/ Biological Factors in Family Violence


Biological factors in violence are important but complex and often misunderstood. This article provides a framework for understanding an evolutionary analysis of human behavior and its potential contribution to understanding the role of the family in the development of aggressiveness. An example is given of the evolutionary analysis of family patterns in the perpetuation of criminal violence.

Key words: aggression, violence, family, kinship

    1. Glenn Weisfeld, Ph.D., is Associate Professor, Department of Psychology, Wayne State University, Detroit, Michigan, 48202.return to text

    2. Donald M. Aytch, Ph.D., is a Certified Forensic Examiner, Detroit Recorder's Court and Assistant Professor, Department of Psychology, University of Detroit Mercy, Detroit, Michigan, 48219.return to text

    Biologists have identified several distinct types of aggression which will be reviewed in this article. Consideration of the biological functions of these discrete forms may help family professionals to understand aggression as a whole and to analyze particular instances of it (e.g., family patterns in the perpetuation of criminal violence). However, in order to appreciate biological approaches to the study of aggression, practitioners first need to understand the rationale of the much-misunderstood evolutionary perspective.[3]


    Ethology and its congeners (e.g., sociobiology, evolutionary psychology) study behavior from an evolutionary standpoint. They reason that behavior is a property of living organisms and hence within the scope of biology, whose central theory is Darwinism. Since human beings have evolved, our behavior must have been guided by the forces of natural selection. Attempts have been made to deny or minimize the role of genes in human behavior, but the evidence for their role is overwhelming. We share over 98% of our genes with the chimpanzee, our closest relative; thus our behavior cannot be divorced from that of other species. Even our capacity for language, which mediates the transmission of culture, depends on brain structures that develop via genes. Furthermore, genes influence behavior continuously throughout development. For example, the heritability of personality traits—the amount of variance across individuals due to their genetic differences—remains stable from early childhood into adulthood. About half of individual variance in personality traits is due to genetic differences and half to environmental ones—even in adults (Plomin, 1980). In short, all human behavior is mediated by the brain, which develops through the influence of genes that are the products of natural selection.

    On the other hand, evolutionists recognize that environmental factors are indispensable to every behavior: Darwinians are not genetic determinists. Every behavior takes place in an environment that might, if altered, change the behavior. Even before birth, intrauterine factors can influence brain development and hence behavior. Furthermore, behaviors that are fairly hard-wired and stereotypic, such as imprinted responses, depend upon crucial experiences such as exposure to the mother. Genes and behavior interact inextricably. Both are essential, and hence neither is more important than the other: We must study both.

    One common objection to studying the genetic basis of human behavior is that since genes cannot be altered, we ought to concentrate on environmental factors that can be manipulated. The evolutionists' answer is that we usually need to understand how a behavior works before we can intervene effectively, and a full understanding must take genetic factors into account.


    How do ethologists proceed to study behavior, and why? A two-step process is generally followed. The first goal is to identify species-wide behaviors, so that the broadest possible generalizations about human behavior can be established. Thus, human ethologists emphasize our commonalities as a species, rather than individual or group differences among people. Human behavior is indeed highly variable, but there are quite a few universals (e.g., care of children by both parents, favoritism of relatives over nonkin, appreciation of the arts, sexual jealousy, and friendship). These are basic observations about human behavior that hold true everywhere. They tell us about—indeed, they comprise—our human nature.

    A general model of human behavior provides a framework for understanding variation, including pathological variation. It is easier and more systematic to establish general laws than to focus initially on differences. Analogously, physiologists describe and explain normal bodily processes. These descriptions provide normative models for understanding pathological processes, or pathophysiology. Another reason to seek generalizations about human behavior is that species-wide traits are almost sure to have biological functions. Evolutionists believe that it is particularly important to identify the adaptive function of a given behavior, to answer the "why" question about it.

    How is this goal of identifying species-wide, evolved traits reached? One method is to test for universality by means of cross-cultural research. If a behavior occurs in all of the hundreds of far-flung cultures that have been described, it probably originated when humans were still a single population in East Africa. It is unlikely that the behavior arose in a single culture and then spread throughout the world by cultural diffusion. This biological theory is particularly supportable if the behavior is also seen in our primate relatives, as is often the case. These and other research strategies for identifying evolved traits in humans were developed by Darwin himself, and are described elsewhere (Weisfeld, 1982).


