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Biology 202
2004 First Web Paper
On Serendip

Does Testosterone Really Lead to Aggression?

Student Contributor

It is a common myth that testosterone causes aggression but is there biological reason to back up this assertion? Some say there is, while emphatically rattling off statistics and experimental evidence, while still others are armed with ambiguous or even refuting information with which to contest this argument. The bottom line is that we do not know for sure whether or not testosterone causes aggression (how problematic the idea of cause and effect can be in biology!) and so at this point we must turn away from the enticing idea that there exists a clear and definitive answer to this question. We must instead turn our attention to the evaluation of available information, in order to better understand the role of Testosterone in guiding behavior.

According to theory from evolutionary biology, aggression serves an important function in terms of both individual survival as well as procreation potential. In terms of this evolutionary biological theory, what it comes down to is this: competition arises when resources are limited and therefore animals/species must actively compete in order to increase their own fitness. It does not take a biologist to then infer that aggression is advantageous at both the individual and genetic levels. (1).

Hormones are inextricably linked to behavior as seen by the impact that its presence or absence has on an organism. In terms of aggression, there exists intriguing evidence that there is a definite connection between the hormonal effects of testosterone and the outward expression of aggressive behavior (1). For example, castration leads to a marked decrease in aggression as shown by castration experimentation on various species. Furthermore, when testosterone is replaced through hormone therapy in these castrated animals, the amount of aggression increases and is restored to its original pre-castration level (1). Taken together, this seems to present a strong argument for the role of testosterone in aggression. However, the story does not end here: if we are to suppose that testosterone does in fact lead to aggressive behavior we must then necessarily ask how and why it does. In doing so, we might just find that the original supposition falls through.

Testosterone exerts its hormonal and behavioral effects upon interaction with androgen receptors (i.e., when converted into 5-alpha-dihydrotestosterone) or with estrogen receptors (i.e., when converted into estradial by aromatase) (2). . According to some, there exists a "critical time period" (i.e., during development) when testosterone serves to "sensitize" particular neural circuits in the brain. Presumably, this sensitization allows for the effects of testosterone that manifest in adulthood. A recent theory builds upon this story, adding the idea that almost immediately after birth, testosterone leads to the establishment of an "androgen-responsive system" in males. And what about females? It is presumed that a similar androgen system is set-up in females, "although a greater exposure to androgens is required to induce male-like fighting" (2).

Although not the primary function of most hormones, neural activity can be modulated as a result of their presence. For example, it has been shown that some hormones can modify cell permeability and therefore have a crucial impact on ion concentration, membrane potential, synaptic transmission and thus neural communication and behavioral outcomes (2). More specifically, when a hormone such as testosterone acts on a target neuron, the amount of neurotransmitter that is release is significantly affected. For example, it has been suggested (i.e., with experimental data) that testosterone acts on serotonergic synapses and lowers the amount of 5-HT available for synaptic transmission. This is important when coupled with the fairly well accepted idea that the presence of 5-HT serves to inhibit aggression, as shown convincingly in studies done on male rhesus monkeys: Serotonin reuptake inhibitors such as Fluoxentine and several other antidepressants lead to a significant decrease in aggression in both monkeys and humans (2).

Although convincing relationships have been found between testosterone and aggression, hormones in general cannot cause a particular behavioral outcome; they can only facilitate or inhibit the likelihood that such an outcome will occur. For example, the mere presence or level of testosterone is not sufficient in invoking aggressive behavior, as seen by a significant population of males that are not aggressive. There must therefore be other factors involved: at the hormonal level, what about the effects of noradrenaline, acetylocholine or glutamate? It is important to remember here that the endocrine system consists of a complex array of communication pathways, none of which act independently (2).

Furthermore, we know that biological factors do not act in a vacuum and we must therefore concede significant impact and effect from environment and social factors as well. For example, some studies have found that it is not testosterone level that is the best predictor of aggression, but that obesity and lower levels of "good" cholesterol tend to be the best predictors of aggressive behavior in human males (3). Additionally, it has been shown that social status greatly influences the presence/degree of aggressive behavior in both animals and humans. The facts are that higher levels of social status correspond to higher levels of testosterone, although the quandary remains: is this elevated status a result of elevated testosterone levels and the evolutionarily advantageous aggressive behavior it might influence, or is the testosterone level a result of the heightened social status (i.e., building upon the well-supported idea that "winning" social competition leads to an increase in testosterone levels) (4)? It is the age-old nature versus nurture debate, or perhaps more appropriately, nature and nurture discussion.

To come full circle and reiterate this discussion's opening declaration: we do not know for sure whether or not testosterone leads to aggression. Therefore, any assertion of a causal relationship between the two is instantly problematic. Instead, we must continue to learn and to discuss the various possibilities with an open mind, in order to come to a better understanding of the role that testosterone and other hormones play in aggressive behavior.


1)Gender Website, a comprehensive cross- disciplinary approach to gender difference, touching upon areas such as Psychology, Genetics, Neurobiology, and Development to name a few.

2) Simpson, Katherine. The Role of Testosterone in Aggression. McGill Journal of Medicine, 2001. A thorough biological examination of aggression and the role that hormones play in facilitating/inhibiting aggressive behaviors. Many studies sited, comprehensible graphs presented. As found from the website:

3)DeNoon, Daniel. Don't Blame Testosterone for Aggression: Angry, Hostile Men Don't Have Extra Sex Hormone. WebMD Medical News, November 11, 2003. A newspaper article reporting on recent findings that Testosterone might not be the most important factor in aggression.

4)Steroids Website, a website dedicated to education regarding anabolic-androgenic steroids. Informative articles available such as: "Psychological and Behavioral Effects of Endogenous Testosterone Levels and Anabolic-Androgenic Steroids Among Males: A Review".



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