Human Aggression and the Interplay Between Hormones and Neurons

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Scientists from Duke University review studies of testosterone level fluctuations in response to competitive social interactions.

Image by MARIO OLAYA from Pixabay

Written by Brett Weiss

December 25, 2020

Certain cells called neuroendocrine cells release hormones into the blood in response to neural stimulation. In turn, the released hormones control body functions and behaviors. An example could be a social challenge that stimulates the brain and a concomitant release of testosterone into the blood that may affect behavior.

Hariri and colleagues from Duke University published a review in Psychoneuroendocrinology where they discuss neuroendocrine function and how it relates to human aggression. Through their research of scientific studies, they identify the hormone testosterone as having a seminal role in facilitating aggressive behavior. Moreover, the studies they looked at suggest testosterone levels fluctuate rapidly in response to aggressive and competitive interactions. This means that short-term changes in testosterone levels from aggressive interactions rather than typical levels under resting conditions shape ongoing and future aggressive behaviors.

Aggression entails “any form of behavior directed toward the goal of harming or injuring another living being who is motivated to avoid such treatment.” Researchers tend to classify aggressive behavior as reactive or proactive. Reactive aggression is typically hostile and may be a defensive response to provocation and may involve retaliation. Proactive aggression occurs in the absence of provocation and is goal-oriented behavior aimed toward acquiring a valued resource.

Significant fluctuations in blood testosterone concentrations resulting from changing social scenarios and aggressive interactions has become increasingly apparent to experts. Not only that but blood testosterone levels fluctuate rapidly during individual competitive interactions. Observations of testosterone fluctuations have led scientists to propose explanations of varying levels of testosterone and how they affect behavior.

Researchers developed one such explanation, the “Challenge Hypothesis,” to explain the relationship between fluctuating testosterone levels and aggression in birds. According to this proposal, blood testosterone concentrations fluctuate around three different levels: baseline, breeding baseline, and physiological maximum. Males providing paternal care have testosterone levels near baseline during the non-breeding season, but the concentrations increase to breeding baseline at the start of breeding season to initiate sperm formation and mating behavior. Blood testosterone reaches the physiological maximum in response to inter-male competitive interactions to facilitate aggressive behaviors. Importantly, the costs that go with maintaining higher testosterone concentrations throughout a year, like decreased paternal care, could have led to a highly flexible neuroendocrine system through time. Such flexibility may allow testosterone levels to responsively fluctuate to a changing social environment. Although scientists developed the “Challenge Hypothesis” for birds, some researchers support the notion of its applicability to humans.

Another proposal called the “Biosocial Model of Status” says that blood testosterone levels fluctuate based on the outcome of competitive interactions. It holds that winners will experience increased testosterone whereas losers will experience lower blood testosterone. Purportedly, winners of competitive interactions require increased testosterone levels to promote competitive and aggressive behaviors to defend their status. The decreased testosterone levels of losers in contrast serve to promote submissive behavior in order to avoid further loss of status or physical injury.

(Carre et al., 2011 | Psychoneuroendocrinology) Following competitive interactions, winners typically have elevated testosterone levels. Winners have elevated testosterone levels in comparison to losers, however, there is substantial variation in individual testosterone level response to victory or defeat. Figuring out where this individual variation comes from will help scientists to better understand and predict aggressive behavior.

A key prediction from both models is that resulting fluctuations in testosterone from competition confers adaptive behavioral responses that allow organisms to rapidly adjust their social behavior. The outcome of competition can also moderate testosterone levels, and individual differences in the way people respond to these testosterone levels may determine aggressive responses to competition and social behavior.

To better understand how individual differences mediate the effects of testosterone on aggressive behavior, researchers have studied the neural mechanisms of aggression. Based on clinical data, researchers have developed a neural model where decreased cross-talk between regions toward the front and middle of the brain, the orbitofrontal cortex and amygdala, respectively, results in an increased propensity to engage in reactive aggression. Reduced crosstalk between these regions may occur when the brain processes a threat-related stimulus such as an angry face, which activates the amygdala. The amygdala can then send neural messages to other regions of the brain with receptors for testosterone (androgen receptors). A functional orbitofrontal cortex can curtail the activity of the amygdala and dampen an aggressive behavioral response.

(Carre et al., 2011 | Psychoneuroendocrinology) The brain processes threat signals like an angry face in the amygdala (AMY), which ultimately mediates reactive aggression through its interactions with other brain regions. The amygdala processes threat signals and communicates with other regions that have receptors for testosterone called androgen receptors (black dots in the figure) to mediate aggressive behavior. The orbitofrontal cortex (OFC) inhibits aggression by reducing neural activity in the amygdala.

“Both animal and human studies provide compelling support for the idea that testosterone concentrations fluctuate rapidly in response to competitive/aggressive interactions,” stated the scientists in their publication. Adding observations of testosterone level fluctuations in aggression to the effects of typical, resting-state testosterone levels can enhance future human studies of this behavioral phenomenon.

Story Source

Carré JM, McCormick CM, Hariri AR. The social neuroendocrinology of human aggression. Psychoneuroendocrinology. 2011 Aug;36(7):935-44. doi: 10.1016/j.psyneuen.2011.02.001. Epub 2011 Mar 2. PMID: 21367531.

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