Differences in patterns over time
Personality change refers to the different forms of change in various aspects of personality. These changes include how we experience things, how our perception of experiences changes, and how we react in situations. An individual's personality may stay somewhat consistent throughout their life. Still, more often than not, everyone undergoes some form of change to their personality in their lifetime.
Personality refers to individual differences in characteristic thinking, feeling, and behavior patterns.
Every person has their own "individual differences in particular personality characteristics" that separate them from others. The overall study of personality focuses on two broad areas: understanding individual differences in personality characteristics, and understanding how the various parts of a person come together as a whole.
Social interactions, age, personal experiences, and significant events (especially traumatic events) can all alter a person's perceptions and cause their personality to change.
Each person has their own unique personality, and as a result, the many differences and changes that occur, may be confusing. Even psychologists are still studying and researching to fully understand what personality means and why personality changes. The development of personality is often dependent on the stage of life a person is in. Most development occurs in the earlier stages of life and becomes more stable as one grows into adulthood.
While still uncertain, research suggests that genetics play a role in the change and stability of certain traits in a personality. They have also discovered that environmental sources affect personality too. The debate over nature versus nurture have pervaded the field of psychology since its beginning. Cultural is also a large factor in personality trait differences as well.
Definition of personality.
Personality, one's characteristic way of feeling, behaving and thinking, is often conceptualized as a person's standing on each Big Five personality trait (extraversion, neuroticism, openness to experience, agreeableness and conscientiousness). A person's personality profile is thus gauged from their standing on five broad concepts which predict, among other life outcomes, behavior and the quality of interpersonal relationships. Initially, it was believed that one's Big Five profile was static and dichotomous in that one was either at one extreme of each trait or another For example, people are typically categorized as introverted or extraverted. Personality was therefore assessed in terms of generalities or averages. Some researchers also noticed significant inconsistencies in how people behaved across situations, challenging the traditional notion of stable personality traits.
This school of thought attributes human behavior to environmental factors, relegating individual differences to situational artifacts and contesting the existence of individual predispositions. It was led by situationists like Walter Mischel (1968). Their contention held that personality was a fictitious concept. For them, the discrepancies observed across one's behaviors were evidence that inter-individual differences did not exist
Some aspects of the situationist perspective even suggest that all human beings are the same and that the differences we observe are simply illusory byproducts of the environment. 
However, personality experts (sometimes referred to as personologists) soon integrated these inconsistencies into their conceptualization of personality. They modified the old, more monolithic construct by measuring how people differ across situations. Their new methods of personality assessment describe fluctuations in personality characteristics that are consistent and predictable for each person, based on his predispositions and the environment they are in. Some work suggests that people can adopt different levels of a personality dimension as the social situations and time of day change.
Therefore, someone is not conscientious all of the time, but can be conscientious at work and a lot less so when they are home. This work also suggests that intrapersonal variations on a trait can be even larger than interpersonal variations. Extraversion varies more within a person than across individuals, for example. This work was based on individual self-ratings during the day across a long period of time. This allowed for researchers to assess moment-to-moment and day to day variations on personality attributes.
The impact of social roles.
In addition, social roles (e.g. employee) have been identified as potential sources of personality change. Researchers have found strong correspondences between the demands of a social role and one's personality profile. 
If the role requires that the person enacting it be conscientious, her standing on this trait is more likely to be high. Conversely, once he leaves that role or takes on another which entails less conscientiousness, he will manifest a lower level standing on that trait. Longitudinal research demonstrates that people's personality trajectories can often be explained by the social roles they adopted and relinquished throughout their life stages. Thus social roles are often studied as fundamental predictors of personality. 
The goals associated with them elicit the appropriation of certain personality profiles by the people enacting them. For example, employees judged effective by their peers and superiors are often described as conscientious. 
Personality also changes through life stages. This may be due to physiological changes associated with development but also experiences that impact behavior. Adolescence and young adulthood have been found to be prime periods of personality changes, especially in the domains of extraversion and agreeableness. It has long been believed that personality development is shaped by life experiences that intensify the propensities that led individuals to those experiences in the first place, which is known as the Correspondence Principle.
Subsequent research endeavors have integrated these findings in their methods of investigation. Researchers distinguish between mean level and rank order changes in trait standing during old age. 
Their study of personality trajectories is thus contingent on time and on age considerations. Mottus, Johnson and Geary (2012) found that instability engendered by aging does not necessarily affect one's standing within an age cohort. Hence, fluctuations and stability coexist so that one changes relative to one's former self but not relative to one's peers. Similarly, other psychologists found that Neuroticism, Extraversion (only in men), and Openness decreased with age after 70, but Conscientiousness and Agreeableness increased with age (the latter only in men). Moreover, they suggest that there is a decline on each trait after the age of 81.
Inconsistency as a trait.
