Guilt is a cognitive or an emotional experience that occurs when a person realizes or believes - whether justified or not - that he or she has violated a moral standard, and is responsible for that violation. It is closely related to the concept of remorse.
In psychology, as well as in ordinary language, guilt is an affective state in which one experiences conflict at having done something that one believes one should not have done (or conversely, having not done something one believes one should have done). It gives rise to a feeling which does not go away easily, driven by 'conscience'. Sigmund Freud described this as the result of a struggle between the ego and the superego parental imprinting. Freud, an atheist, rejected the role of God as punisher in times of illness or rewarder in time of wellness. While removing one source of guilt from patients, he ironically added another. This was the unconscious force within the individual that may contribute to illness and also to the kind of so called accident that, until then had been attributed to God's will or simply bad luck. Today, as a result of Freud's views, even the victim of someone else's accident or bad luck may be offered criticism rather than comfort. The theory is that the victim may be at fault for having attracted the other person's hostility. Guilt and its causes, merits, and demerits are common themes in psychology and psychiatry. It is often associated with anxiety, and sometimes depression. The philosopher Martin Buber underlined the difference between the Freudian notion of guilt, based on internal conflicts, and existential guilt, based on actual harm done to others.
Guilt has been used as a tool of social control, because people who feel guilty are less likely to assert their rights. Some evolutionary psychologists theorize that guilt and shame helped maintain beneficial relationships, such as reciprocal altruism. If a person feels guilty when he harms another, or even fails to reciprocate kindness, he is more likely not to harm others or become too selfish. In this way, he reduces the chances of retaliation by members of his tribe, and thereby increases his survival prospects, and those of the tribe or group. As with any other emotion, guilt can be manipulated to control or influence others. As a highly social animal living in large groups that are relatively stable, we need ways to deal with conflicts and events in which we inadvertently or purposefully harm others. If someone harms another, and then feels guilt and demonstrates regret and sorrow, the person harmed is likely to forgive.
Thus, guilt makes it possible to forgive, and helps hold the social group together. Guilt is founded on our empathy system and mirror neurons. When we see another carrying out an action, we carry out the action ourselves in neuronal activity, though not in overt action. The neurons that mirror others are called "mirror neurons". When we see another person suffering, we can feel their suffering as if it is our own. This constitutes our powerful system of empathy, which leads to our thinking that we should do something to relieve the suffering of others. If we cannot help another, or fail in our efforts, we experience feelings of guilt. From the perspective of group selection, groups that are made up of a high percent of co-operators outdo groups with a low percent of co-operators in between-group competition. People who are more prone to high levels of empathy-based guilt may be likely to suffer from anxiety and depression; however, they are also more likely to cooperate and behave altruistically. This suggests that guilt-proneness may not always be beneficial at the level of the individual, or within-group competition, but highly beneficial in between-group competition.
Another common notion is that guilt is assigned by social processes, such as a jury trial; i.e. that it is a strictly legal concept. Thus, the ruling of a jury that O.J. Simpson or Julius Rosenberg was "guilty" or "not guilty" is taken as an actual judgment by the whole society that they must act as if they were so. By corollary, the ruling that such a person is "not guilty" may not be so taken, due to the asymmetry in the assumption that one is assumed innocent until proven guilty, and prefers to take the risk of freeing a guilty party over convicting innocents. Still others -- often, but not always, theists of one type or another -- believe that the origin of guilt comes from violating universal principles of right and wrong. In most instances, people who believe this also acknowledge that even though there is proper guilt from doing 'wrong' instead of doing 'right,' people endure all sorts of guilty feelings which do not stem from violating universal moral principles.
According to the Myers Briggs indicator, guilt occurs heavily in INF types, who normally hold themselves to very high moral standards. (source: www.personalitypage.com).
Collective guilt, like guilt, is the unpleasant emotional reaction that results among a group of individuals when it is perceived that the group illegitimately harmed members of another group. It is often the result of “sharing a social identity with others whose actions represent a threat to the positivity of that identity”. Different intergroup inequalities can result in collective guilt; such as receiving unearned benefits and privileges or inflicting more extreme forms of harm on an outgroup (including genocide). Individuals are generally motivated to avoid collective guilt in order to maintain a positive social identity. There are many ways of decreasing collective guilt, such as denying harm or justifying actions. Collective guilt can also lead to positive outcomes; such as promoting intergroup reconciliation and reducing negative attitudes towards the outgroup.
