symptoms of the myth of pure evil is to identify violence as an animalistic impulse, as we see in words like beastly, bestial, brutish, inhuman, and wild, and in depictions of the devil with horns and a tail. But while violence is certainly common in the animal kingdom, to think of it as arising from a single impulse is to see the world through a victim’s eyes. Consider all the destructive things that members of our species do to ants. We eat them, poison them, accidentally trample them, and deliberately squish them. Each category of formicide is driven by an utterly distinct motive. But if you were an ant, you might not care about these fine distinctions. We are humans, so we tend to think that the terrible things that humans do to other humans come from a single, animalistic motive. But biologists have long noted that the mammalian brain has distinct circuits that underlie very different kinds of aggression.
The most obvious form of aggression in the animal kingdom is predation. Hunters such as hawks, eagles, wolves, lions, tigers, and bears adorn the jerseys of athletes and the coats of arms of nations, and many writers have blamed human violence, as William James did, on “the carnivore within.” Yet biologically speaking, predation for food could not be more different from aggression against rivals and threats. Cat people are well aware of the distinction. When their animal companion sets its sights on a beetle on the floorboards, it is crouched, silent, and intently focused. But when one alley cat faces off against another, the cat stands tall, fur erect, hissing and yowling. We saw how neuroscientists can implant an electrode into the Rage circuit of a cat, press a button, and set the animal on attack mode. With the electrode implanted in a different circuit, they can set it on hunting mode and watch in amazement as the cat quietly stalks a hallucinatory mouse.45
Like many systems in the brain, the circuits that control aggression are organized in a hierarchy. Subroutines that control the muscles in basic actions are encapsulated in the hindbrain, which sits on top of the spinal cord. But the emotional states that trigger them, such as the Rage circuit, are distributed higher up in the midbrain and forebrain. In cats, for example, stimulating the hindbrain can activate what neuroscientists call sham rage. The cat hisses, bristles, and extends its fangs, but it can be petted without it attacking the petter. If, in contrast, they stimulate the Rage circuit higher up, the resulting emotional state is no sham: the cat is mad as hell and lunges for the experimenter’s head.46 Evolution takes advantage of this modularity. Different mammals use different body parts as offensive weapons, including jaws, fangs, antlers, and in the case of primates, hands. While the hindbrain circuits that drive these peripherals can be reprogrammed or swapped out as a lineage evolves, the central programs that control their emotional states are remarkably conserved.47 That includes the lineage leading to humans, as neurosurgeons discovered when they found a counterpart to the Rage circuit in the brains of their patients.
Figure 8–1 is a computer-generated model of the brain of a rat, facing left. A rat is a sniffy little animal that depends on its sense of smell, and so it has enormous olfactory bulbs, which have been amputated from the left-hand side of the model to leave room for the rest of the brain in the picture. And like all quadrupeds, the rat is a horizontal creature, so what we think of as the “higher” and “lower” levels of the nervous system are really laid out front to back, with the rat’s high-level cogitation, such as it is, located at the front (left) end of the model and the control of the body at the rear (right), extending into the spinal cord, which would spill out of the right edge of the picture if it were shown.
FIGURE 8–1. Rat brain, showing the major structures involved in aggression Source: Image derived from the Allen Mouse Brain Atlas, http://mouse.brain-map.org.
The Rage circuit is a pathway that connects three major structures in the lower parts of the brain.48 In the midbrain there is a collar of tissue called the periaqueductal gray—“gray” because it consists of gray matter (a tangle of neurons, lacking the white sheaths that insulate output fibers), “periaqueductal” because it surrounds the aqueduct, a fluid-filled canal that runs the length of the central nervous system from the spinal cord up to