cognitive control processes required to switch back and forth between categorizing based on concept versus based on attribute leads to reduced speeds in the critical blocks of the test. In other words, it is much less mentally demanding to sort concepts in the compatible block when only one aspect of the concept must be focused upon. Comparatively, sorting concepts in the incompatible block, which requires focusing on both concept and attribute not only takes longer to process because of the increased complexity, but also because the previous concept may have required a different cognitive effort. Finally, Rothermund and Wentura propose a figure-ground model of explanation for the IAT. In essence, this theory suggests that IAT respondents simplify their task by relying on salience. For example, negative is salient for most people, so if a respondent is to press the right key for negative words, the individual will plan to press the right key for all negative words (figure), and the left key for any other (non-negative) words (ground). This leads to fast decision-making in the compatible task and two of the critical tasks, but not for the third critical task, in which two salient categories require different keys pushed. There is empirical support for all these explanations of the IAT's effects, but this is not necessarily evidence against the IAT's overall validity, as these theories are not mutually exclusive. Furthermore, regardless of the fundamental cognitive processes of the IAT, studies show that multiple implementations of the test validly measure their targeted constructs, and the psychometric value of each implementation varies as a function of its individual characteristics (e.g., construct measured, participant characteristics, testing environment). Balanced-identity theory and design Heider's balance theory In 1958, Fritz Heider proposed the balance theory, which stated that a system of liking and disliking relationships is balanced if the product of the valence of all relationships within the system is positive. In the theory, there are concepts and associations. Concepts are persons, groups, or attributes; and among attribute concepts, there are positive and negative valences. Associations are relations between pairs of concepts, and the strength of association is the potential for one concept to activate another, either by external stimuli or by excitation through their associations with other, already active, concepts. The theory followed the assumption of associative social knowledge: an important portion of social knowledge could be represented as a network of variable-strength associations among