They discovered that trait consistency increased throughout one's life span, becoming steadily more consistent with age (Roberts & DelVecchio, 2000). .
Trait theory has also provided significant evidence to suggest that many maladaptive personality characteristics are consistent across time (Paunonen, 2003; Janis, Mahl, Kangan & Holt, 1969; and Myers, 1990). Explosive temper tantrums and aggression in childhood and adolescence are often consistent traits, producing aggressive, and potentially violent adults (Capsi, 1993). One study found that adolescents who were using illegal drugs by age 14 were having significant problems at age four (Block; Block, & Keyes; Shedler & Block cited Funder, 2001,). This suggests that regardless of current environmental influences, the maladaptive personality characteristics seen in childhood were found to be carried into early adolescence and adulthood. Conversely, studies on children who were high in agreeableness, extraversion or sergent engagement, were found as adolescents and young adults demonstrating high levels confidence, social acceptance and emotional stability. (Shinner, 2000; Shinner, Masten & Tellegen 2002). However, while trait theories are a good means of measuring personality characteristics and consistency, they provide little evidence for why individuals possess certain traits, or why some people are more consistent than others.
The biological approach complements trait theory by offering genetics as a possible explanation for consistency. There is substantial evidence to support the notion that, like physical characteristics, personality is genetically influenced. Some of the strongest evidence in this field comes from studies on twins. Even when twins were raised apart many personality characteristics were found to be consistent over time, especially in the case of identical twins (Tellegen, Lykken, Bouchard, Wilcox and Rich, 1988 cited Weston, 2002).
