Rank-order stability refers to the maintenance of the relative standing of individuals on a trait dimension within a population over time. Mean-level change refers to change in the average trait levels of a population over time. Individual difference in change refers to individual deviations from population mean-level patterns of change.

The cumulative continuity principle of personality development refers to personality and temperament being moderately stable in preschool years and becoming increasingly stable until middle adulthood. The maturity principle of personality development refers to the fact that, on average, young adults increase in their absolute levels of agreeableness (1), emotional stability (2), conscientiousness (3) and social dominance (4). These aspects are related to being mature and thus maturing is associated with increases in these factors.

The disruption hypothesis states that adolescents tend to experience temporal dips in personality maturity as a result of biological, social, and psychological transitions from childhood to adolescence. Adolescents may also not fit the maturity principle because they temporarily conform to deviant peer norms and experience difficulties in adjusting to increasingly mature expectations.

There seems to be an U-shaped change in the mean levels of most Big Five traits. There may be temporary mean-level decreases in conscientiousness (1), openness (2), extraversion (3) and emotional stability (4) in early adolescence and increases in late adolescence with the emotional stability aspect specifically holding true for girls.

Peers may play an important role in explaining individual differences in adolescents’ personality trait change (i.e. transactional model). Co-development refers to the tendency of group members to show interrelated development on a trait because of their social connectedness. There are several possible trajectories of co-development:

1. Convergence
   This occurs when dyad members become more similar over time.
2. Correlated change
   This occurs when the change trajectories of dyad members are correlated.
3. Time-lagged partner effects
   This occurs when one dyad member’s change is associated with the other’s previous trait level.

Personality trait change and personality trait co-development may result from social learning processes (1), conformity to social norms for behaviour and other personality expressions (2) or shared environmental experiences (3).

Adolescents tend to become more stable in their ranking on personality trait dimensions and tend to achieve greater psychological maturity, either linearly or through the U-shape. Adolescents differ regarding their personality trait trajectories but the individual differences in change are not related to personality trajectories of their friends and siblings.

The 1-year rank order stability of Big Five traits increased substantially in early and middle adolescence. Rank-order stability levels do not increase further in late adolescence. This is mostly due to stability of environmental influences. This may be stability of social relationships (1), identity maturation (2) and decreasing brain development of areas associated with personality.

Throughout adolescence and early adulthood, boys and girls showed increasing agreeableness and girls showed increasing conscientiousness. There is a temporal decline in boys’ conscientiousness and girls’ emotional stability. The mean-level increases in conscientiousness may depend on the continuous improvements in effortful control in childhood. It may be driven by further increases in self-regulation capacity in adolescence. Personality maturation among adolescents may also be driven by increasing expectations regarding their behaviour. The temporal dip may be explained by a temporary mismatch between external expectations and actual behaviour, affect and cognition.