There are several causes of collective guilt, salient group identity, collective responsibility, and perception of unjust ingroup actions.In order for an individual to experience collective guilt, they must identify themselves as a part of the ingroup. “This produces a perceptual shift from thinking of oneself in terms of ‘I’ and ‘me’ to ‘us’ or ‘we’” Only when an individual is salient with the ingroup can they perceive responsibility for the harmful actions of the group, past and present.In addition to ingroup salience, an individual will only feel collective guilt if they view the ingroup as responsible for the harmful actions done to the outgroup. “For instance, in two studies, racial inequality in the US was framed as either “Black Disadvantage” or “White Privilege”. When the term “black disadvantage” was used to describe racial inequality, white participants felt less collectively responsible for the harm done to the outgroup, which lessened collective guilt. In comparison, when “white privilege” was used white participants felt more collectively responsible for the harm done, which increased collective guilt.
Lastly, an individual has to believe the actions caused by the ingroup were unjustifiable, indefensible and unforgivable. If an individual can justify the actions of the ingroup this will lessen collective guilt. Only when an individual views the ingroup actions as reprehensible will that individual feel collective guilt. Collective guilt is not only a result of feeling empathy for the outgroup. It can also be caused by self-conscious emotion that stems from the questioning of the morality of the ingroup.
There are various methods of reducing collective guilt. Some of these methods are denying ingroup’s harmful actions, denying responsibility, claiming actions by ingroup were just, and focusing on positive aspects caused by the harmful action. First, by denying the ingroup’s harmful actions, or downplaying the severity of the harm, the effect of collective guilt is lessened. If the individual or group can neglect to observe the harm caused by their actions, either consciously or unconsciously, then the individual will not feel collective guilt. If a person does not feel that the ingroup is responsible for the harm caused by actions, collective guilt will be lessened. Additionally, if a person believes that only individuals are responsible for their own actions, and not a collective group, than they can deny the existence of collective responsibility, thereby reducing feelings of collective guilt. An individual can rationalize the actions of the ingroup. If the individual believes that there were just reasons for the harm inflicted, collective guilt is likely to be reduced. For instance, outgroup dehumanization is one effective means towards justifying the ingroup’s actions. By focusing on the positive aspects of the ingroup’s actions rather than the harmful effects, collective guilt can be reduced. For instance, an individual or group may choose to focus on the benefits of high levels of production and consumption, and not on its harmful effects on the environment.
Traditional Japanese society, Korea society and Ancient Greek society are sometimes said to be "shame-based" rather than "guilt-based", in that the social consequences of "getting caught" are seen as more important than the individual feelings or experiences of the agent. This may lead to more of a focus on etiquette than on ethics as understood in Western civilization. This has led some[who?] in Western civilizations to question why the word ethos was adapted from Ancient Greek with such vast differences in cultural norms. Christianity and Islam inherit most notions of guilt from Judaism, Persian and Roman, mostly as interpreted through Augustine, who adapted Plato's ideas to Christianity. The Latin word for guilt is culpa, a word sometimes seen in law literature, for instance in mea culpa meaning "my fault (guilt)".
Guilt was a main theme in John Steinbeck's East of Eden, Fyodor Dostoevsky's Crime and Punishment, Tennessee Williams' A Streetcar Named Desire, William Shakespeare's play Macbeth, Edgar Allan Poe's "The Tell-Tale Heart", and many other works of literature. It was a major theme in many works by Nathaniel Hawthorne, and is an almost universal concern of novelists who explore inner life and secrets.
Psychopaths typically lack any sense of guilt or remorse for harm they may have caused to others. Instead, they rationalize their behavior, blame someone else, or deny it outright. This is seen by psychologists as part of a lack of moral reasoning (in comparison with the majority of humans), an inability to evaluate situations in a moral framework, and an inability to develop emotional bonds with other people.
Guilt can sometimes be remedied by: punishment (a common action and advised or required in many legal and moral codes); forgiveness (as in transformative justice); or sincere remorse (as with confession in Catholicism or restorative justice). Guilt can also be remedied through intellectualization or cognition  (the understanding that the source of the guilty feelings was illogical or irrelevant). Law does not usually accept the agent's self-punishment, but some ancient codes did: in Athens, the accused was permitted to propose his or her own remedy, which might in fact be a reward, while the accuser proposed another, and the jury chose something in-between. This forced the accused to effectively bet on his support in the community, as Socrates did when he proposed "room and board in the town hall" as his fate. He lost, and drank hemlock, a poison, as advised by his accuser.
The evolutionary soil in the mind of man in which the seed of revealed religion germinates is the moral nature that so early gives origin to a social consciousness. The first promptings of a child's moral nature have not to do with sex, guilt, or personal pride, but rather with impulses of justice, fairness, and urges to kindness--helpful ministry to one's fellows. And when such early moral awakenings are nurtured, there occurs a gradual development of the religious life which is comparatively free from conflicts, upheavals, and crises. 
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- see cognitive therapy under