    The second goal of ethologists is to identify the functions of these evolved behaviors. If we understand why a behavior evolved, we can appraise its utility under current circumstances, and possibly intervene constructively. For example, if homicides occur frequently because of sexual jealousy, which is true throughout the world (Daly & Wilson, 1988), then we can perhaps take preventive measures. We can advise people of the danger posed by sexual jealousy, both as potential victims and as victimizers. This is not to condone these homicides, but to understand their causation so as to intervene more effectively. Prevention of a behavior may be preferable to punishment after the fact, and prevention rests upon identifying precipitating causes.

    How are functions identified? Ethologists determine the function of a species-wide behavior or anatomical trait by comparative analysis. They try to figure out what distinguishes those species with the trait from those without it, just as an epidemiologist isolates the cause of an outbreak. For example, biparentalism—care of offspring by both parents—occurs in those animals with highly dependent young that apparently require both parents' efforts to survive. Since children need many years to reach maturity and even then continue to associate with their parents, it is not surprising that biparentalism is a human universal.

    In determining the function of a given behavior, evolutionists try to discern its utility in enhancing survival and reproduction; for example, parental care functions to enhance the survival of offspring. Evolutionists limit their use of the notion of function to direct biological exigencies—such as feeding, energy conservation, defense, and reproduction. Thus, explanations of a behavior in terms of self-fulfillment, self-expression, energy release, developmental stage attainment, resolution of psychological crises, and the like are not considered true biological explanations. A proper evolutionary explanation must show how the trait enables its bearer to pass on its genes more successfully to the next generation. Biological fitness is measured as number of offspring.

    Another important consideration is that functional explanations must refer to the fitness of the individual, not the group or species. Traits do not usually evolve for the good of the group, but for the good of the individual. A trait must benefit the individual for the genes that underlie it to be selected. Biological competition takes place mainly among members of the same species, who have similar needs. Those individuals who reproduce most successfully are the fittest. One important modification of this original, Darwinian definition of fitness concerns behavior toward relatives. If a man saves his sister from drowning and if, as a result, she lives to produce two children, he will have helped to pass on his type of genes to the next generation. In this example, he will have passed on as many of his type of genes as if he had had a child himself—including genes for aiding one's kin. Thus, a tendency toward kin altruism will be favored by natural selection under some circumstances, and has been observed in many animals from bees to bats. This evolutionary reasoning makes sense of the universal tendency of people to favor their blood relatives—the closer the better. It also illustrates the advantage of comparative analysis: that we can learn from studying the behavior of other species.

    This introduction may suffice to allow the reader to follow the evolutionary analysis of aggression below. Many good introductions to ethology and sociobiology are now available, including a bestseller by Wright (1995). The evolutionary approach is gaining adherents within traditional social science as well as among the lay public. It complements other approaches in its emphasis on generalizations about human behavior rather than differences, on the role of genes rather than learning, and on function—the "why" of behavior rather than just the "how" and "when."

    Types of Aggression

    Comparative research reminds us that aggression is—in a statistical sense—normal behavior: It is found in virtually all animals and presumably has some general utility. Cross-species comparisons of aggressive behavior suggest that animals and humans exhibit several distinct functional types of aggression. Recognizing the adaptive value of particular categories of aggression may be desirable when contemplating intervention. In considering a particular sort of aggression, practitioners may find it useful to determine what this aggression is designed to accomplish, both in the eyes of the aggressor and in terms of the adaptive value of that type of aggression. It may sometimes be more effective to try to alter the form or severity of the aggression, to modify or redirect it, than to try to suppress it entirely.

    The first step, then, in analyzing violence ethologically is to identify some of the basic types of aggression and their likely adaptive value. The best known typology of animal aggression is that of Moyer (1976). Brain research reviewed by Valzelli (1984) has confirmed Moyer's behavioral and neurological distinctions among the types of aggression. Each type of aggression involves characteristic neural pathways and communication displays, and an evolutionary function. Some types of animal aggression, such as predatory aggression, will be ignored here because of their peripheral importance to human family aggression.


    Defensive aggression arises from fear of or actual attack. The adaptive value of this type of aggression is obvious; it is said that self-preservation is the first law of nature. Thus, the spouse who defends herself or himself from verbal or physical abuse is to some extent acting normally. This point may seem trivial, but it represents a departure from the simplistic view that violence is always unjustified and that the cure is to suppress it categorically without trying to understand what precipitates it. In this example, a spouse might be advised on how to prevent such attacks rather than simply defending oneself or escaping. Some of the common precipitating issues in spousal violence are mentioned below.

