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Biological Components of Human Aggression

This was my term paper for Biology of the Brain at Empire State College (SUNY) in pursuit of a B.A. in Human Development.


Herein is presented a brief definition and typology of aggression, relevant brain physiology, and the primary sub-cortical neurobiological components of human aggression. It is demonstrated that human aggression is dependent upon three biological systems: the fight – flight system, the behavioral approach system, and the behavioral inhibition system. Explained are the, physiology, function, and aggression mediation of each of these systems.

Definition of Aggression

Aggression has been part of the human condition through out history and continues as a major cause of human conflict to this day. Science has studied aggression in vertebrates for decades, and researchers continue to make new and important findings relative to human aggression. Although there are many different types and causes of aggression, they all have common biological components.

Human aggression is complicated, ranging from mild verbal anger to vicious murder and everything in between. Conflict styles and aggression vary across individuals, sexes, genders, societies, and eras. Aggression is an important part of being human, an aspect of our complicated and diverse selves.

— Agustin Fuentes

The word aggression can describe many different types of acts from shootings and bombings to insults and undeserved criticism (Geen, 2001). Researchers have studied aggression for decades, yet are unable to decide upon a common definition. Johnson says that the complexities of human aggression defy definition (1972, in Renfrew, 1997). Different branches of science define aggression in their own way. One can describe aggression as a behavior, but also as a feeling that can motivate behavior. There are numerous definitions and types of aggression; however, the Oxford Dictionary of Biology provides a definition that conveys a generally accepted meaning of aggression in the field of human behavior: a “behaviour aimed at intimidating or injuring another animal of the same or a competing species" (2000).

However, we do not consider unintentional harm to be aggression (Anderson & Bushman, 2002). There is no scientific consensus on the various types of aggression, but most classifications are similar to or based upon Moyer’s eight types: predatory, inter-male, fear-induced [defense], territorial, maternal, irritable, sex-related, and instrumental (goal oriented) (1976, in Franken, 2002). Numerous factors are involved in the influence and acting out of human aggression, including genetic, environmental, cognitive, neurological, and biological (Geen, 2001; Raine, 2002,).

Brain Physiology and Function

The human brain consists of three primary sections (vesicles) and five secondary vesicles. The forebrain (prosencephalon) consists of the telencephalon (composed primarily of the cerebrum and basal ganglia, which includes the amygdala), and the diencephalon (composed primarily of the thalamus, hypothalamus, pituitary gland, and sensory processing areas). The midbrain consists of the mesencephalon, which is composed of portions of the brain stem. The hindbrain (rhombencephalon) consists of the metencephalon (composed primarily of the pons portion of the brain stem and the cerebellum), and the myelencephalon (composed primarily of the medulla oblongata portion of the brain stem) (Pack, 2001).

Neurobiological components are multi-functional and not limited to activity in only one functional system. The limbic system, coined by MacLean in 1952, provides such an example. The limbic system controls basic emotions, and although scientists do not agree upon its components (Buckley, 2001), it is believed to consist of the amygdala, cingulate gyrus, fornix, hippocampus, mammillary bodies (in the hypothalamus) (Colman, 2001b), parahippocampal gyrus, septum pellucidum, and thalamus (Colman, 2001a).

Function of Neurotransmitters in the Brain

Neurotransmitters are an important part of the central nervous system and enable nerve impulses to bridge the synaptic cleft (Postlethwait & Hopson, 2003).

Some neurotransmitters are important in regulating mood. Researchers have found that serotonin, dopamine, gamma—aminobutyric acid (GABA), norepinephrine, and epinephrine are relevant to human aggression (Renfrew, 1997). In addition to mood regulation, serotonin is also involved in several other physiological processes, such as muscle contraction, the regulation of appetite and perception, and blood platelet grouping. To accommodate these many functions, there are at least fifteen different serotonin receptors (Kroeze, Kristiansen & Roth, 2002).

The reticular activating system (RAS) is part of the reticular formation, a neural network in the brain stem responsible for arousing the cerebral cortex (Colman, 2001c; Tyson, 1988). To conserve energy, the information processing centers of the brain shut down when not needed. The RAS only activates the portions of the brain needed to process current sensory perceptions (Franken, 2002). Although not known for sure, scientists believe that thalamic relay neurons constitute a significant portion of the RAS. These neurons can remain in a state of depolarization, but when the RAS receives new sensory information, the neurons polarize and send the information to the cortex (Reiner, 1995). The thalamus directly connects to both the neocortex and the amygdala. Input from the sensory organs first goes to the thalamus, which acts as a switching station and controls information exchange between the neocortex and basal ganglia [which the amygdala is a part of (“amygdala n.,” 2002)] (Postlethwait and Hopson, 2003; Williams, 1990, in Panksepp, 1998).

Neurobiological Mediation of Aggression

As multi faceted as aggression is, without the biological components, aggression would not occur. Gray has said, “In the long run, any account of behaviour which does not agree with the knowledge of the neuro-endocrine systems…must be wrong” (1972, in Corr, 2004, p. 318).

