Short answers, pulled from the story.
Happiness has no single, universally accepted definition and is described as a polyseme and a fuzzy concept. It is used two main ways: the current experience of an emotion such as joy, which Daniel Kahneman called "what I experience here and now," and the appraisal of life satisfaction, which Ruut Veenhoven called the "overall appreciation of one's life as-a-whole."
Happiness is typically measured using self-report surveys and several established scales, including the Subjective Happiness Scale from 1999, the Positive and Negative Affect Schedule from 1988, and the Satisfaction with Life Scale developed by Ed Diener. The World Happiness Report, published since 2012, uses the Cantril ladder method, which asks people to rate their life from 0 to 10.
Sonja Lyubomirsky has estimated that 50 percent of a person's happiness level could be genetically determined, 10 percent is affected by life circumstances, and the remaining 40 percent is subject to self-control. Heritability estimates vary widely by study, ranging from 12 to 18 percent in one study of 11,500 genotypes up to 70 to 90 percent when subjective well-being is measured as a trait.
Research by George Vaillant and Robert J. Waldinger at Harvard found that the happiest and healthiest people reported strong interpersonal relationships, and that good relationships and mental health contribute more to happiness than income does. A 2023 meta analysis found reasonably solid evidence that expressing gratitude and enhancing sociability improve well-being, but no convincing evidence that exercise, mindfulness, or walks in nature do.
Aristotle, in the Nicomachean Ethics written in 350 BCE, said happiness is the only thing humans desire for its own sake, unlike riches, honour, or health. He used the Greek term eudaimonia, treating it as an activity rather than an emotion, and concluded through his Function Argument that the happy life is the life of excellent rational activity.
Friedrich Nietzsche critiqued the English Utilitarians, stating that "Man does not strive for happiness, only the Englishman does," and arguing that making happiness one's ultimate goal "makes one contemptible." He introduced the figure of the "last man" to represent people who seek only pleasure and safety while avoiding struggle, and instead valued what can be earned through difficulty and pain.
June Gruber argued that happiness may make a person more sensitive, more gullible, less successful, and more likely to undertake high-risk behaviours. Iris Mauss has shown that the more people strive for happiness, the more likely they set standards too high and feel disappointed, and a 2012 study found psychological well-being was higher for people who experienced both positive and negative emotions.