This is the first part of an editorial series on nepotism in the White House. Read more on Jared Kushner’s role here, and on Ivanka Trump’s role here.

Donald Trump has broken with so many conventions of the American presidency that it can be hard to track them all. Some of his supporters would argue that they elected him to explode outmoded Washington ways. But many of these conventions were meaningful curbs on presidential behavior and power.

One of Mr. Trump’s most disturbing departures from tradition was his appointment of close relatives to positions at the very apex of government power. We are now seeing consequences of that ethical break, as the extraordinary access to government secrets of his son-in-law, Jared Kushner, has finally been constrained by White House officials concerned about his lack of full security clearance. Meanwhile, The Times has reported that Mr. Kushner’s family business has received loans from the companies of people with whom he met in the White House. The president’s repeated deployment of his daughter Ivanka as a diplomat has also come under heavy criticism.

Mr. Trump was, of course, accustomed to relying on family members in the private sector, a world in which relatives can be literally reared in the business. A legacy of family control has helped sustain many private companies, including The New York Times. But it has never been embraced in public service by Americans, who left family governance behind when they rejected a monarchy to form the United States.