Political families, similar in some ways, differ in the politicians they produce. The Adamses gave us two indifferent presidents and three brilliant diplomats. The Kennedys gave us one president, three senators, and two martyred icons. The Bushes have given us, so far, one senator, one president, and two governors.

But no family has yet matched the Roosevelts, whose two presidents (fifth cousins, linked by an outsized first lady) occupied the White House for twenty years. The Roosevelts defined the twentieth century.