in discounting curves are affected by personality characteristics such as self-reports of impulsivity and locus of control; personal characteristics such as age, gender, IQ, race, and culture; socioeconomic characteristics such as income and education; and many other variables. to drug addiction. Lesions of the nucleus accumbens core subregion or basolateral amygdala produce shifts towards choosing the smaller-sooner reward, suggesting the involvement of these brain regions in the preference for delayed reinforcers. There is also evidence that the orbitofrontal cortex is involved in delay discounting, although there is currently debate on whether lesions in this region result in more or less impulsivity. Economic theory suggests that optimal discounting involves the exponential discounting of value over time. This model assumes that people and institutions should discount the value of rewards and punishments at a constant rate according to how delayed they are in time. While economically rational, recent evidence suggests that people and animals do not discount exponentially. Many studies suggest that humans and animals discount future values according to a hyperbolic discounting curve where the discount factor decreases with the length of the delay (for example, waiting from today to tomorrow involves more loss of value than waiting from twenty days to twenty-one days). Further evidence for non-constant delay discounting is suggested by the differential involvement of various brain regions in evaluating immediate versus delayed consequences. Specifically, the prefrontal cortex is activated when choosing between rewards at a short delay or a long delay, but regions associated with the dopamine system are additionally activated when the option of an immediate reinforcer is added. Additionally, intertemporal choices differ from economic models because they involve anticipation (which may involve a neurological "reward" even if the reinforcer is delayed), self-control (and the breakdown of it when faced with temptations), and representation (how the choice is framed may influence desirability of the reinforcer), none of which are accounted for by a model that assumes economic rationality. One facet of intertemporal choice is the possibility for preference reversal, when a tempting reward becomes more highly valued than abstaining only when immediately available. For example, when sitting home alone, a person may report that they value the health benefit of not smoking a cigarette over the effect of smoking one. However, later at night when the cigarette is immediately available, their subjective value of the cigarette may rise and they may choose to smoke it. A theory