"Hedonic treadmill" is a term coined by Brickman and Campbell in their article, "Hedonic Relativism and Planning the Good Society" (1971), describing the tendency of people to keep a fairly stable baseline level of happiness despite external events and fluctuations in demographic circumstances. The idea of relative happiness had been around for decades when in 1978 Brickman, et al., began to approach hedonic pleasure within the framework of Helson's adaptation level theory, which holds that perception of stimulation is dependent upon comparison of former stimulations. The hedonic treadmill functions similarly to most adaptations that serve to protect and enhance perception. In the case of hedonics, the sensitization or desensitization to circumstances or environment can redirect motivation. This reorientation functions to protect against complacency, but also to accept unchangeable circumstances, and redirect efforts towards more effective goals. Frederick and Lowenstein classify three types of processes in hedonic adaptation: shifting adaptation levels, desensitization, and sensitization. Shifting adaptation levels occurs when a person experiences a shift in what is perceived as a "neutral" stimulus, but maintains sensitivity to stimulus differences. For example, if Sam gets a raise he will initially be happier, and then habituate to the larger salary and return to his happiness set point. But he will still be pleased when he gets a holiday bonus. Desensitization decreases sensitivity in general, which reduces sensitivity to change. Those who have lived in war zones for extended periods of time may become desensitized to the destruction that happens on a daily basis, and be less affected by the occurrence of serious injuries or losses that may once have been shocking and upsetting. Sensitization is an increase of hedonic response from continuous exposure, such as the increased pleasure and selectivity of connoisseurs for wine, or food.