Aggression: Biological Theories

Behavior is the result of the complex interactions of many variables—psychological attributes such as one's temperament or tolerance to frustration; social forces including racism, poverty, and the economy more broadly; and situational factors such as loss of a job or the death of a child—but it is ultimately how these factors act on and with various aspects of the biological substrate of the individual that determines how they will or will not manifest as specific behaviors. To a large extent, the biological makeup of the individual determines behavioral probabilities and thus behavioral patterns. Aggression is generally understood as any form of behavior intended to gain power or dominance over and/or to cause harm to another. In this regard, specific instances of aggression are generally viewed as the product of volition; the behavior is seen as a matter of choice. However, when our focus turns to patterns of aggressive behavior, including episodes of impulsive violence, a more complex picture emerges, one with decidedly biological underpinnings.

Aggression is not synonymous with violence, for people can behave aggressively without committing violent acts. A correlation between the two, however, is undeniable. The more aggressive a person is, the greater the likelihood will be for that person to be involved in, if not create, circumstances that situate him or her against others, thus increasing the possibility of a violent outcome. The word violence derives from the Latin root vio, denoting “force.” The etymological origin of the term biological is the Greek vios, meaning “mode of life.” In its broadest context, then, aggression is a life force: purposive as an agent of vitality.

Within species, aggressive behaviors confer reproductive advantage to males in their competition for territory and access to females. The propensity toward aggression in male members of mammalian species is well established as an adaptive strategy serving individual and species survival. In accordance with these principles, theories of evolutionary psychology begin with the premise that human behavior has archaic biologic origins and propose that mental functions undergo natural selection over time. To the extent that reproductive success is enhanced by aggressive traits, they are selected and carried over into the next generation. Examining the basic mechanisms involved in aggression through comparative animal studies better positions us to understand how more recently evolved capacities (notably language and consciousness) contribute to aggressive behavior in humans.

In human embryogenesis, a gonad appears after 6 weeks of cellular differentiation. This gonad has the potential to develop into testicular or ovarian tissue, depending on the presence of a single (SRY) gene on the Y chromosome. If this gene is present, the gonad develops into a testis. In general, in humans and other mammals, it is the presence or absence of a Y chromosome that determines the sex of offspring. During critical periods of prenatal development (in humans, this begins in the 7th week), hormonal secretions by an embryonic testis determine the sexual fate of all sexual structures by way of masculinizing the body—and the brain.

This biological disposition of human males to aggression, compounded by the differential socialization of humans according to gender, has led to [Page 6]significant disparity between the sexes in terms of aggressive behaviors. Crime statistics make clear the preponderance of males in the population of violent offenders. Despite this and conventional wisdom, researchers such as Björkqvist and Niemelä propose that females may be as aggressive as males, but not recognized as such because their modes of aggression tend to be less physical and less direct. For example, verbal hostility, as in shouting obscenities or rumor mongering, constitutes a prevalent mode of aggression in females. However, females can behave in individual cases as violently as males. This fact notwithstanding, males have been the main perpetrators of violence across cultures and across time and thus the focus of most studies on aggression.

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