The central nervous system

1. The central nervous system is the brain and the spinal cord.
2. Nerves carry signals into it from your eyes, ears, skin and internal organs.
3. It processes those signals, and that processing is where cognitions such as attention, thinking and deciding happen.
4. It then sends instructions back out along motor nerves to your muscles and glands, and that output is your behaviour.
5. Eysenck argued that people inherit different settings in this system, so the same situation is handled differently by different brains.

The three divisions of the brain

1. Cerebrum: the large upper part of the brain, divided into two hemispheres.
   1. Its wrinkled outer layer is the cerebral cortex, which handles thinking, language, perception and voluntary movement.
   2. Underneath the cortex sit the structures of the limbic system, including the hippocampus and the amygdala, which deal with emotion and memory.
2. Cerebellum: tucked underneath at the back of the brain.
   1. It controls balance, co-ordination and smooth, well-practised movement such as riding a bike.
   2. Damage here makes movement clumsy and unsteady.
3. Brainstem: the stalk that joins the brain to the spinal cord.
   1. Its sub-parts include the medulla, which runs breathing and heart rate, and the pons, which relays signals between the brainstem and the rest of the brain.
   2. It also holds the reticular activating system, the network that decides how much stimulation reaches the cortex.

Arousal levels

1. Your arousal level is how alert and active your brain is at a given moment.
2. Everyone has a baseline they are comfortable at, and Eysenck argued that this baseline is largely inherited.
3. The reticular activating system in the brainstem sets the baseline by filtering how many incoming signals get through to the cortex.
4. Drop below your baseline and the brain feels under-stimulated, so you go looking for something to raise it.
5. Climb above it and you feel tense, so you back away from anything that would add more.

Why low arousal raises the risk of offending

1. Someone born with a low baseline needs a lot happening before they feel anything, so a normal evening is flat and boring to them.
2. So they chase strong stimulation, and noise, speed and risk all raise arousal fast.
3. This hunger for stimulation is the biology behind extraversion and sensation-seeking.
4. Most under-aroused people meet that need lawfully, through sport, gaming or a busy social life.
5. For some, offending delivers the same jolt, so shoplifting or taking a car feels rewarding rather than frightening.
6. A low-arousal nervous system also learns badly from punishment, because being told off produces too little anxiety for the lesson to stick.

Arousal and the criminal personality

1. The criminal personality is a high score on extraversion, neuroticism and psychoticism at the same time.
2. Eysenck traced all three dimensions back to how the central nervous system handles arousal and stimulation.
3. Low cortical arousal accounts for the extraversion half, because the person is permanently hunting for stimulation.
4. A nervous system that overreacts to stress accounts for the neuroticism half, because the person flares up fast and settles slowly.

Turning this into marks

1. Build the chain in separate steps, from the reticular activating system to low cortical arousal to sensation seeking, so each step earns its own credit instead of restating the one before.
2. Name the structure, such as the reticular activating system in the brainstem, because writing that the brain wants more stimulation does not secure the knowledge mark.
3. The mark most often lost is describing low arousal as a calm personality, when it means an under-stimulated brain that produces restless, impulsive behaviour.