Biological Roots of Aggression

Although aggressive acts are usually triggered by environmental factors, laboratory
research suggests that aggression has biological roots. Various experiments
point to the fact that the three basic types of aggression are controlled
by different mechanisms in the midbrain. It has been demonstrated in laboratory
animals that offensive aggression has intimate connections to neurons
in the ventral tegmental area of the midbrain. When lesions occur in this
section of the brain, offensive aggression decreases markedly or disappears
altogether, although defensive and predatory aggression are not affected.
Conversely, when parts of the anterior hypothalamus are stimulated, offensive
behavior increases, and attack may ensue. The brain appears in these
experiments to be programmed in such a way that defensive aggression is
controlled by the periaqueductal gray matter (PAG) found in the midbrain.
So specialized are the neural activities of the midbrain that defensive aggression
involving perceived threats emanates from a different part of the brain
than defensive aggression that involves an actual attack. Acid-based amino neurons from the medial hypothalamus are known to trigger defensive aggression.
Alcoholic intake often intensifies aggressive behavior because alcohol reduces
the inhibitions that the cerebral cortex controls while stimulating the
neural pathways between the medial hypothalamus and the PAG. Although
alcohol does not increase aggressive behavior in all humans, many people
react aggressively when they consume alcoholic beverages.