Alcohol's positive and negative motivational effects are believed to be important influences on alcohol-seeking behavior and, therefore, key factors among the many and varied causes of alcohol abuse and dependence. Alcohol's positive effects, such as enhanced mood, and negative effects, such as hangover, are considered important factors in motivating drinkers to increase or decrease their drinking. Scientists have developed a variety of animal behavioral models to study alcohol's motivational effects. These models include "self-administration models," in which the animal controls the exposure to alcohol, and "conditioning models," in which the researcher controls the animal's exposure to alcohol. Such models have been used to study the influence of genetic differences on sensitivity to alcohol's positive and negative motivational effects, the brain mechanisms underlying alcohol's motivational effects, as well as relapse and craving.