Within any group at every age, human beings vary dramatically among themselves on any given psychological construct, structure, function, or process (let ^ stand for any such entity). It is commonly understood that variation in ^s, whether physical attributes or personality traits, appear in normal distributions in the population. It is important to assess whether differences between individuals in a ^ are fleeting or are stable through time. Thus, psychological science is centrally concerned with assessment of the stability of individual variation. Stability is consistency in the relative standing or rank of individuals in a group on some ^ through time (Figure 1). Stability is particularly pertinent to developmental science and the study of individual variation within a developmental trajectory. An individual with a stable personality trait (say, openness) will display a relatively high level of openness at one point in time vis-a-vis his or her peers and will continue to display a high level at a later point in time, where other individuals display lower levels at both times. Individuals show instability in a given ^ if they do not maintain their relative standing or order in the group through time.

Although developmental science emphasizes overall change and growth, the study of psychological stability is important for several reasons. First, the physiological systems of humans and other living organisms require certain stable conditions—physical, chemical, psychological, and environmental—to survive. These systems also allow for the maintenance of these conditions even in the face of ever-changing circumstances; stability in living organisms is generally in a state of dynamic and adaptive equilibrium. Second, stability provides basic information about the overall developmental course of a given insofar as individuals do or do not maintain their standing or order on ^ vis-a-vis others in the group through time. Third, it is generally assumed that to be meaningful, a ^ should show substantial consistency across time: a major predictor of ^ at a given age is ^ at an earlier age. Fourth, psychological stability affects the environment: Interactants often adjust to match a consistent ^ in another individual.

Figure 1

Figure 1

Development is governed by genetic and biological factors in combination with environmental influences and experiences. Stability of any ^ could be attributable to genetic or biological factors in the individual, or stability might emerge through the individual’s transactions with a consistent environment. Personality traits tend to be stable across time and can serve as examples of a develop-mentally stable Current models describe personality as consisting of five factors, openness, neuroticism, extraversion, agreeableness, and conscientiousness. According to this five-factor trait theory, traits are endogenous dispositions that follow intrinsic paths of development essentially independent of environmental influences.

In the context of developmental science, consistency may have many applications and its evaluation may take many different forms. Longitudinal developmental design is requisite to addressing questions about consistency. Furthermore, it is desirable to distinguish between consistency among individuals, or individual order consistency (stability/instability), and consistency in a group, or group mean level consistency (continuity/discontinuity). Order consistency and mean consistency are independent or orthogonal constructs. Conceptually and empirically, the two can exist simultaneously, and both have been reported in the same longitudinal study of different ^s. Normally, stability is assessed using (Pearson or Spearman) statistical correlation.

It is further desirable to distinguish among types of stability. There are three prominent models of possible temporal association. One model describes homotypic (sometimes called complete) stability: the maintenance of order among individuals in the same ^ through time. This applies to some personality traits; individuals displaying high levels of openness in relation to others at Time 1 will display relatively high levels of openness relative to others at Time 2. A second model describes heterotypic stability, which is the maintenance of order among individuals on different manifest ^s through time where the different ^s are theoretically related and presumably share the same latent process (e.g., positive affectivity in personality, say,


Why might stability fail? If a is not stable, there are two possible explanations: Either ^ is not stable, or ^ has not been assessed adequately. Barring measurement imprecision, we attribute instability to genetic and biological as well as environmental and experiential forces. The timing of biological phenomena is partially driven by genetic factors and tends to happen within a specific period of the life course for most people in any particular population. Change also arises because people engage in normative life tasks and roles, such as leaving home, establishing a family, and starting a career. The life span perspective in psychology specifies that human beings are open systems, and the plastic nature of psychological functioning ensures that people exhibit both stability and instability in many ^s throughout the life course.

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