Gratitude, thankfulness, or appreciation is a positive emotion or attitude in acknowledgment of a benefit that one has received or will receive. The experience of gratitude has historically been a focus of several world religions, and has been considered extensively by moral philosophers such as Adam Smith. The systematic study of gratitude within psychology only began around the year 2000, possibly because psychology has traditionally been focused more on understanding distress rather than understanding positive emotions. However, with the advent of the positive psychology movement, gratitude has become a mainstream focus of psychological research. The study of gratitude within psychology has focused on the understanding of the short term experience of the emotion of gratitude (state gratitude), individual differences in how frequently people feel gratitude (trait gratitude), and the relationship between these two aspects.