    Each of the basic types of aggression seems to arise from a particular emotion, or motive—in this case—fear. Recognizing its emotional basis helps to identify the type(s) of aggression operating in a given situation. For example, defensive aggression is characterized by nonverbal signs of fear and by initial attempts at avoidance or escape. Practitioners can, in theory, utilize nonverbal cues, interviews, and other observations to identify the types of aggression and the precipitating factors in a given instance. In the case of defensive aggression, the predominant emotion is fear, which can interfere with calm, systematic thought and adversely affect health.


    This is sometimes called intermale aggression because, unlike other types, it is much more common in males than in females. In many animal species the mature males fight for social rank, or dominance. High-ranking males usually secure more prerogatives of rank, including material resources and mates. Any pacifistic male that declined to fight would risk failure to reproduce, and its genes for pacifism would be selected out of the population. Thus, males in these species seem to possess a "dominance drive" that impels them to compete for social rank. Sociobiologists have observed further that any female who mated with a male without a dominance drive would risk having sons lacking in reproductive success, and hence would defeat her purpose of passing on her genes to her grandoffspring.

    In our species too, reproductive success for males is associated with gaining high social rank. Around the world, women tend to prefer a high-ranking, wealthy husband (Buss, 1994), so men's dominance motivation presumably leads to biological success. Thus, dominance behavior relates to marriage and hence to family formation and maintenance. The dominance motive is enhanced by testosterone, thus helping to account for the sex difference in dominance aggression that is seen cross culturally (Rohner, 1976).

    Young boys, too, seem to exhibit a tendency to fight for dominance. In various cultures, at around three years of age boys begin to fight for rank and take pride in being "tough" (Omark, Strayer & Freedman, 1980). In male primates, this interest in fighting is enhanced by prenatal testosterone, as well as by pubertal surges in this hormone. The same seems to hold for men (Dabbs, 1990).

    Because of its motivational basis, dominance competition—especially among males—is hard to suppress. But it need not be injurious. Most play fighting among boys is just that—mutually enjoyable roughhousing that does little harm and in fact helps to establish group leadership. In adult male animals, dominance aggression is usually limited to dominance displays and contests of strength in which the use of dangerous weapons (reserved for predators) is inhibited. Similarly, dominance aggression is constrained in human societies by cultural controls, as well as by compassion for one's rival. These cultural controls include opportunities to compete non-violently, such as by procuring wealth that will allow a person to gain resources and by the availability of rewards for prosocial behavior.

    One obvious threat to this system of controlled dominance competition in humans is the availability of firearms, which can overwhelm our evolved anatomical and emotional protections (Lorenz, 1966). In a Detroit study, many homicides were found to have stemmed from a "trivial" public altercation, with an acquaintance rather than a stranger, that escalated to the use of firearms because neither man was willing to "lose face" (Wilson & Daly, 1985). Unemployed, unmarried, young men committed a disproportionate number of homicides; these are men of low status rank who are, perhaps, desperate to maintain or enhance their meager social position. These "social conflict" homicides outnumbered those associated wiih committing a property crime by 214 to 166.

    But property crime itself often constitutes a form of competition to gain sexual opportunities. Around the world, men who do not earn enough to help support a family are unlikely to marry or to stay married (Goode, 1993). Most women expect a man to contribute to the family income, not merely to support himself. Lacking a job, a man may turn to illegitimate means of advancement and sexual access, such as property crime or rape (Daly & Wilson, 1988). In all cultures these crimes tend to be committed by unmarried, impoverished young men; for example, rape is relatively common in polygynous cultures with large numbers of unmated men (Thornhill & Thornhill, 1983). Furthermore, most homicides are committed by young men against young men—probably a reflection of this desperate rivalrousness (Daly & Wilson, 1988). These researchers are not trying to excuse rape, but rather to explain it so it can be more effectively combatted. Of course, many destitute young men do not resort to crime; perhaps they are less impulsive or violent, or have been raised to believe that poverty is more desirable than criminality. The dominance hierarchy model may also help in understanding some recent physiological research on aggression pertaining to a neurotransmitter: brain serotonin. The level of serotonin is high both in dominant vervet monkeys and in men occupying leadership positions (Masters & McGuire, 1994). It is low in individuals with depression, which often entails feelings of failure. Low serotonin is also correlated with committing impulsive acts of homicide—why? Low ranking monkeys, children, and men tend to exhibit rather high levels of aggression, probably because they get picked on and have to retaliate (Weisfeld, 1994). Thus, Prozac and its congeners (which raise serotonin levels) reduce aggressiveness and increase self-esteem, presumably by enhancing feelings of dominance, or pride.


