Affective forecasting, also known as hedonic forecasting or the hedonic forecasting mechanism, is the prediction of one's affect (emotional state) in the future. As a process that influences preferences, decisions, and behavior, affective forecasting is studied by both psychologists and economists, with broad applications.

In The Theory of Moral Sentiments (1759), Adam Smith observed the personal challenges, and social benefits, of hedonic forecasting errors:

[Consider t]he poor man's son, whom heaven in its anger has visited with ambition, when he begins to look around him, admires the condition of the rich …. and, in order to arrive at it, he devotes himself for ever to the pursuit of wealth and greatness…. Through the whole of his life he pursues the idea of a certain artificial and elegant repose which he may never arrive at, for which he sacrifices a real tranquillity that is at all times in his power, and which, if in the extremity of old age he should at last attain…, he will find to be in no respect preferable to that humble security and contentment which he had abandoned for it. It is then, in the last dregs of life, his body wasted with toil and diseases, his mind galled and ruffled by the memory of a thousand injuries and disappointments..., that he begins at last to find that wealth and greatness are mere trinkets of frivolous utility….

[Yet] it is well that nature imposes upon us in this manner. It is this deception which rouses and keeps in continual motion the industry of mankind.

In the early 1990s, Kahneman and Snell began research on hedonic forecasts, examining its impact on decision making. The term "affective forecasting" was later coined by psychologists Timothy Wilson and Daniel Gilbert. Early research tended to focus solely on measuring emotional forecasts, while subsequent studies began to examine the accuracy of forecasts, revealing that people are surprisingly poor judges of their future emotional states. For example, in predicting how events like winning the lottery might affect their happiness, people are likely to overestimate future positive feelings, ignoring the numerous other factors that might contribute to their emotional state outside of the single lottery event. Some of the cognitive biases related to systematic errors in affective forecasts are focalism, hot-cold empathy gap, and impact bias.

While affective forecasting has traditionally drawn the most attention from economists and psychologists, their findings have in turn generated interest from a variety of other fields, including happiness research, law, and health care. Its effect on decision-making and well-being is of particular concern to policy-makers and analysts in these fields, although it also has applications in ethics. For example, one's tendency to underestimate one's ability to adapt to life-changing events has led to legal theorists questioning the assumptions behind tort damage compensation. Behavioral economists have incorporated discrepancies between forecasts and actual emotional outcomes into their models of different types of utility and welfare. This discrepancy also concerns healthcare analysts, in that many important health decisions depend upon patients' perceptions of their future quality of life.

Affective forecasting can be divided into four components: predictions about valence (i.e. positive or negative), the specific emotions experienced, their duration, and their intensity. While errors may occur in all four components, research overwhelmingly indicates that the two areas most prone to bias, usually in the form of overestimation, are duration and intensity. Immune neglect is a form of impact bias in response to negative events, in which people fail to predict how much their recovery will be hastened by their psychological immune system. The psychological immune system is a metaphor "for that system of defenses that helps you feel better when bad things happen", according to Gilbert. On average, people are fairly accurate about predicting which emotions they will feel in response to future events. However, some studies indicate that predicting specific emotions in response to more complex social events leads to greater inaccuracy. For example, one study found that while many women who imagine encountering gender harassment predict feelings of anger, in reality, a much higher proportion report feelings of fear. Other research suggests that accuracy in affective forecasting is greater for positive affect than negative affect, suggesting an overall tendency to overreact to perceived negative events. Gilbert and Wilson posit that this is a result of the psychological immune system.

While affective forecasts take place in the present moment, researchers also investigate its future outcomes. That is, they analyze forecasting as a two-step process, encompassing a current prediction as well as a future event. Breaking down the present and future stages allow researchers to measure accuracy, as well as tease out how errors occur. Gilbert and Wilson, for example, categorize errors based on which component they affect and when they enter the forecasting process. In the present phase of affective forecasting, forecasters bring to mind a mental representation of the future event and predict how they will respond emotionally to it. The future phase includes the initial emotional response to the onset of the event, as well as subsequent emotional outcomes, for example, the fading of the initial feeling.