behavior, Humanistic psychology integrates these elements as functions of being in a greater encompassing system such as societies, cultures, or interpersonal relationships. Consequently, personality development is subjected to shifts in personal meaning and individual goals of achieving an ideal self. Influencing factors 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. Genetics Twin and adoption studies have demonstrated that the heritability of personality traits ranges from .3-.6, with a mean of .5, indicating that 50% of variation in observable personality traits is attributable to genetic influences. In contrast, family and adoption studies have demonstrated a low heritability factor of .22. A study conducted on German women using an IAT (implicit association test), shows a connection between the function of specific neurotransmitters and the predisposition to have certain personality traits like anxiety or extraversion. With the effects of genetic similarity 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. 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. Environmental The weakness of shared environmental effects in shaping personality surprised many psychologists, spurring research into non-shared environmental effects, the environmental influences that distinguish siblings from one another. The non-shared environment may include differential treatment by parents, individually-distinct reactions to the shared family environment, peer influences, experiences outside the family, and test error in measurement. In adults, the non-shared environment may also include the unique roles and environments experienced after leaving the family of origin. Further effects of environment in adulthood are demonstrated by research suggesting that different work, marital, and family experiences are associated with personality change; these effects are supported by research involving the impact of major positive and negative life events on personality. Gene-environment interactions A culmination of research suggests that the development of personality occurs in relation to one's genetics, one's environment, and the interaction between one's genetics and environment. Van Gestel and Van Broeckhoven (2003) write, “Almost by definition, complex traits originate from interplay between (multiple) genetic factors and environment.” The corresponsive principle of personality development states that “life experiences