Calculation Method

LifeCourse Associates has compiled a database of the nearly 14,000 leaders who have served since 1789 as U.S. President or Vice President; as U.S Supreme Court Justice; as a member of the U.S. Congress; or as a state governor. The database also includes, from 1775 to 1787, delegates to the Continental Congress and state governors. For nearly every person, the database records the leader’s name, party, state, year of birth, year of death, and years or Congresses of service. For a few persons, some data are unknown — most commonly year of birth for persons born before 1830; but in no year do these leaders comprise more than one percent of leaders serving.

Definition of Generation and Leadership Share

Each leader belongs to a generation as defined by his or her birthyear—from the Awakening Generation (born 1701–1723) to Generation X (born 1961–1981). See the accompanying key (in sidebar) for defining dates for every generation. A generation’s leadership share of the House, Senate, governorships, or Continental Congress is defined as the number of leaders belonging to that generation over a two-year period as a share of all leaders during that two-year period. Since 1787, a generation’s overall “national leadership share” is defined as the simple average of the House share, the Senate share, and the governorship share; from 1775 to 1785, it is defined as a simple average of the Continental Congress share and the governorship share.

Odd-Numbered Years

In all of the tables and charts, results for generational share or average age are tabulated by odd-numbered year. For all members of Congress, the results for each odd-numbered year refer to all Representatives or all Senators serving in the House or Senate (however briefly) during the entire subsequent two-year Congress. The percentages for the Senate in 1927, for example, refer to the percentages of all Senators who served at any time during the 70th Congress, which convened in the spring of 1927 and adjourned in the spring of 1929.

However, for state governors, the results refer only to governors who were serving during the midterm of the two year calendar period. The percentages for governors in 1927, for example, refer to the percentages for the governors who served at midnight on December 31, 1927. The same is true for Supreme Court Justices, although we do not present percentages for that office, only absolute counts.

Because of turnover (due to deaths, vacancies, impeachments, resignations, appointments, special elections, contested seats, etc.) during each two-year period and because of missing data (e.g., when birthyears are unknown), the total of 100 percent is often slightly greater or less than the statutory number of House seats, Senate seats, and governorships (today, 435, 100, and 50, respectively) for any given two-year period.

Definition of Leadership Index

A generation's Leadership Index is calculated by summing all leadership shares recorded for odd-numbered years. This value is then multiplied by two, to account for the two-year period each share represents. This final sum is then divided by the generation's total number of birthyears. Essentially, the Leadership Index is the integral of the leadership share curve in the chart, divided by the number of birthyears for the generation.

Average Ages and Longevity

The average age of each Congress or two-year period is calculated in a similar fashion: for each body, it is the average age of all leaders serving in that two-year period (or Congress). The average age of national leadership is, similarly, a simple average of all three bodies (since 1789) or two bodies (before 1789). Longevity, for each birth cohort, is tabulated as the average longevity of all leaders born in the cohort year. The longevity of national leadership is an average for all individual leaders in all bodies—not a simple average of longevity for each body.

Sources

For all members of both houses of the U.S. Congress (Senators and Representatives), our data are imported directly from the Biographical Directory of the United States Congress, 1774-Present, now published on-line through the joint efforts of the U.S. Senate Historical Office and Legislative Resource Center of the U.S. House of Representatives. This database, now continuously updated, used to be published in multi-volume hard-copy form. The first edition was published in 1859, under the title Biographical Directory of the United States Congress.

For all U.S. state governors, Presidents, Vice Presidents, and Supreme Court Justices, our data are compiled from Congressional Quarterly, American Political Leaders, 1789–2000 (2000 or more recent editions). With U.S. governors, in questionable cases, we also make use of Larry Kestenbaum’s valuable website, PoliticalGraveyard.com.

For revolutionary-era leaders, including colonial governors and delegates to the Continental Congress, please see the biographical references in Neil Howe and William Strauss, Generations (1991).