The broad question in psychology is "What is the function of the nervous system?" and the broad answer provided by the theory of evolution is "To enable the organism to survive." The theory of evolution could thus be considered as the basic theory of psychology.^₁

The next question is How does the nervous system enable the organism to survive? and the classic answer is It ensures that the organism will approach things which are good for it (for example, things that it eats) and that it will avoid things which are bad for it (for example, things that eat it). The need-reduction theory explains the former mechanism and the activation theory explains the latter mechanism. Thus, the need-reduction theory and the activation theory could be considered as the means of fitting psychology within the basic framework of the theory of evolution. Let us look at each theory in turn.

You are alive. You are in a precarious state. Life is a narrow tightrope with death on either side. To stay alive, you must maintain yourself within a narrow range of temperature, blood-sugar concentration, metabolic rate, and so on. Let us focus on temperature.

You have been set by the great temperature-setter-in-the-sky at 98.6 Fahrenheit (or at 37 Centigrade if God has gone metric). You are allowed to vary a little bit around this optimal temperature. But, a bit too low, you die; a bit too high, you die. Certain physiological mechanisms enable you to maintain your optimal temperature despite variations in the temperature of your environment. If it gets too cold, you shiver; if it gets too hot, you sweat.

Consider, however, the alligator. It shivers not, neither does it sweat. Yet all alligators are not frozen alligators or boiled alligators. A group of alligatorologists organized an expedition to Africa to find out why. A few thousand miles and several thousand dollars later, they discovered the answer. When an alligator gets too warm, it slides into the cool water; when an alligator gets too cold, it climbs on to a hot rock. Thus, the alligator maintains its optimal temperature by adjusting the environment to itself rather than by adjusting itself to the environment. It behaves.

The process by which an organism maintains itself in its optimal state is called homeostasis. When it deviates from this optimal state, it can return to it either by adjusting itself to the environment or by adjusting the environment to itself. Our species, of course, uses both mechanisms. We shiver and sweat and we buy furnaces and air-conditioners. Adjusting ourselves to the environment is the province of physiology; adjusting the environment to ourselves is the province of psychology.

Let us take a closer look at the psychological mechanism. Imagine a hypothetical contented organism which has just been wined and dined. It is in its optimal state. However, it can not remain thus for long. The mere passage of time conspires against its bliss. It gets thirsty. It gets hungry. This physiological state of deprivation is called a need. The need can be satisfied by appropriate behavior with respect to some appropriate object in the environment - by drinking water in the case of thirst and by eating bread in the case of hunger. Since the nervous system is the only system within the organism which knows the environment, the physiological state of deprivation in the digestive system must be transformed into some psychological counterpart in the nervous system. A need must be transformed into a drive. The drive orients the organism to some appropriate thing in the environment - the goal. By making the appropriate response to the goal, the drive is removed, the need is satisfied, and the optimal state is regained.

Let us turn now from the positive to the negative drives, from the tendency to approach things that are good for us to the tendency to avoid things that are bad for us, from the need-reduction theory to the activation theory.

There are two ways we can avoid things that are bad for us. We can remove the thing or we can remove ourselves. The first involves fight and the second involves flight. The emotion underlying the former is rage and the emotion underlying the latter is fear. Such primitive emotions must have played a dominant role in the early history of our species. Consider one of our remote ancestors confronted by a saber-toothed tiger. She has a tiger in her subjective map. She can remove it or remove herself. She can kill it or she can run away. The only good tiger is a dead tiger or a distant tiger.

An emotion-arousing stimulus has three broad effects - experiential (we feel angry or afraid), physiological (there are certain changes in our bodies), and behavioral (we fight or flee). Discovery of the function of a structure in the lower brain called the amygdala has clarified the interaction among those three effects.

The emotion-arousing stimulus, like all stimuli, acts directly on the cortex. The stimulus is transformed at the appropriate receptor (a set of cells specialized for this purpose) into nerve impulses, which are transformed at the appropriate projection area of the cortex (a set of cells specialized for this purpose) into a perception. This cue function of the stimulus has long been known. However, what is less known is that the emotion-arousing stimulus also acts indirectly on the cortex to perform an arousal function. It switches on the amygdala which projects diffusely on to the cortex, to alert you that something is happening in your environment.

Thus, the arousal function alerts you that something is happening (the amygdala responds in the same way to sights, sounds, tastes, smells, and touches) whereas the cue function informs you precisely what is happening. The arousal function prepares you for an emergency. It acts upward on the cortex to produce the experiential effects (fear or rage) and downward on the autonomic nervous system and the endocrine system (responsible respectively for the physical and chemical aspects of your internal environment) to produce the physiological effects (increased heart rate, injection of adrenaline, and so on), to provide the motivation and the energy for the behavioral effects (fight or flight).

The cue function informs you whether there is indeed an emergency. Most stimuli are not worth getting emotional about. In such cases, the cortex acts downward on the amygdala to inhibit the arousal function. Animals without a cortex get mad at every little thing. The cue function also informs you of the nature of the emergency so that you can respond appropriately. Otherwise, you might attack tigers and run away from rabbits.

The need-reduction theory and the activation theory are diagrammed together to clarify the similarities and differences between them (see Figure 2-1). Both theories involve a negative feedback loop to maintain the organism in its optimal state. Both theories describe the nervous system as a mediator between the internal environment (that is, the other subsystems within the organism) and the external environment. According to the need-reduction theory, the function of the nervous system is to mediate between a state of deprivation in the internal environment (need) and a thing in the external environment which will satisfy that need (positive goal), so that the organism will approach that thing; according to the activation theory, the function of the nervous system is to mediate between a thing in the external environment (negative goal) and a state of the internal environment (an emotion), so that the organism will avoid that thing.

Since the nervous system is merely a mediator between internal and external environments, the person is extrinsically motivated. The person is pushed and pulled by external forces - pushed by needs and pulled by satisfiers of those needs, pushed by threatening things and pulled by emotions generated by those things. Behaviorists conclude that all human behavior is determined by those extrinsic needs.

Secondary drives can however he established through association with those primary drives. Thus, monkeys will work for tokens if those tokens can be exchanged for food. Capitalism is established by making money the means to the end of satisfying the basic biological needs. The behaviorist would thus explain your behavior in reading this book by saying that you are reading this book to pass a course to get a degree to get a job to get money to buy food to remove your hunger drive to satisfy your hunger need to return to your optimal state to survive.