Personality traits are defined as the relatively enduring patterns of thoughts, feelings, and behaviors that distinguish individuals from one another. The dominant view in the field of personality psychology today holds that personality emerges early and continues to change in meaningful ways throughout the lifespan. Evidence from large-scale, long-term studies has supported this perspective.
Adult personality traits are believed to have a basis in infant temperament, meaning that individual differences in disposition and behavior appear early in life, possibly even before language or conscious self-representation develop. The Five Factor Model of personality has been found to map onto dimensions of childhood temperament, suggesting that individual differences in levels of the “big five” personality traits (neurotic-ism, extraversion, openness to experience, agreeableness, and conscientiousness) are present from young ages.
A fascinating evolutionary perspective in particular can attempt to explain simplistically why humans have personality and individuality. This perspective traces personality and individuality back to when the early humans were learning how to function in complex social groups. Many specialists from different fields have a general agreement that early humans saw themselves as a part of the group to which they belonged, rather than seeing themselves as individuals with independent personalities. In terms of personality at this time, the whole group was identical.  A member of the group associated themselves as one with the tribe and therefore the responsibility rested in the group and not the individual. Kropotkin explained the important of this by stating that because the primitive man identified his existence with the existence of his tribe it has allowed for mankind to reach the remarkable level present today. A small step of differentiation that later led to personality and individuality was the division of labor. This differentiation was necessary in order for the group to function in a much more efficient way. This differentiation became adaptive since it increased the groups functionality. These early humans then continued to develop personality and individuality, which stemmed from their group and the social interactions they encountered. Individual life, and thus individuality and personality essentially arose from collective life.  In order to explain some of the variations in human personality and individuality it's possible to look at the evolutionary process of natural selection. Evolution introduced variations of the human mind, natural selection acted on these by choosing which were the most beneficial and which led to a greater fitness. Since humans are so complex, many opposing personality traits proved to be beneficial in different ways. An example of this is that in some situations a more aggressive personality could be beneficial, yet a more submissive personality could be beneficial in another situation. Another type of selection helps to take on the evolutionary aspect of human personality and individuality. This type of selection is referred to as emotional selection. It considers emotions as the core emergence of humans in the world. The emotions of humans are what have led to the evolution of human personality and individuality. “The ability to adapt to all conditions of life is usually called, ‘intelligence,’ but is founded in the complexity and flexibility of the emotional system. The concept of emotional representation as a way of selectively modeling the environment is the key idea underlying our understanding of human individuality.”  With these basic understandings introduced, hopefully it will help make more sense out of the development of personality.
Classic theories of personality, such as Freud’s tripartite theory, and post-Freudian theory, including developmental stage theories and type theories, have often held the perspective that most personality development occurs in childhood, and that personality is stable by the end of adolescence. As recently as the 1990s, modern personality theorists concurred with William James’ 1890 assertion that, by age 30, personality is “set like plaster”. Currently, lifespan perspectives that integrate theory and empirical findings dominate the research literature. The lifespan perspective of personality is based on the plasticity principle, that personality traits are open systems that can be influenced by the environment at any age. This interactional model of development emphasizes the relationships between an individual and her environment, and suggests that there is a dialectic between continuity and change throughout the lifespan. Large-scale longitudinal studies have demonstrated that the most active period of personality development appears to be between the ages of 20-40. Personality grows increasingly consistent with age and plateaus sometime around age 50, but never reaches a period of total stability. Although change is less likely later in life, individuals retain the potential for change from infancy to old age.
Personality traits demonstrate moderate levels of continuity, smaller but still significant normative or mean-level changes, and individual differences in change, often late into the life course. This pattern is influenced by genetic, environmental, transactional, and stochastic factors.
Twin and adoption studies have demonstrated that the heritability of personality traits ranges from .3-.6, with a mean of .5. Heritability of .5 means that 50% of variation in observable personality traits is attributable to genetic influences. But a given genotype will lead to a certain phenotype only under the right environmental circumstances. In other words, the heritability of a trait may change depending on an individual’s environment and/or life events. An example of the way environment can moderate the expression of a gene is the finding by Heath, Eaves, and Martin (1998) that marriage was a protective factor against depression in genetically identical twins, such that the heritability of depression was as low as 29% in a married twin and as high as 51% in an unmarried twin. Ultimately, emerging evidence suggests that genetic and environmental influences on personality differ depending on other circumstances in a person’s life.
With the effects of genetic similarity are removed, children from the same family often appear no more alike than randomly selected strangers; yet identical twins raised apart are nearly as similar in personality as identical twins raised together. What these findings suggest is that shared family environment has virtually no effect on personality development, and that similarity between relatives is almost entirely due to shared genetics. Although the shared environment (including features like the personality, parenting styles, and beliefs of parents; socioeconomic status; neighborhood; nutrition; schools attended; number of books in the home; etc.) may have a lasting impact at the extremes of parenting practice, such as outright abuse, most personality researchers have concluded that the majority of “average expectable environments” do not have an effect on personality development.
The weakness of shared environmental effects in shaping personality came as a surprise to many psychologists, and spurred research into nonshared environment, or the environmental influences that make siblings different from one another instead of similar. Nonshared environmental effects encompass the variability in behavioral outcomes that is not explained by genetic and family environmental influences. The nonshared environment may include differential treatment by parents, individually distinct reactions to the shared family environment, peer influences, and experiences that occur outside the family. In adults, nonshared environment also encompasses the unique roles and environments experienced after leaving the family of origin. Further effects of environment in adulthood are demonstrated by findings that different work, marital, and family experiences are associated with personality change, and by the impact of major positive and negative life events on personality.
Van Gestel and Van Broeckhoven (2003) write, “Almost by definition, complex traits originate from interplay between (multiple) genetic factors and environment.” Interactions between genetic predisposition and the environment are a major factor in personality development. The corresponsive principle of personality development states that “life experiences may accentuate and reinforce the personality characteristics that were partially responsible for the particular environmental elicitations in the first place”. This principle is summarizes how gene-environment interactions (also called person-situation transactions) maintain and reinforce the continuity of personality throughout the lifespan. Three main types of gene-environment interactions are active (the process by which individuals with certain genotypes select and create environments that facilitate the expression of those genotypes), passive (the process by which genetic parents provide both the genes and the early environmental influences that contribute to the development of a characteristic in their children), and reactive (the process by which non-family individuals respond to the behavior produced by a genotype in characteristic ways).
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