Subjective well-being (SWB) refers to how people experience the quality of their lives and includes both emotional reactions and cognitive judgments. Psychologists have defined happiness as a combination of life satisfaction and the relative frequency of positive and negative  affect. SWB therefore encompasses moods and emotions as well as evaluations of one's satisfaction with general and specific areas of one's life. Concepts encompassed by SWB include positive and negative affect, happiness, and life satisfaction. Positive psychology is particularly concerned with the study of SWB. SWB tends to be stable over time and is strongly related to personality traits. There is evidence that health and SWB may mutually influence each other, as good health tends to be associated with greater happiness, and a number of studies have found that positive emotions and optimism can have a beneficial influence on health.