Hormonal influence
Testosterone
Raleigh et al. (1991): monkey’s who fed on experimental diets that are high in tryptophan (increases serotonin levels) - showed less aggression, and vice versa.
Rosado et al (2010): 80 dogs (various breeds) referred to Spanish vets because of their aggressive behaviour towards humans. They were compared to 19 dogs (various breeds) that didn’t show this aggressive behaviour.
Aggressive dogs = 278 units of serotonin.
Non-aggressive dogs = 387 unit of serotonin.
THEREFORE, different levels of aggression can be attributed to animals’ serotonin levels.
From young adulthood onwards, it produces male characteristics including aggression - this is because it affects parts of the brain involved in controlling aggression.
Sapolsky (1998) - removing the source of testosterone in various species typically leads to significantly lower aggression levels, and then injecting synthetic testosterone to increase its levels back to normal led to the aggressive behaviour returning.
Random Research on testosterone
Archer (2009) - men are usually more aggressive than women.
Dabbs (1990) - men have much higher concentrations of testosterone than women.
Daly and Wilson (1998) - ages 21-35 = highest testosterone concentrations and increased male-on-male aggressive behaviour.
Dabbs et al (1987) - salivary testosterone measured in violent and non-violent criminals. Highest testosterone levels = history of mostly violent crimes; Lowest testosterone levels = only non-violent crimes.
Carre and Olmstead (2015) - testosterone levels fluctuate rapidly to respond to the social environment.
Changes in testosterone levels seem to influence aggressive behaviour by increasing amygdala reactivity whilst processing social threats (e.g. angry face).
Evaluation
The evidence to show a positive relationship between testosterone and aggression is inconsistent.
E.g. Albert et al. (1994) reported positive correlations between testosterone levels and self-reported levels of aggression among prison inmates.
Olweus et al (1988) found positive correlations between testosterone levels and the likelihood of responding aggressively when provoked.
However, no correlation was found between testosterone levels and actual violent behaviour among male prison inmates.
THEREFORE, the relationship between testosterone and aggression in humans isn’t clear.
Mazur (1985): it’s important to distinguish between aggression (intending to inflict injury) and dominance (wishing to achieve/maintain status over someone).
Aggression is a form of dominance (but dominance comes in many forms).
In non-human animals, testosterone influences dominance and this can be shown through aggressive behaviour.
However, in humans, this is more likely to be shown in a more varied and subtle way, e.g. status-striving behaviour.
Eisenegger et al. (2010): depending on the situation, testosterone could actually make women act ‘nicer’ rather than more aggressively.
What is nicer? Nicer than what? How do we measure it?
THEREFORE, testosterone may not directly increase aggression, but rather it promotes status-seeking behaviour, of which aggression is one type of this.