Regret is a negative conscious and emotional reaction to personal past acts and behaviors. Regret is often expressed by the term "sorry" whereas "I'm sorry" can express both regret and sympathy. Regret is often a feeling of sadness, shame, embarrassment, depression, annoyance, or guilt, after one acts in a manner and later wishes not to have done so. Regret is distinct from guilt, which is a deeply emotional form of regret — one which may be difficult to comprehend in an objective or conceptual way. In this regard, the concept of regret is subordinate to guilt in terms of its emotional intensity. By comparison, shame typically refers to the social (rather than personal) aspect of guilt or (in minor context) regret as imposed by the society or culture (enforcement of ethics, morality), which has substantial bearing in matters of (personal and social) honor.
It is also distinct from remorse, which is a more direct and emotional form of regret over a past action that is considered by society to be hurtful, shameful, or violent. Unlike regret, it includes a strong element of desire for apology to others rather than an internal reflection on one's actions, and may be expressed (sincerely or not) in order to reduce the punishment one receives.
Regret can describe not only the dislike for an action that has been committed, but also, importantly, regret of inaction. Many people find themselves wishing that they had done something in a past situation.
There are extremely specific models of regret mostly in economics and finance under a new field called behavioral economics. Of these, the most clearly emotional is buyer's remorse, also called buyer's regret. Other examples include regret aversion or more generally, regret (decision theory).
Existential regret has been specifically defined as 'a profound desire to go back and change a past experience in which one has failed to choose consciously or has made a choice that did not follow one’s beliefs, values, or growth needs'.
Meta-analysis involving what we regret most has concluded that overall, Americans regret choices regarding their education the most. Subsequent rankings then include decisions about career, romance, and parenting. Education was the forerunner of regret in a number of different studies. This finding can be attributed to the opportunity principle.
Opportunity principle defines people's biggest regrets as those marked by the greatest opportunity for corrective action. When the opportunity to improve conditions is nonexistent, cognitive processes proceed to mitigate regret. Education is the forerunner of what we regret most because it is seen as something where circumstances could be changed: "In contemporary society, education is open to continual modification throughout life. With the rise of community colleges and student aid programs in recent decades, education of some sort is accessible to nearly all socioeconomic groups." 
Regret pushes people toward revised decision making and corrective action that often bring improvement in life circumstances. A study measured regret in accordance to negative reviews with service providers. It was concluded that regret was an accurate predictor of who switched providers. Regret can be seen as an evolutionary development. As more intense regret is experienced, the likelihood of initiating change is increased. Consequently, the more opportunity of corrective action available, the larger the regret felt and the more likely corrective action is achieved. People learn from their mistakes.
The lost opportunity principle
In response to the opportunity principle, the lost opportunity principle directly opposes its views. The lost opportunity principle states that regret should intensify, not diminish, when people feel that they could have made better choices in the past but now perceive limited opportunities to take corrective action in the future. "People who habitually consider future consequences (and how they may avoid future negative outcomes) experience less, rather than more, intense regret after a negative outcome."  This principle offers another reason as to why education is the most regretted aspect in life. Education becomes a more limited opportunity as time passes. Aspects such as making friends, becoming more spiritual, and community involvement tend to be less regrettable which makes sense because these are also aspects in life that do not become limited opportunities. As the opportunity to remedy a situation passes, feelings of hopelessness may increase. An explanation of the Lost Opportunity Principle can be seen as a lack of closure. Low closure makes past occurrences feel unresolved. Low closure is associated with "reductions in state self-esteem and persistent negative affect over time". Because high closure is associated with acceptance of lost opportunity, low closure is then associated with the realization and regret of lost opportunity.
The lost opportunity principle suggests that regret does not serve as a corrective motive (which the opportunity principle suggests). Instead, regret serves as a more general reminder to seize the day. Feeling regret will spur future action to make sure other opportunities are taken so that regret will not be experienced again.
People appear to overestimate how much regret they will feel in the future. In particular, anticipated regret (how much regret one thinks one will feel) appears to be overestimated for actions and choices. This appears to be, in part, due to a tendency to underestimate the extent to which people attribute bad outcomes to external factors than to internal factors (i.e., themselves).
A study conducted in 2014 by neuroscientists based at the University of Minnesota show that rats are capable of feeling regret about their own actions. This emotion had never previously been found in any other mammals apart from humans. The study reported in the journal Nature Neuroscience, found that rats expressed regret through both their behavior and their neural activity. Those signals were specific to situations the researchers set up to induce regret, which led to specific neural patterns in the brain and in behavior.
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