Raymond Cattell's trait theory offers a structured framework for understanding personality by distinguishing between two critical traits: surface and source traits. Surface traits are observable patterns of behavior, such as indecisiveness, anxiety, and irrational fears. These traits are less stable, varying across situations and over time. This means that they are less helpful in understanding the deeper aspects of an individual's personality.

In contrast, source traits are the fundamental, underlying factors that shape an individual's behavior. These traits are more stable and consistent across various situations, providing a reliable foundation for understanding personality. Cattell believed these source traits represent the core dimensions of personality and influence behavior more than surface traits.

Cattell developed the 16 Personality Factors (16PF) Questionnaire to measure source traits, a tool designed to evaluate personality across 16 key dimensions. These include warmth, intellect, emotional stability, aggressiveness, liveliness, dutifulness, social assertiveness, sensitivity, paranoia, abstractness, introversion, anxiety, open-mindedness, independence, perfectionism, and tension. For instance, warmth can range from being reserved and detached to being outgoing and supportive, while emotional stability can vary from reactive and moody to calm and composed. Each trait is measured along a continuum, allowing for a meaningful understanding of personality rather than reducing traits to binary categories.

With its focus on surface and source traits, Cattell's theory remains an influential tool in psychological research and applied settings, such as clinical psychology and occupational testing. His factor-analytic approach laid the groundwork for a scientific and detailed exploration of personality, emphasizing that personality traits exist on a continuum, and their varied expression shapes individual behavior across different contexts.