​​ Stems from the testosterone peak produced halfway through pregnancy in boys. Girls with adrenal gland abnormalities that produce too much testosterone before birth are also more aggressive later.

​​ Less serotonin also causes more aggression.

​​ Malnourishment in the womb may cause aggression in later life.

​​ Mother's consumption of nicotine, alcohol, medication during pregnancy may put the offspring at a higher risk of being more aggressive.

​​ As testosterone levels rise during puberty, so does the incidence of murder. It peaks between the ages of 20 and 24 and has its lower values at 50 to 54. The declining levels in the late twenties are not due to less testosterone in the body, but do to the completion of the prefrontal cortex development. The prefrontal cortex function may be further dampened by alcohol.

​​ Violent video games, religious texts read by religious people, high temperatures also may cause higher levels of aggression.

​​ Amygdala stimulation can stop aggression, shown to be able to halt a charging bull and tame sewer rats. Some psychopathic individuals have malfunctions of their amygdala which prevent them from understanding their victims facial expressions .

​​ Sleep can be compared to a psychiatric and neurological disorder in the sense that:

- higher visual centres n the brain are activated and we hallucinate as though schizophrenic

- the amygdala is activated during emotional or aggressive dreams

- we make up stories like people with alcohol dementia

- we then forget everything like from sever dementia

- we lose muscle tension, like narcolepsy suffered with cataplexy when awake

​​ If you do not lose muscle tension when you sleep, you end up sleepwalking.