Chapter 4 introduces key theoretical and measurement issues in trait psychology. The authors begin with a review of several important theoretical issues, including a review of the key assumptions that most trait psychologists share. These include that there exist meaningful differences between individuals, and that these differences are stable across time and across some situations. The authors review the history of trait psychology, noting that a heavy emphasis on the primacy of traits in explaining behavior has now been tempered with the recognition that situations and traits interact to produce behavior.

Next, the authors review three key ways in which traits and situations interact to produce behavior, including situation selection, evocation of responses from others, and manipulation of others. Another lasting change in trait psychology caused by the challenges of situationists is the reliance on aggregation to measure the average tendencies of individuals. Aggregation refers to averaging several single observations and produces a more reliable measure of a personality trait than a single observation.

Finally, the authors review several key measurement issues in trait psychology, including carelessness, faking, and response sets. One such response set that has received a great deal of theoretical and empirical attention is socially desirable responding. The authors next review the use of trait measures to predict performance, including the use of trait measures in employment selection, and educational selection. A key problem reviewed by the authors in using trait measures to predict performance is the restriction of range. This occurs when the range of one or both variables being correlated is restricted, producing a smaller correlation than would be the case if no range restriction occurred.