Introduction

A traditional and important distinction in the study of most individual differences is that between a trait and a state. For psychologists, traits are relatively stable and enduring characteristics that tend to be considered more as causes of behaviours (including the selection of situations) than as outcomes. The adverb ‘relatively’ is included as a qualifier because traits may change over an extended period of time (e.g. over a decade or a lifetime), but for most intents and purposes, it is assumed that traits do not change. In contrast, states are presumed to change and are more often thought of as outcomes or reactions to circumstances, although it is entirely possible to think of states as causes of behaviours including the selection of situations and circumstances. There is no fixed time period over which a state must or should exist – the length of a state can vary from minutes to hours, perhaps encompassing an entire day. Regardless, the assumption is that states change, and such changes are meaningful and represent something other than random fluctuation.

The state–trait distinction is particularly important because it cannot be assumed that constructs at the two levels of analysis function in the same ways. That is, state- and trait-level phenomena may be governed by, or reflect, different psychological processes (e.g. Tennen et al., 2005). Consistent with this distinction, state- and trait-level relationships are mathematically independent (e.g. Nezlek, 2001).