    Ethologists have wondered why animals restrict their severity of fighting when competing for dominance—why not go all out? The answer seems to be that such actions would prompt a "rage reaction" in the victim, which would need to oppose any such attack vigorously. Animals tend to stay within the rules because the rules are enforced by the victim. In fact, fights are relatively rare once the animals work out their dominance hierarchy. Only when a subordinate tries to usurp a superior's prerogative or a dominant animal continues to attack a defeated animal that has submitted does retaliation typically occur. Because such retaliatory attacks are intense and feature threatening displays, they seem to be prompted by rage, or anger. Various sorts of violation of individual prerogatives seem to trigger angry aggression in animals: violations of territory, unprovoked attack, threats to offspring, attempts to seduce one's mate, and sexual infidelity.

    Similarly, human anger seems generally to be a response to violation of some social expectation or norm. Subjects report that they would be angered by being passed up by a bus driver, but only if the driver is supposed to stop there (Pastore, 1952). Thus, frustration by itself—being thwarted in our purpose—does not consistently anger us, but being wronged does. This "transgression-aggression hypothesis" (Weisfeld, 1972) is confirmed by noting that anger is reduced by acts that restore equity: retribution by the victim or a third party, apology and restitution by the culprit, or a plausible excuse (see Hokanson, 1970; Bramel, Taub, & Blum, 1968; Baker & Schaie, 1969). Note that anger can be dissipated even if the subject takes no action. However, aggressing against an "innocent" third party does not usually reduce anger. This and other evidence has discredited the "catharsis theory" of aggression, the notion that aggressive energy can be drained off in harmless ways. Attacking an inanimate object, engaging in sports, or observing film violence does not reduce aggressiveness—in fact, it usually increases it (Geen & Quantry, 1977). The motivation to attack is aimed at punishing the transgressor; it would be maladaptive if the individual attacked any convenient target. Only by attacking the transgressor successfully or otherwise restoring equity does systolic blood pressure—the measure of anger used in much of this research—fall.

    What sorts of offense provoke angry aggression in humans? Violent attacks are often precipitated by being treated in ways contrary to one's biological interests. For example, spousal abuse is frequently caused by sexual problems, financial disputes, and sexual jealousy (Figueredo & McCloskey, 1993; Frude, 1994; Russell & Wells, 1993); these issues are salient to reproductive interests. Sexual jealousy commonly underlies spousal abuse and homicide around the world (Daly & Wilson, 1988), as in a Detroit study (Daly, Wilson, & Weghorst, 1982). Losing a mate is often a disaster to one's success in raising offspring, so it is not surprising that various mate-guarding behaviors to prevent such an outcome have evolved in pair-bonding species. But mate guarding is not identical in the sexes (Wright, 1994). Women in many cultures guard their sexual reputations assiduously; women may fear desertion by their mates and loss of the man's contributions. Consistent with this perspective, most fights between adolescent girls in a low-income neighborhood were found to stem from sexual insults (Campbell, 1995).

    What about aggression directed against one's own children—how can this be explained in evolutionary terms? Doubtless, like all other mammals, humans have evolved parental tendencies. However, parental behavior has evolved to benefit one's own offspring, not those of others. Animals and people are more solicitous of their own flesh and blood than of others' offspring (Hepper, 1991). Thus, cross-national research reveals that child homicide, battering, and sexual abuse are much more likely if an adult other than a biological parent tends the child (Daly & Wilson, 1988, 1994). For example, the risk of child homicide by stepparents was about 100 times that by biological parents in a U.S. study that controlled for social class, and about 70 times in a Canadian one (Daly & Wilson, 1988). Furthermore, children who are handicapped and, consequently, less likely to be reproductively successful themselves, are more prone to abuse and infanticide. In many mammals the mother assesses her newborns, and abandons those that appear abnormal. Likewise, the stimulus characteristics of abnormal babies may cause their parents to bond less strongly to them, and not breastfeed them. (See Daly & Wilson, 1988; and Eibl-Eibesfeld, 1989, for a discussion of the early bonding research.) These instances of abuse and homicide of children also may be conceptualized as occurring because the adult has been angered by the child. Child abuse often occurs when the adult has been angered by the child's behavior, especially when the parent has unrealistic expectations of the child's abilities. But to some extent the child's abilities may be poorly developed because of a congenital condition, so parental genes may be involved.

