Some facets of the U.S. government—like presidential terms and post offices—were written into the original Constitution after (often lengthy) deliberations by the Founding Fathers. The number of Supreme Court justices was not one of those things.
The document did establish a Supreme Court, and it stated that the president should appoint its judges; it also mentioned that a “Chief Justice shall preside” if the president gets impeached. Since it was left up to Congress to work out the rest of the details, they passed the Judiciary Act of 1789, which outlined an entire court system and declared that the Supreme Court should comprise one chief justice and five associate justices. As History.com explains, they landed on six because the justices would have to preside over federal circuit courts, one of which was located in each state. Traveling wasn’t quick or easy in the horse-and-carriage days, so Congress wanted to minimize each justice’s jurisdiction. They split the courts into three regions, and assigned two justices to each region.
According to Maeva Marcus, director of the Institute for Constitutional History at George Washington University Law School, the even number of justices was a non-issue. “They never even thought about it, because all the judges were Federalists and they didn’t foresee great disagreement,” she told History.com. “Plus, you didn’t always have all six justices appearing at the Supreme Court for health and travel reasons.”
Over the next 80 years, the number of Supreme Court justices would fluctuate for two reasons: the addition of federal circuit courts, and presidents’ partisan motives. John Adams and his Federalist Congress reduced the number to five with the Judiciary Act of 1801, which they hoped would prevent Democratic-Republican Thomas Jefferson from getting to fill a seat after he took office that year. By the following year, Jefferson’s Congress had passed another judicial act that returned the number of justices to six, and they upped it to seven after forming another circuit court in 1807.
The nation grew significantly during the early 19th century, and Congress finally added two new circuit courts—and with them, two new Supreme Court seats—during Andrew Jackson’s presidential tenure in 1837. Republican Abraham Lincoln then briefly increased the number of justices to 10 in order to add another abolitionist vote, but Congress shrunk it to seven in 1866 to keep Andrew Johnson from filling seats with Democrats. As soon as Republican Ulysses S. Grant succeeded Johnson, Congress set the number back to nine, where it’s remained ever since.
In 1911, Congress did away with circuit courts altogether, so the number of Supreme Court justices stopped being contingent upon their expansion (though each justice does still oversee a region to help with occasional tasks). As for presidents shifting the number to serve their own goals, it’s now looked down upon as “packing the court.” When Franklin D. Roosevelt tried to increase it to 15 in the 1930s to push his New Deal through the Supreme Court, the Senate opposed the bill by a whopping 70 to 20 votes.
In short, the depth of the Supreme Court’s bench changed a lot in America’s early years not only because the country was expanding, but also because the federal government was still testing out its system of checks and balances. And though presidents do still appoint justices based on their own political party, we’ve gotten used to the idea that the Supreme Court is, at least ideologically, supposed to be unbiased. If Congress and the president kept up the habit of adding and subtracting justices at will, it would tarnish this ideal.
“If Congress increases the size of the Supreme Court for transparently partisan political reasons, it would cement the idea the justices are little more than politicians in robes, and that the court is little more than an additional—and very powerful—arm through which partisan political power can be exercised,” Steve Vladeck, a professor at the University of Texas School of Law, wrote for NBC News. “Indeed, that Congress has not revisited the size of the court in 150 years is a powerful testament to just how ingrained the norm of nine has become—and how concerned different political constituencies have been at different times about preserving the court’s power.”