Personality inconsistency has become such a prevalent consideration for personologists that some even conceptualize it as a predisposition in itself. Fleisher and Woehr (2008) suggest that consistency across the Big Five is a construct that is fairly stable and contributes to the predictive validity of personality measures. Hence, inconsistency is quantifiable much like a trait, and constitutes an index of - and enhances - the fit of psychological models.
To accommodate the inconsistency demonstrated on personality tests, researchers developed the Frame Of Reference principle (FOR). Frame of Reference (FOR) refers to the set of conjectures an individual or group of individuals uses to judge ideas, actions, and experiences to create meaning. FOR's include beliefs, values, schemas, preferences and culture. This can lead to prejudice, biases, and stereotypes due to the limited view an individual has. According to this theory, people tend to think of their personality in terms of a specific social context when they are asked to rate them. Whichever environment is cognitively salient at the time of the personality measurement will influence the respondent's ratings on a trait measure.
If, for example, the person is thinking in terms of their student identity, then the personality ratings he reports will most likely reflect the profile he espouses in the context of student life. Accounting for the FOR principle aims at increasing the validity of personality measures. This demonstrates that the predictive validity of personality measures which specify a social context is a lot higher than those measures which take a more generic approach.
This point is substantiated by yet another body of work suggesting that FOR instructions moderated the link between extraversion and openness scores on manager ratings of employee performance
This research thus recognizes that the importance of intrapersonal fluctuations contingent on personality is context specific and is not necessarily generalizable across social domains and time.
There are several different FOR's:
Process of change.
Personality is one of the strongest and most consistent predictors of subjective well-being while simultaneously subject to change.
Most people, in their lifetime, will experience an event that opens their eyes to a new understanding of the world. For example, someone who is carefree and happy might become more serious and stern after experiencing abuse in a relationship. Another who is serious and stern might become more happy and interested in life after finding a religion that provides them with closure and answered questions. Each day of life is met with events and situations that result in a response from those who experience them - and sometimes, these events can change who we are and how we think at the core. Human beings all go through different changes with personality, and with morality throughout their lifetime.
Research has found a correlation between being multilingual and personality, specifically how one may change personality based on the language currently being spoken. One who is raised bilingual or lived a number of years in a foreign country and learned the language of the land not only experience personality change but often adopt different personalities based on the language they are speaking. These changes are often based on cultural norms of the language's origin.
A study published in 2012 found that "personality does change and that the extent to which personality changes is comparable to other characteristics, such as income, unemployment and marital status". Some of the biggest concerns faced in life are the previously listed factors - how much money does one make (income)? Does one have a job or not (unemployment)? Does one have a lifelong companion (marital status)? These situations can lead to bigger, more complex situations. If one seeks to be married but is not, they may become cold. If one has no job but then gets hired somewhere, they may become grateful and filled with hope. When positive changes happen, "personality... meaningfully predicts changes to life satisfaction". Simply, when one experiences a personality change, it can strongly determine how that person will then feel about life.
Change over a lifetime.
There are two very specific types of change that researchers tend to focus on: rank-order change and mean-level change. A "rank-order change" refers to a change in an individual's personality trait relative to other individuals; such changes do not occur very often.
A "mean-level change" refers to an absolute change in the individual's level of a certain trait over time. Longitudinal research shows that mean-level change does occur. However, some traits tend to change while some traits tend to stay stable.
During adolescence there are many increases or rapid changes in hormones, societal pressures, and environment factors, among other things. These things theoretically factor into significant personality changes as one progresses through adolescence. As a person progresses through adulthood, their personality becomes more stable and predictable because they establish patterns of thinking, behaving, and feeling.
Personality does not stop changing at a specific age. Biological and social transitions in life may also be a factor for change. Biological transitions are stages like puberty or giving birth for the first time. Social transitions might be changes in social roles like becoming a parent or working at a first job. These life transitions do not necessarily cause change, but they may be reasons for change. As humans we do not adapt just in our body. Our mind also makes changes to itself in order to thrive in our environment. One theory says that whether or not these life transitions cause personality change is based on whether the transition was expected based on age or was unforeseen. The events that are expected will cause personality change because those events have common scripts. However, events that are unexpected will give prominence to the traits that already exist for the individual. Historical context also affects personality change. Major life events can lead to changes in personality that can persist for more than a decade. A longitudinal study followed women over 30 years and found that they showed increases in individualism. This may have been due to the changes that were occurring in their country at the time.
Stressful life events and trauma.
Negative life events, long-term difficulties, and deteriorated life quality, all predict small but persistent increases in neuroticism, while positive life events, and improved life quality, predict small but persistent decreases in neuroticism. There appears to be no point during the lifespan that neuroticism is immutable, which is known as the Plasticity Principle.
While extreme, traumatic brain injury can impact