    As described above, we also serve our biological interests by aiding and defending other relatives with whom we share genes by common descent. For example, identical twins, who share all of their genes, have been shown to cooperate more than fraternal twins in a test situation (Segal, 1988). Thus, it is not surprising that the relatives of a murder victim are often infuriated at the perpetrator. The other side of this nepotism coin is that we are liable to aggress against nonkin. In a Detroit study, nonkin were 11 times more likely to be killed by someone who lived with them than were relatives (Daly & Wilson, 1982).

    An Example of Biological Factors in Aggression

    To try to confirm the point that an evolutionary analysis of aggression may carry potentially fruitful, if complex, implications for family issues, we will describe a recent study (Aytch & Weisfeld, 1994). In this research the aggression of initial interest was violent crime, which may involve various types of aggression. In this case no attempt was made to identify these types of aggression. Instead, the emphasis was on the role of kinship in the cross-generational perpetuation of criminal aggression.

    A triracial sample of 130 Detroit felons was studied. As predicted, these men tended to report that, as children, they had had poor relationships with their mothers: an anxious attachment pattern (p < .01). Moreover, anxious (especially ambivalent) attachment was associated with having more felony convictions (p < .01).

    It is known that a history of anxious maternal attachment in childhood is associated with forming unstable, turbulent romantic relationships in adulthood (Hazan & Shaver, 1987). Therefore, a cycle of criminality might be perpetuated across generations involving these family dynamics, as follows: Anxiously attached children might become criminals and have unstable romantic lives, leading to another generation of children from unstable families. Many of these children would grow up in families with rivalrous half-siblings and stepsiblings of low consangunity. Their mother might resort to frequent punishment to settle these disputes, resulting in the pattern of anxious attachment that is associated with this parental style. Furthermore, the fathers might make little positive contribution due to family instability.

    Consistent with this model, these felons tended to come from families with stepsibs, (35% of the sample) or half-sibs (45%), as would result from short-lived romantic relationships. Moreover, number of half siblings was associated with maternal punitiveness (p < .01). Further supporting the model, maternal punitiveness and paternal inattentiveness were both associated with number of convictions. Data by other researchers confirm these relationships between variables.

    This study points out the importance of kinship in violence and—more broadly—of evolutionary principles in the study of behavior. It also illustrates the interaction of presumably evolved affinities for close kin with experiential factors such as maternal punitiveness, paternal inattentiveness, and family structure. Of course, other environmental factors such as role models, television exposure, and unemployment also affect violent tendencies. But these seem to interact with biologically based processes too, such as parent-child bonding. Take poverty for instance. In a sample of college students, Aytch and Weisfeld (1994) found that lower-income families tended to have more anxiously attached children. Presumably, typical circumstances associated with low-income families interfere with the parent-child bonding process. In the sample of male offenders, those subjects who were anxiously attached were found to have more felony convictions. Poverty also tends to lower the marriage rate and hence undercut family integrity, thus contributing to the number of violence-prone men (Wilson, 1987).

    Implications for Intervention

    The extent to which evolutionary and experiential factors influence aggressive behavior is of great concern to both developmental and forensic psychologists. The aforementioned findings demonstrate the influences of genes on human aggression. Therefore, we cannot hope to make much progress in reducing aggression without taking these factors into account. For instance, intervention programs to reduce violence have focused primarily on removing violent offenders from society. However, in spite of increased allocation of resources for detaining violent offenders, the number of offenders convicted and incarcerated each year steadily increases. Perhaps what should be investigated more thoroughly is how certain environmental factors can precipitate biological factors that lead to a propensity toward aggressive and/or criminal behavior. Specifically, low family consanguinity associated with half-siblings and stepsiblings has been found to cause anxious attachment (Aytch & Weisfeld, 1994). These anxiously attached children exhibit an increased propensity toward criminal behavior when they reach adulthood.

    Based on these findings, efforts to stabilize families in ways that enhance the long-term viability of marital relationships (i.e., marital counseling, adolescent pregnancy prevention, economic empowerment, and male responsibility programs), and thereby increase family consanguinity, would likely aid in the reduction of aggressive behavior. If such interventions prove to be successful, there may be as well a substantial drop in the number of successive generations of children from unstable families that seem to perpetuate a cycle of cross-generational families with similar dynamics.


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      1. The brief evolutionary analysis of behavior summarized in this article necessarily contains some simplifications. For further information, readers are urged to consult the articles cited, plus Archer (1994) and Eibl-Eibesfeldt (1989). Physiological research on aggression—such as the roles of hormones, neuro transmitters, brain lesions, impulsiveness, and prenatal insults—is probably more familiar conceptually to most readers, and so will not be emphasized here. (See Turner, 1994, for a review of this important topic.).return to text