Leader-Member Exchange Theory became the focus of research in the late 1970s. Before the LMX theory, researchers thought of leadership as a process with consistent relationships between a leader and every follower. LMX changed that idea by theorizing that a leader has different relationships with each of their followers. Early research focused on the formation of in-groups and out-groups while later research shifted the focus to how LMX changed organizational effectiveness.
Illinois Mathematics and Science Academy, "08. Leader-Member Exchange Theory" (2019). CORE. 20.