Happiness is a state of mind or feeling characterized by contentment, love, satisfaction, pleasure, or joy. A variety of biological, psychological, religious, and philosophical approaches have striven to define happiness and identify its sources.
While direct measurement of happiness presents challenges, tools such as The Oxford Happiness Questionnaire have been developed by researchers. Positive psychology researchers use theoretical models that include describing happiness as consisting of positive emotions and positive activities, or that describe three kinds of happiness: pleasure, engagement, and meaning.
Research has identified a number of attributes that correlate with happiness: relationships and social interaction, extraversion, marital status, employment, health, democratic freedom, optimism, endorphins released through physical exercise and eating chocolate, religious involvement, income and proximity to other happy people.
Philosophers and religious thinkers often define happiness in terms of living a good life, or flourishing, rather than simply as an emotion. Happiness in this older sense was used to translate the Greek Eudaimonia, and is still used in virtue ethics.
Happiness economics suggests that measures of public happiness should be used to supplement more traditional economic measures when evaluating the success of public policy.
The evolutionary perspective offers an alternative approach to understand what happiness or quality of life is about. Briefly, the questions to be answered are: What features are included in the brain that allows humans to distinguish between positive and negative states of mind, and how do these features improve the survivability of humans? Answering these questions points towards an understanding of what happiness is about and how to best exploit the capacities of the brain with which humans are endowed. The perspective is presented in detail by the evolutionary biologist Bjørn Grinde in his book Darwinian Happiness, as well as in a more formal way.
Some researchers, such as David T. Lykken, have found that about 50% of one's happiness depends on one's genes, based on studying identical twins, whose happiness is 50% correlated even when growing up in different houses. About 10% to 15% is a result of various measurable life circumstances variables, such as socioeconomic status, marital status, health, income, sex and others. The remaining 40% is a combination of unknown factors and the results of actions that individuals deliberately engage in to become happier. These actions may vary between persons; extroverts, for example, may benefit from placing themselves in situations involving large amounts of human interaction. Also, exercise has been shown to increase one's level of momentary subjective well-being significantly.
Michael Argyle developed the Oxford Happiness Questionnaire as a broad measure of psychological well-being. This has been criticized as an aggregate of self-esteem, sense of purpose, social interest and kindness, sense of humor and aesthetic appreciation.
Though it may be impossible to achieve any comprehensive measure of happiness objectively, some physiological correlates to happiness can be measured through a variety of techniques. Stefan Klein, in his book The Science of Happiness, links the dynamics of neurobiological systems (i.e., dopaminergic, opiate) to the concepts and findings of positive psychology and social psychology.
Human relationships are consistently found to be the most important correlation with human happiness.
A widely-publicized study from 2008 in the British Medical Journal reported that happiness in social networks may spread from person to person. Researchers followed nearly 5000 individuals for 20 years in the long-standing Framingham Heart Study and found clusters of happiness and unhappiness that spread up to 3 degrees of separation on average. Happiness tended to spread through close relationships like friends, siblings, spouses, and next-door neighbors, and the researchers reported that happiness spread more consistently than unhappiness through the network. Moreover, the structure of the social network appeared to have an impact on happiness, as people who were very central (with many friends and friends of friends) were significantly more likely to be happy than those on the periphery of the network. Overall, the results suggest that happiness might spread through a population like a virus.
Research in the US has found that older Americans are generally happier than younger adults. The effect does not appear to be generational, because longitudinal research found that happiness increased over time for the older people who were studied. While older individuals reported more health problems, they reported fewer problems overall. Young adults reported more anger, anxiety, depression, financial problems, troubled relationships and career stress.
Studies are contradictory as to whether parents are more likely to report being happier than non-parents.  One study found having up to three children increased happiness among married couples, but not among other groups with children.
Happiness is also correlated with the ability to "rationalize or explain" social and economic inequalities.
There is now extensive research suggesting that religious people are happier and less stressed. It is not clear, however, whether this is because of the social contact and support that result from religious activities, the greater likelihood of behaviors related to good health (such as less substance abuse), indirect forms of psychological and social activity such as optimism and volunteering, psychological factors such as "reason for being," learned coping strategies that enhance one's ability to deal with stress, or some combination of these and/or other factors.
Surveys by Gallup, the National Opinion Research Center and the Pew Organization conclude that spiritually committed people are twice as likely to report being "very happy" than the least religiously committed people. An analysis of over 200 social studies contends that "high religiousness predicts a lower risk of depression and drug abuse and fewer suicide attempts, and more reports of satisfaction with sex life and a sense of well-being," and a review of 498 studies published in peer-reviewed journals concluded that a large majority of them showed a positive correlation between religious commitment and higher levels of perceived well-being and self-esteem and lower levels of hypertension, depression, and clinical delinquency. A meta-analysis of 34 recent studies published between 1990 and 2001 found that religiosity has a salutary relationship with psychological adjustment, being related to less psychological distress, more life satisfaction, and better self-actualization. Finally, a recent systematic review of 850 research papers on the topic concluded that "the majority of well-conducted studies found that higher levels of religious involvement are positively associated with indicators of psychological well-being (life satisfaction, happiness, positive affect, and higher morale) and with less depression, suicidal thoughts and behavior, drug/alcohol use/abuse."
