When the original instinct theory of motivation collapsed, it was replaced by drive-reduction theory—the idea that a physiological need (such as for food or water) creates an aroused state that drives the organism to reduce the need. With few exceptions, when a physiological need increases, so does a psychological drive—an aroused, motivated state.

The physiological aim of drive reduction is homeostasis—the maintenance of a steady internal state. An example of homeostasis (literally “staying the same”) is the body’s temperature-regulation system, which works like a room’s thermostat. Both systems operate through feedback loops: Sensors feed room temperature to a control device. If the room’s temperature cools, the control device switches on the furnace. Likewise, if our body’s temperature cools, our blood vessels constrict to conserve warmth, and we feel driven to put on more clothes or seek a warmer environment (FIGURE 33.1).

Not only are we pushed by our need to reduce drives, we also are pulled by incentives—positive or negative environmental stimuli that lure or repel us. This is one way our individual learning histories influence our motives. Depending on our learning, the aroma of good food, whether fresh roasted peanuts or toasted ants, can motivate our behavior. So can the sight of those we find attractive or threatening.

When there is both a need and an incentive, we feel strongly driven. The food-deprived person who smells pizza baking feels a strong hunger drive and the baking pizza becomes a compelling incentive. For each motive, we can therefore ask, “How is it pushed by our inborn physiological needs and pulled by incentives in the environment?”