The environmental factors that exert pressures on our personality formation are the culture in which we are raised, our early conditioning, the norms among our family, friends and social groups, and other influences that we experience. The environment to which we are exposed plays a substantial role in shaping our personalities.
Most of us believe that we are like our parents because of some combination of genes and, more important, of nurture--that parents, to a large extent, raise us in their own image. But if that is the case, if nurture matters so much, then why did the adopted kids not resemble their adopted parents at all? The Colorado study isn't saying that genes explain everything and that environment doesn't matter. On the contrary, all of the results strongly suggest that our environment plays as big--if not bigger--a role as heredity in shaping personality and intelligence. What it is saying is that whatever that environmental influence is, it doesn't have a lot to do with parents.
Needs can be triggered by external as well as internal stimuli, so personality cannot be studied in isolation from environmental forces. 'At every moment, an organism is within an environment which largely determines its behavior ... (usually) in the guise of a threat of harm or promise of benefit... The press of an object is what it can do to the subject or for the subject--the power it has to affect the well-being of the subject in one way or the other' (Murray, 1938, p.39-41, 121). Thus, press refers to those aspects of the environment that facilitate or obstruct a person's efforts to reach or avoid a given goal.
These new conceptualizations of the role of genetic contributions as constraining environmental influences have implications for preventing unwanted personality traits or temperaments and even psychological disorders. That is, it seems that environmental manipulations, particularly early in life, may do much to override the genetically influenced tendency to develop undesirable behavioral and emotional reactions. Although current research suggests that the influence of everything in our environment, such as peer groups and schools, in its totality affects this genetic expression, the strongest evidence exists for the effects of early parenting influences and other early experiences (Cameron et al., 2005; Collins, Maccoby, Steinberg, Hetherington & Bornstein, 2000).
The general environmental model that I have suggested (Lewis, 1997) holds that children's behavior always is a function of the environment in which the behavior occurs, because the task of the individual is to adapt to its current environment. As long as the environment appears consistent, the child's behavior will be consistent; if the environment changes, so, too, will the child's behavior. It is the case that maladaptive environments produce both normal and abnormal behavior. From a developmental point of view, I would hold that maladaptive behavior is caused by maladaptive environments; if we change those environments, we may be able to alter the behavior.
It should be emphasized that the word environment, whether shared or nonshared, is broadly construed in quantitative genetics to include all nongenetic factors. In other words, environmental influence could be biological (e.g., hormones) as well as social (e.g., parents, peers, and television). That is, because twins share the same womb as well as their home environment, the shared environment can include prenatal hormonal influences, or other aspects of the physical environment, as well as social influences.
Genetic factors explain only about half of the variance of personality development. The other half is due to environmental factors. These environmental factors, however, tend to make children growing up in the same family different, rather than alike, and have, therefore, been called "nonshared environment" (Plomin & Bergeman, 1991; Rowe, 1994; Turkheimer & Waldron, 2000). As an alternative to parental environmental influence, Harris (1995, 1998) proposes that children's peers are the primary context for the development of individual differences in personality.
As an alternative to theories which promote the important of universal traits and behaviors for leaders, some have proposed that the effectiveness of leader behaviors depends on the environment. These "contingency" theories contend that leader behaviors may be helpful or harmful, depending on the characteristics of subordinates and the situation. For instance, the Vroom-Yetton model of supervisory decision-making promotes a careful consideration of situational factors before determining the most effective decision-making strategy (Vroom & Yetton, 1973). Also, the Path-Goal theory calls for leaders to consider the needs of subordinates before deciding on a leadership strategy (House, 1996).
Adults also change in response to changes in the environment, including major life events, changes in social and vocational roles, and therapy (Robert et al., 2008; Maiden et al., 2003). For example, attaining a responsible position at work can result in increased social dominance (an aspect of extraversion), conscientiousness, and emotional stability (Roberts et al., 2008).
What makes a personality stable? First, heredity is at work. As we have noted, genes contribute to individual differences in all five of the Big Five personality factors (Borkenau et al., 2001; Krueger & Johnson, 2008). Second, lasting effects of childhood experiences may contribute; you have seen, for example, that parents can either help a child overcome a difficult temperament or contribute to its becoming an enduring pattern of response. Third, traits may remain stable because people's environments remain stable; playing consistent social roles like mother or engineer may be especially important in creating consistency in personality (Roberts, Wood, & Caspi, 2008).
Caspi and his colleagues elegantly demonstrated experimentally the long-held notion that the inclusion of environmental information in the genetic model could be critical in showing an association between genes and behavioral traits (Caspi et al., 2002, 2003). Following Caspi and colleagues, the budding personality geneticist needs to decide whether early environmental information can be assessed in his or her sample.