The individual level of happiness and religiosity correlations show up when measuring within the United States, a predominantly religious country where people without religion are outsiders. According to a 2007 paper by Liesbeth Snoep in the Journal of Happiness Studies, there is no significant correlation between religiosity and individual happiness in Netherlands and Denmark, countries that have lower rates of religion than the United States so that being without religion is normal. When measuring between countries, the least religious industrialized countries such as in northern Europe have a much higher happiness than the most religious industrialized country, the US, so cross country comparisons on religiosity and happiness seem to show a societal level correlation of increased secularization and decreased religiosity to increased happiness. It may be simply that non-religious people are less happy in a religious country, but everyone is happier in more secular, less religious countries.
Happiness forms a central theme of Buddhist teachings. For ultimate freedom from suffering, the Eightfold Path leads its practitioner to Nirvana, a state of everlasting peace. Ultimate happiness is only achieved by overcoming craving in all forms. More mundane forms of happiness, such as acquiring wealth and maintaining good friendships, are also recognized as worthy goals for lay people (see sukha). Buddhism also encourages the generation of loving kindness and compassion, the desire for the happiness and welfare of all beings. According to the Buddha, "Mind is the forerunner of states of existence. Mind is chief, and (those states) are caused by the mind. If one speaks and acts with a pure mind, surely happiness will follow like one's own shadow!" In Buddhism, the third of the Four Noble Truths states "to eliminate suffering, eliminate craving," thus establishing happiness as beyond material and emotional possession and attainable only through an attentive practice leading to extinguishing of craving and aversion. In SGI Buddhism happiness is obtained by getting yourself aligned with the universe by chanting Nam-myho-renge-kyo to overcome your sufferings and that of others. It teaches that one must never look outside of oneself for happiness.
In Catholicism, the ultimate end of human existence consists in felicity (Latin equivalent to the Greek eudaimonia), or "blessed happiness", described by the thirteenth-century philosopher-theologian Thomas Aquinas as a Beatific Vision of God's essence in the next life. According to Augustine's Confessions, he lived much of his life without God. He sinned much and recognized his sinfulness. As a youth, he sinned for its own sake, and later, in the pursuit of a perceived good. When he lost a dear friend to death, it troubled him greatly, and he turned to God for answers. He turned to God to find true happiness and was converted to Christianity. He found that true happiness can only come from a relationship with God and appreciating God's creation for His sake, not its own.
The Chinese Confucian thinker Mencius, who 2300 years ago sought to give advice to the ruthless political leaders of the warring states period, was convinced that the mind played a mediating role between the "lesser self" (the physiological self) and the "greater self" (the moral self) and that getting the priorities right between these two would lead to sagehood. He argued that if we did not feel satisfaction or pleasure in nourishing one's "vital force" with "righteous deeds", that force would shrivel up (Mencius,6A:15 2A:2). More specifically, he mentions the experience of intoxicating joy if one celebrates the practice of the great virtues, especially through music.
Al-Ghazali (1058-1111) the Muslim Sufi thinker wrote the Alchemy of Happiness, a manual of spiritual instruction throughout the Muslim world and widely practiced even now.
In the Nicomachean Ethics, written in 350 B.C.E., Aristotle stated that happiness (also being well and doing well) is the only thing that humans desire for its own sake, unlike riches, honor, health or friendship. He observed that men sought riches, or honor, or health not only for their own sake but also in order to be happy. Note that eudaimonia, the term we translate as "happiness", is for Aristotle an activity rather than an emotion or a state. Happiness is characteristic of a good life, that is, a life in which a person fulfills human nature in an excellent way. People have a set of purposes which are typically human: these belong to our nature. The happy person is virtuous, meaning they have outstanding abilities and emotional tendencies which allow him or her to fulfill our common human ends. For Aristotle, then, happiness is "the virtuous activity of the soul in accordance with reason": happiness is the practice of virtue.
Many ethicists make arguments for how humans should behave, either individually or collectively, based on the resulting happiness of such behavior. Utilitarians, such as John Stuart Mill and Jeremy Bentham, advocated the greatest happiness principle as a guide for ethical behavior.
Common market health measures such as GDP and GNP have been used as a measure of successful policy. On average richer nations tend to be happier than poorer nations, but this effect seems to diminish with wealth. This has been explained by the fact that the dependency is not linear but logarithmic, i.e., the same percentual increase in the GNP produces the same increase in happiness for wealthy countries as for poor countries. 
Well-known philosopher and author, Dev K., stated in the prologue of his book "Can money buy happiness?"(2010) as follows: "Can money buy happiness? No, because “happiness” is not an entity that is produced and sold but rather a mental and emotional state. Happiness, however, is a spectrum and money can expand on existing happiness. Even in people without existing happiness, money has potential to lead to opportunity for happiness. One thing is for certain – money cannot make you depressed". This view has been seen by some as the perfect answer to this age-old debate, but also deemed non-sensical by others.
Economic freedom correlates strongly with happiness while social security not at all, and socialist East European countries were less happy than Western ones, even less happy than other equally poor countries.
It has been argued that happiness measures could be used not as a replacement for more traditional measures, but as a supplement. According to professor Edward Glaeser, people constantly make choices that decrease their happiness, because they have also more important aims. Therefore, the government should not decrease the alternatives available for the citizen by patronizing them but let the citizen keep a maximal freedom of choice. 
Reported in Josiah Hotchkiss Gilbert, Dictionary of Burning Words of Brilliant Writers (1895).