish the diversity of human cultures, and thus seriously cut down man's spectrum of adaptability. War fulfills certain functions, similar to those found in animals. It is mainly a mechanism for preserving and extending one's territory, and a means of getting access to scarce resources. It is therefore dangerous to consider war merely as a pathological form of human behavior because this may distract our attention from the fact that, h order to overcome war, the functions of war have to be fulfilled by nonviolent means. Cultural evolution phenocopies biological evolution, due to similarities in the selection pressures shaping its course. This allows us to define the point of the evolutionary spiral we are at currently and to predict our future course.     

Intruders of differing reproductive status alter aggression differentially in early and late pregnant mice

Independent groups of early and late pregnant mice, housed individually, were observed with an intruder in their home cage. The intruders were either males, virgin, early, or late pregnant females. Male intruders were sniffed less, but more frequently and more severely attacked, than any type of female intruder, both by early and by late pregnant residents. While early pregnant females behaved similarly with the three types of female intruders, late pregnant animals treated them differently: they showed practically no aggression to late pregnant intruders, more to early pregnant ones and were most aggressive towards virgin female intruders. The possible relation of these findings to reproductive behaviour is discussed.     

The medial amygdala: Serotonergic inhibition of shock-lnduced aggression and pain sensitivity in rats

Two aspects of the amygdaloid complex (corticomedial and basolateral) were examined with reference to serotonergic inhibition of shock-induced aggression. Fighting was significantly depressed by serotonergic stimulation (5-HT, 10 μg bilateral) in the corticomedial amygdala while serotonergic blockade (methysergide, 5 μg bilateral) in this region increased levels of fighting. No consistent effects were obtained with serotonergic manipulation of the basolateral amygdala. Further investigation revealed that the state of serotonergic activity in medial amygdaloid sites was associated with concomitant alterations in the animals' sensitivity to footshock. Results are discussed in relation to a) a general inhibitory role of serotonin in behavioural mechanisms and b) a dopaminergic-serotonergic balance for behavioural arousal involving medial amygdaloid nuclei.     

Outline of a denotative definition of aggression

Aggression is defined as generic assertiveness which includes both constructive and destructive behaviors. An attempt is then made to classify the distinctive operational settings (phenotypic situations) which trigger aggression. The classes of such settings are: 1) privation - the frustration of vital needs and the frustration due to inner conflict of needs; 2) conflict (social) - situation-specific competition and intragroup rivalry for dominance; and 3) victimization — predator-prey relations and vandalism per se. Critical questions which emerge from the definition and classification are posed.     

Visual target control of schedule-induced aggression in white king pigeons (columba livia)

Visual target control of schedule-induced attack was studied in domesticated pigeons by exposing them to successive and simultaneous target preference procedures involving a fixed-time food schedule and projected target images. Pigeons preferred attacking an image of a conspecific head over a headless bird regardless of the height of the latter. Images of an intact bird and of a head alone were equally effective in controlling attack and more effective than the headless bird. Neither the eye nor four. other head-related features exclusively controlled attack. The combined results suggest that the head of an intact conspecific target selectively controls schedule-induced attack and that the effectiveness of the head in directing attack is inversely related to its physical integrity as a unit without regard to specific features. These results are consistent with reports that the head and head-related features of an intruder control reproductive aggression in birds.     

The reinforcing value of several types of aggressive behavior: A review

Field observations of “surplus killing” and laboratory studies of operant performance rewarded by prey-killing opportunities suggest that predatory behavior is positively reinforcing. Similarly, both repeated encounter and operant performance studies suggest that intraspecific aggression can be positively reinforcing for successful aggressors. While a few studies suggest that defensive aggression under aversive conditions may also be positively reinforcing, it appears that when appropriate response modes are available escape and/or avoidance are preferred to attack. Studies of the reinforcing properties of aggression-eliciting brain stimulation are in general agreement with these conclusions, but methodological problems with these latter observations render them less compelling. The progressive escalation of aggression seen in “warm-up effects” of birds and fish, “priming effects” of mice, and ecstatic violence of humans may be analogous processes based on the positively self-reinforcing characteristics of some kinds of aggression. The transient reductions of aggression which appear as refractory periods and satiation effects in a variety of species may reflect temporary reductions in the reinforcing value of aggression. All these temporal effects must be considered in the evaluation of experiments on the reinforcing value of aggression. More generally, it is possible that these temporal fluctuations reflect the operation of common motivational processes (aggressive states) which regulate overt aggression by changing its reinforcing value.     

An evaluation of contingency strength and response suppression

Typically, functional analyses of severe problem behavior have been conducted in two ways: (a) The target response is reinforced immediately after it occurs, or (b) the target response is reinforced on some schedule thought to mimic a naturally occurring schedule. We evaluated the effects of contingency strength in reducing levels of problem behavior with 2 participants who had been diagnosed with developmental disabilities. Results showed that under a neutral contingency, one in which the probability of reinforcement for aggression was equal to the probability of reinforcement for the nonoccurrence of aggression, rates of aggression were suppressed to low levels for both participants.     

Do Positive Self-Perceptions Have a “Dark Side”? Examination of the Link between Perceptual Bias and Aggression

The hypothesis that positive self-perceptions may have a dark side was investigated in the present study by examining the relationship between positively biased self-perceptions and aggression. Ratings of actual and perceived social acceptance of third-grade (n = 278), fourth-grade (n = 260), and fifth-grade (n = 321) students were compared to form a measure of perceptual bias. Peers provided nominations for overt and relational aggression. Gender differences were found for aggression (males were more overtly and relationally aggressive than females) but not perceptual bias. African-American children held more positive perceptions of their social acceptance and were perceived by peers as more aggressive than Caucasian children. Even after controlling for the effects of gender and ethnicity, more positively biased perceptions were associated with more peer nominations for overt and relational aggression. Contrary to an optimal range of bias hypothesis, even moderately positive self-perceptions were associated with elevated levels of aggression.