Function, rather than physiology, defines some neurobiological systems in the brain, and in some cases, science better understands function than the specific physiology that is responsible. In 1964, Jeffrey Gray developed a neuropsychological personality theory known as the reinforcement sensitivity theory (RST) (in Corr, 2004). This theory explains three neurobiological systems that mediate human aggression: the fight – flight system (FFS), the behavioral activation system (BAS), and the behavioral inhibition system (BIS) (Gray, 1991; in Franken, 2002). Many researchers have either modified or built entirely new theories from Gray’s RST. In 2000, Gray and McNaughton revised the RST and changed the FFS to the fight – flight – freeze system (FFFS) and made additional clarifications about the types of behavior each system governs (Corr, 2004). [Note: terms FFS and FFFS are used interchangeably based upon the use by the particular source that is referenced; however, both terms refer to the same system].

Researchers continue to make new proposals involving the RST up to the present day. Therefore, study of the neurobiology of human behavior results in varied and often contradictory findings. However, despite modification of personality theories and re-arranging and re-naming of functional systems, the neurobiological components of human behavior remain the same. Although the names of the functional systems of Gray’s RST are used, concentration is placed upon the neurobiological components that mediate aggression. Researchers may change their beliefs about the triggers and responses of the RST systems; however, the neurobiological components themselves will continue to function the same.

The Fight – Flight System (FFS)

The FFS is associated with the fear emotion. It is responsible for governing avoidance or escape behavior toward aversive stimuli (Franken, 2002; Corr, 2004). This is the same system that makes us jump and our heart start to race when we have been startled. When we experience a real or perceived threat, including pain (Franken, 2002) or punishment (Corr, 2004), the sympathetic nervous system (SNS) activates the body’s fight or flight response (Franken, 2002). LeDoux suggests that humans tend to treat novel stimulus as potentially threatening, which can also lead to FFS activation (1986,1992,1993,1996, in Franken, 2002).

The FFS is comprised of the brain stem, midbrain, amygdala, and hypothalamus as well as many sub-systems and responses (Figure 1) (Sergeant, Van De Poll, & Van Goozen, 1994; Franken, 2002).

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The amygdala receives sensory information a few milliseconds before the neocortex. This allows us to react before we are fully conscious of the threat (LeDoux, in Franken, 2002). Pontius has identified the amygdalar visual pathway. This pathway extends directly from the retina, through the thalamus, and to the amygdala and motor system, bypassing the parieto–occipital cortex and cognitive evaluation (2002; in Pontius 2004). Both the amygdala and the neocortex can activate the FFS. The amygdala can also deactivate sub-cortical responses (Blair, 2004). When the FFS activates, the hypothalamus causes the adrenal medulla to start pumping epinephrine into the blood stream to get the body into its highest state of readiness (Purves, Orians, Heller, and Sadava, 2004). Epinephrine causes the heart to speed up, raises blood sugar levels, and causes blood flow to be re-directed from the digestive system to the muscular system (Postlethwait and Hopson, 2003). These changes prepare the body to either stay and fight the perceived threat (aggression), or flee.

If the FFS remains activated, the hypothalamus causes the release of corticotrophin-releasing hormone (CRH) into the bloodstream. CRH binds to the anterior pituitary, causing the release of adrenocorticotropic hormone (ACTH) into the bloodstream. ACTH then stimulates the adrenal cortex to produce cortisol, which causes the liver to make immediate energy available by changing proteins and lipids into glucose. When the threat is gone and the body no longer needs to maintain maximum readiness, the FFS deactivates. This occurs when cortisol circulating in the blood stream triggers the hypothalamus and pituitary to cease production of CRH and ACTH (Postlethwait and Hopson, 2003).

Several researchers have linked the FFS to aggression (Lorimer, 1972, in Renfrew, 1997; Zillmann, 1979; Tyson, 1998). The amygdala is able to generate fear responses (Blair et al, 1999, Hornak et al, 1996, Young et al, 1996, in Critchley, Simmons, Daly, Russell, van Amelsvoort, Robertson, et al, 2000), and is linked to the expression and regulation of aggression (Renfrew, 1997; Bear, 1991, Fuster, 1989, in Cairns & Stoff, 1996; Zillmann, 1979). The hypothalamus has also been linked to aggression (George, Rawlings, Williams, Phillips, Fong, Kerich, Momenan, Umhau, & Hommer, 2004; Veenema, Meijer, de Kloet, Koolhaas, & Bohus, 2003;Yao, Rameshwar, Donnelly, & Siegel, 1999). Ramamurthi found that bilateral amygdalectomies reduced aggressive behavior in 70-76% of cases (1988, in Blair, 2004). Animal research reveals that stimulation of the amygdala results in aggression (Siegel & Brutus, 1990, and Shaikh, Schubert, and Siegel, 1994, in Siegel & Shaikh, 1997) and lesions to or removal of the amygdala leads to passivity or reduced aggression (McGregor and Herbert, 1992; in Critchley, Simmons, Daly, Russell, van Amelsvoort, Robertson, et al, 2000). Research indicates that in both animals and humans, the frontal cortex is able to mediate sub-cortical aggression (Anderson, Bechara, Damasio, Tranel, & Damasio, 1999; Grafman, Schwab, Warden, Pridgen, & Brown, 1996; Gregg & Siegel, 2001; Panksepp, 1998; Pennington & Bennetto, 1993, in Blair, 2004).