Aggressiveness is a major cause of disruptive behavior, crimes, robberies, fights, and all sorts of conflicts. It is most commonly encountered among the strong, and can take various forms, such as verbal or physical. The causes of aggressive behavior are the sum of genetic and environmental factors. These days, the only dispute comes from the importance given to these two factors.
Abnormalities on the neuroanatomic level―on the neurophysiologic and endocrine system―affect the probability of developing aggressive behavior. Although these factors do not seem to represent direct causes, the biological differences between children interact with the early exposure to different types of learning environment creating an individual pattern of social behavior. These are called biosocial interactions.
Studies, such as MRI and PET, have shown significant differences between the brain of a more aggressive person and another one who has a diminished level of aggression. Together with the prefrontal cortex, the hypothalamus and the amigdala seem to represent extremely important places where to find the anatomic differences which affect the level of aggressiveness. Electric stimulation and lesions of these nuclei can increase or reduce the tendency of an individual to be violent or not. So, different traumas, lesions, or similar affections can lead to important changes in the behavior of a person, which can include a serious form of aggressiveness. Nevertheless, the occurrence of these changes also depends on situational factors. For example, research made throughout electrical stimulation on this cortical zone on animals, show that the same kind of stimulation which triggers aggression against a weak opponent might not lead to the same result when it is involved a stronger one.
Persons presenting a low level of serotonin, which is a neurotransmitter involved in the restraining of impulsive reactions, seem to develop a higher risk for developing aggressive behavior. If serotonin is taken out from the bodies of animals, through diets or drugs, these animals manifest violent behavior. Researchers have discovered that men arrested for murders have more reduced levels of serotonin compared to non-impulsive offenders. In addition, it is proven that children having a low level of serotonin also develop violent behavior.
High levels of testosterone during the intrauterine period and the early years of life seem to lead to predispositions for high forms of aggression. Reinisch discovered that girls whose mothers where administrated with testosterone during their pregnancy have shown more aggressive behavior in their adulthood, than girls from the control group. Analogous, teenager boys having high levels of testosterone respond more aggressively when they are provoked, though the effects are not unilateral. Studies show that dominating others and manifesting aggressive behavior increases the level of testosterone.
Furthermore, genetic variation plays an important role regarding the occurrence of biologic differences. Studies made on twins brought up separately show a higher correlation on monozygotic twins than on non-identical twins. Longitudinal studies made on boys adopted from their birth also show a significant correlation between them and their natural father, regarding the probability to be convicted for violent acts.
These genetic influences can be very well expressed through the biological differences presented above, testosterone, serotonin, and the neuroanatomy of the limbic system. These biologic predispositions influence the way in which interactions with the environment shape the beliefs and social-cognitive schemes of an individual, and also the way people react, both cognitive and emotional, to provocative and frustrating stimuli coming from the environment.
Even though a person is genetically predisposed towards aggression, and he/she is also capable of doing it, there must be a specific situation to trigger the act of aggression. The probability for the aggressive act to occur, and also its intensity, will be different depending on the type of the instigation and on the potential of aggressiveness that could be manifested.
In conclusion, certain persons having a predisposition for violence will be significantly more aggressive when being attacked than those who lack this predisposition. Another fact is that those who acquired strong aggressive tendencies throughout social learning will react more aggressively than those who haven't learned this specific pattern. Social learning and genetics are complementary factors combined in the human aggression.