Personality traits.

Personality traits are hypothetical constructs. As such, they cannot be measured directly with electrical or mechanical instruments. Instead, the personality psychologist will infer the level of a 
trait within an individual by observing the effects of the trait on the person's behavior. The more of a trait that a person possesses, the more trait-relevant behaviors he or she is likely to display.

Consider a psychological attribute common to many theories of personality: extroversion. This trait is a hypothetical construct because it cannot be observed directly. Yet it has been proposed to be a veridical construct, having utility in describing and explaining consistencies in people's behaviors and in predicting future behaviors.

Personality traits are generally conceived of as forming normally distributed dimensions. Thus, for any particular trait, such as extroversion, humans are thought to vary along a continuum, ranging from low levels of the trait to high levels, with most people falling somewhere in the middle of the dimension. The task of personality assessment is to identify those people who are, for example, more extroverted and those who are less extroverted (i.e., more introverted).