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Aggressive behaviours are said to originate from brain centres that ‘trigger’ or ‘activate’ the metabolic arousal of the neuroendocrine system.
A neuroendocrine system is a group of neurones, glands, and other tissues that regulate homeostasis (this is the normal, steady, and optimal state of the body).
It’s primarily a mechanism involving the hypothalamus and pituitary gland, and it regulates behaviours.
This metabolic arousal results in the expression of aggression through mobilising the body’s muscles (Batrinos, 2012).
DNA strand and male and female icons, Flaticon
As mentioned previously, testosterone is an androgen. It is also an anabolic steroid. Specific genes affect testosterone.
Androgen is a hormone that plays a role in developing male characteristics.
Testosterone is involved mainly in bone and muscle mass (affecting strength), regulating sexual health and drive, and the expression of male characteristics and traits.
Considering the effect of testosterone on the brain and behaviour, it is important to understand its genetic origins and the research surrounding it, especially concerning aggression (for this particular topic).
Is there such a thing as a testosterone gene?
It is difficult to establish a specific gene for testosterone. For instance, the AR gene is involved in the production of androgen receptors (testosterone is an androgen, so this is important in the overall function of testosterone).
Furthermore, the Leydig cells in the testes produce testosterone (remember, testosterone is produced in the gonads for both sexes). The NR2F2 gene specifically (a protein-coding gene) is quite important in normal testosterone production, although that is extremely specific information (Hu et al., 2013).
We primarily care about the evidence of the heritability of testosterone found in humans.
Bogaert et al. (2008): This study measured sex steroid concentrations in men to determine the heritability of these concentrations and the influence genetic and environmental components have on these factors.
674 men aged between 25 and 45 participated in the study.
Researchers measured body composition alongside serum testosterone, SHBG, and other components.
Serum testosterone is the blood test used to measure testosterone levels.
SHBG is a binding agent that carries sex hormones through your blood and controls the amount of testosterone available for the body to use.
They found the sex steroid concentrations and the body composition factor had significant heritability, with testosterone being the highest.
They concluded that these two factors are under strong genetic control.
Meikle et al. (1986): This study measured monozygotic twins (MZ, identical twins) and dizygotic twins (DZ, non-identical twins) for their sex steroid blood content.
75 MZ twins and 88 DZ twins participated in the study.
Multiple blood content components were measured, including testosterone and SHBG.
The familial influence was more significant in MZ twins than DZ twins, which was true for all measurements except for SHBG.
Overall, they found that genes regulate 25% to 76% of the plasma content of the hormones except for dihydrotestosterone and SHBG.
Dihydrotestosterone is a crucial component made through the conversion of testosterone, which increases particularly in men during puberty. It is more potent and more influential than testosterone and is the main driving force behind the effects of testosterone.
Rissman et al. (2006) investigated the Sry gene. This gene is essential in the development of the male gonads, as well as developing high levels of testosterone.
They measured how long it took the mice to develop aggressive behaviours in reaction to another mouse in their territory. These mice either had the Sry gene or did not have it. They also measured it against mice with XX and XY chromosomes.
The gene itself was linked to higher levels of aggression in these mice, as those mice with the Sry gene behaved more aggressively. Female mice with the Y chromosome were aggressive. However, female mice with the XX chromosome were not. Instead, they were more maternal.
This finding suggests that genes play some part in the behavioural instincts of the mice, especially when tied to hormones. Some were more aggressive because of this gene, whereas others were less aggressive without the gene.
Brunner et al. (1993): In this study, researchers assessed a Dutch family and found a genetic issue that caused a mutation in the structure of the MAOA gene. We mentioned this particular study in our research on serotonin, but it is also relevant here.
They found a gene mutation associated with aggressive behaviours.
This study proves genetics influence aggression to some extent, but we now need to ask, is this the case for aggression? Is there a specific gene like this one that can influence aggression to such an extent that we can identify for testosterone?
Testosterone, Flaticon
Whether aggression is associated with testosterone in humans is still up for debate. Through the years, some studies have linked testosterone to aggression, whilst others have suggested otherwise:
Archer (1991): In this review, Archer found ample evidence for the association between testosterone and aggression in animals and investigated the link between humans.
Overall, they found that higher levels of testosterone found in adults were usually indicators of higher levels of aggressiveness.
In many studies on violent male offenders, researchers consistently found that the offenders had higher testosterone levels than less violent offenders and individuals.
Kreuz and Rose (1972): In this study, highlighted by Archer (1991) in their review, they measured testosterone levels in 21 young male prisoners who had a history of aggressive behaviour and displayed aggression whilst in prison.
Those who had committed violent crimes or had a history of aggressive behaviours had higher testosterone levels than those who didn’t have this history.
This finding suggests testosterone affected the prisoners, especially during adolescence, and caused them to be more aggressive.
Albert et al. (1993): Albert suggested that, despite the insistence on the link between aggression and testosterone, studies found:
Those with high and low testosterone levels do not differ enough in serum testosterone levels (remember, this is the blood test that measures testosterone).
Castration (removal of the gonads) does not decrease aggression in human males.
Overall, the evidence suggests that, although aggression in humans does have a biological root in defensive aggression, it’s not dependent on hormones such as testosterone.
Harris et al. (1998) measured the heritability of testosterone in this study.
160 adolescent twin pairs and their parents had blood samples measured.
As we found in other studies mentioned above, there were higher correlations of testosterone in MZ twins.
There was no resemblance between fathers and their children in testosterone levels, but there was a moderate correlation between mothers and daughters. This finding suggests genetic factors influence testosterone in men and women.
In men, 60% of the variance in testosterone levels is heritable, despite the father-son resemblance issues. This suggests that the genetic factors are expressed differently in adolescence and adulthood. In women, it was 40%.
Sluyter et al. (2000) measured the heritability of testosterone and anger, hostility, and aggression (AHA syndrome) levels in middle-aged twins.
Similar to Harris et al., around 60% of the variation in testosterone levels were due to genetic factors.
The genetic contribution to AHA syndrome was around 23% to 53%, which is a moderate heritability level.
Overall, they said that testosterone and aggression traits seem to have a solid genetic basis. However, there is a lack of evidence for genetic relation between the two, so it needs further investigation.
Overall, we can conclude that genetics play a role in the heritability of testosterone. This issue lies in whether or not these differing testosterone levels affect behaviours, specifically aggression. Whilst some studies say that they do, others suggest otherwise. It is not conclusive at this moment in time.
Several studies suggest that testosterone is heritable, i.e., genetic, but the environment affects how these genes are expressed. It is thus both genetic and environmental.
Yes, testosterone has high levels of heritability, and multiple genes seem to be involved in the expression and production of testosterone.
As testosterone levels are heritable, having high testosterone levels can be a genetic trait.
It is difficult to establish a specific gene for testosterone. For instance, the AR gene is involved in the production of androgen receptors (testosterone is an androgen, so this is important in the overall function of testosterone). Similarly, NR2F2 was also involved in testosterone production, specifically in the Leydig cells.
Several studies suggest that testosterone is heritable, i.e., genetic, but the environment affects how these genes are expressed. It is thus both genetic and environmental.
It is involved mainly in bone and muscle mass (affecting strength), regulating sexual health and drive, and the expression of male characteristics and traits.
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