Hormones are definitely at least linked to behaviour although a problem exists in trying to establish cause and effect, which will be discussed later. Much human and non-human research points to a link between the two. For example, when levels of testosterone peak at the time of puberty in young human males, aggression also peaks. Many non-human animal studies support the idea that aggression is somehow caused or linked with testosterone. One such method of testing this is to castrate various animals. This always leads to a marked decrease in aggression. Furthermore, when testosterone is replaced by hormone therapy in the castrated animals, the aggression of the animal returns to its pre-castration level (Simpson, 2001). This seems to support a causation effect or at least a link between the two. A similar study on cockerels by Berhold found the same effect.
More specifically, it seems that androgen stimulation in the early days after birth (up to ten days) causes changes in the neuronal system, which affects the level of aggression of a person through into adulthood. Women are typically less aggressive because their brains contain beta blockers which block most of the effect of the testosterone in the brain. Male mice who are castrated straight after birth throughout their whole life even if they are given hormone therapy, i.e. huge doses of testosterone, later on in life. If the castration occurs 10 days after birth, the difference in aggression levels is much less obvious. This is supposed to be because of the androgen stimulation in the neural circuits during the early days after birth (Motelica-Heino et al 1993).
The usual explanation for the link between testosterone levels and aggression is that testosterone interacts with androgen or oestrogen receptors. During the critical time period, testosterone sensitises certain neural circuits in the brain. This allows for the effects that testerone have which last into adulthood. Testosterone can affect the effect of neurotransmitters, e.g. how fast they move and the amount of neurotransmitter released. It appears to act upon serotonergic synapses, lowering the amount of serotonin available for synaptic transmission. The significance of this is that serotonin inhibits aggression, so less serotonin means more aggression. To prove this, Simpson (2001) carried out an experiment on rhesus monkeys where he gave them serotonin reuptake inhibitors, leading to a marked decrease in aggression. This has been implicated on humans also and has the same effect (Simpson, 2001).
Sapolsky has argued that although there is clearly some kind of link between levels of testosterone and aggression, this tells us nothing about individual differences in how people are affected by testosterone, why men are exceptionally more aggressive than women and does not explain why sometimes the highest levels of aggression are found in those with not an exceptionally large amount of testosterone. The relationship between testosterone and aggression is not straightforward. Does testosterone cause aggression? Does aggression increase testosterone secretion? Does neither have an effect on the other (i.e. a third variable is involved)?
Bernhardt et al (1998) has shown that merely watching participants win or lose in sports competitions increases testosterone secretion in those who are interested in the game. Indeed, Bernhardt did an experiment where he measured participants testosterone levels before and after sports events. Men who watched their team lose had a decrease of testosterone levels by 20% and those whose team won showed a 20% increase in testosterone levels. Clearly, testosterone does not always cause behaviour/emotion, but rather the opposite can occur too. This somewhat dents the argument that hormones explicitly cause aggression.