The Reddin 3-D (Three-Dimension) Leadership Model, developed by William J. Reddin, is a significant contribution to leadership theory because it built upon and extended earlier foundational models, particularly those from the Ohio State Studies and Blake and Mouton's Managerial Grid. These earlier models primarily identified two basic dimensions of leadership behavior.

Ohio State Studies: These studies identified two key dimensions: Initiating Structure (task-oriented behavior, focusing on defining roles, organizing work, and achieving goals) and Consideration (relationship-oriented behavior, focusing on building trust, respect, and rapport with subordinates).

Blake and Mouton's Managerial Grid: This model similarly used two dimensions: Concern for Production (task-oriented) and Concern for People (relationship-oriented). It plotted these on a grid, identifying various leadership styles based on different combinations of these two concerns (e.g., Impoverished, Country Club, Task, Middle-of-the-Road, Team Management).

Both the Ohio State Studies and Blake and Mouton's Grid suggested that a leader's effectiveness could be understood by their balance or emphasis on these two dimensions. However, they often implied that a 'high-high' style (high initiating structure/concern for production and high consideration/concern for people) was generally the most effective, without fully accounting for situational variables.

Reddin's crucial innovation was the addition of a third dimension: Effectiveness. He argued that a leader's style is not inherently good or bad; its effectiveness depends on the situation. Reddin proposed that any of the basic styles (which he categorized as related to task and relationship orientation, similar to the earlier models) could be more or less effective depending on the context. For example, a highly task-oriented style might be very effective in a crisis situation requiring quick decisions, but highly ineffective in a team-building scenario. Conversely, a highly relationship-oriented style might be effective in fostering collaboration but ineffective when strict adherence to procedures is required.

Reddin's model thus moved beyond simply describing leadership styles to evaluating their appropriateness and outcomes in specific situations. He introduced the concept of 'situational demands' and suggested that effective leaders are those who can adapt their style to the demands of the situation, the task, and the subordinates. This addition of the effectiveness dimension made Reddin's model a pioneering contingency theory of leadership, emphasizing that leadership is not about adopting a single 'best' style, but about applying the right style at the right time for optimal results.