Biological Motives

The biological approach to motivation was the earliest serious attempt to explain the causes of behaviour, and its influence can still be traced in most theories that came after it. This approach centres on the idea of an adaptive act: organisms have needs (internal physiological imbalances), which produce a drive, which stimulates behaviour aimed at particular goals, and achieving those goals reduces the drive. The very earliest explanations of motivation relied on the idea of instinct — an inborn pattern of behaviour that is biologically determined rather than learned, found in all members of a species, and that directs behaviour in predictable ways. Curiosity, flight (running from danger), repulsion, reproduction and parental care are commonly cited examples of human instincts; an instinct carries an inner 'impetus' that drives the organism to act in a way that reduces that impetus. Among the basic biological needs explained by this approach, hunger, thirst and sex are the three most fundamental, since they are essential to individual (and, in the case of sex, species) survival.

Hunger dominates behaviour once the body needs food, but what actually makes a person feel hungry is more complicated than an empty stomach alone. Internal signals — stomach contractions, low blood glucose, low protein levels, and the amount of fat stored in the body — combine with a hungry liver's own nerve signals to the brain, and with external cues such as the smell, taste or appearance of food, or simply watching someone else eat. None of these signals alone fully accounts for the feeling of hunger; together they are regulated by a feeding-satiety system centred on the hypothalamus, the liver, and other parts of the body, working alongside cues in the environment.

Thirst works in a broadly similar way. A dry mouth and throat, caused by fluid deprivation, prompts drinking, but a dry mouth by itself doesn't fully explain thirst — the deeper trigger lies within the body's own fluid balance. When body fluids are lost, water moves out of the cells; this cellular dehydration, along with a drop in blood volume, is detected by 'osmoreceptors' — specialised nerve cells in the anterior hypothalamus — which then generate the nerve impulses that motivate drinking. …