	Introduction

This report presents data on average Member tenure over time, analyzes several factors that affect tenure in any given Congress, and examines historical patterns of congressional service, including the distribution of years served within each Congress, and the cross-chamber experience of Representatives and Senators. During much of the 19 th century, the average tenure of Representatives and Senators remained relatively steady, with incoming Representatives generally averaging between two and three years of prior service in most Congresses, and Senators averaging between three and five years. Beginning in the late 19 th and through much of the 20 th century, average tenure for Members in both chambers steadily increased. The average years of service for Members of the 116 th Congress, as of January 3, 2019, when the Congress convened, was 8.6 years for the House and 10.1 years for the Senate. The report also examines two further issues related to Member tenure, including the distribution of Member service over time, as well as Members' cross-chamber experience. Although the average tenure of Members has increased since the late 19 th century, a substantial portion of Representatives and Senators in recent Congresses have served for six years or less in their respective chambers. With respect to cross-chamber experience, while a small proportion of Representatives have historically had previous Senate experience, a sizable percentage of Senators throughout congressional history have had previous House experience. Average Service Tenure

As shown in Figure 1 below, the average years of service of Members in both chambers has generally increased over time. Around the turn of the 20 th century, the average began to increase, rising from just over four years at the beginning of the century, to approximately nine years in each of the three most recent prior Congresses. At the start of the 116 th Congress, the average years of prior service for Representatives was 8.6 years. Factors Affecting Average Tenure

Among the many factors that may affect variation in Member seniority over time, two are examined in this report: the decision of sitting Members whether or not to seek reelection to the next Congress, and the success rate of Members who do seek reelection. Discussion

The data presented in Figure 2 and Figure 3 align with scholarly assessments of congressional history, which conclude that during the early history of Congress, turnover in membership was frequent and resignations were commonplace, and that during the 20 th century, congressional careers lengthened as turnover decreased and Congress became more professionalized.
The average service tenure of Members of the Senate and House of Representatives has varied substantially since 1789. This report presents data on Member tenure over time, analyzes factors that affect average tenure in any given Congress, and examines historical patterns of congressional service, including the distribution of years served in each Congress, and the cross-chamber experience of Representatives and Senators.
During much of the 19th century, the average tenure of Representatives and Senators remained relatively steady, with incoming Representatives generally averaging between two and three years of prior service in most Congresses, and the Senators averaging between three and five years. Beginning in the late 19th and through much of the 20th century, average tenure for Members in both chambers steadily increased. Senators' average years of prior service has increased from just under five years during the early 1880s to approximately 10 years in the most recent Congress. Similarly, the average tenure of Representatives has increased from approximately three years during the early 1880s to approximately nine years in the most recent Congress.
The average years of service for Members of the 116th Congress, as of January 3, 2019, when the Congress convened, was 8.6 years for the House and 10.1 years for the Senate. In comparison, the average years of service for Members of the 115th Congress, as of January 3, 2017, when the Congress convened, was 9.4 years for the House and 10.1 years for the Senate.
This report analyzes two factors that influence variation over time in the average years of service for Members of Congress: the decision of sitting Members whether or not to seek election to the next Congress, and the success rate of Members who do seek reelection. Observed increases in the proportion of Members seeking reelection and decreases in the proportion of Members defeated for reelection align with previous scholarly assessments of congressional history, which conclude that during the early history of Congress, turnover in membership was frequent and resignations were commonplace, and that during the 20th century, congressional careers lengthened as turnover decreased and Congress became more professionalized.
The report also examines two further issues related to Member tenure, including the distribution of Member service over time, as well as Members' cross-chamber experience. Although the average service tenure of Members has increased since Congress's early years, a substantial portion of Representatives and Senators in recent Congresses have served for six years or less in their respective chambers. With respect to cross-chamber experience, while a small proportion of Representatives historically have had previous Senate experience, a sizable percentage of Senators throughout congressional history have had previous House experience.