Scholars differ regarding the reason for the institutionalization of a large, functionally specialized White House-centered presidential staff system during the last six decades. Among the factors cited is a general growth in government size and complexity, increases in the presidential workload, and the institutional rivalry between the president and Congress. However, using new advances in time-series analysis based on fractional integration, we show that these models of staff growth are plagued by conceptual and methodological shortcomings that render their substantive conclusions unreliable. In response, we develop and test a comprehensive explanatory model that combines elements of previous research but uses fractional integration to account more accurately for whether newly created staff positions are institutionalized. We find that presidential staff growth is driven primarily by changes in presidents' bargaining relations with Congress, the media, and the public, and only secondarily by a general growth in government's responsibilities.