Yesterday, in Department of Commerce v. New York, the Supreme Court ruled that the Trump administration's rationale for adding a question about citizenship to the 2020 census was pretextual and therefore invalid under the Administrative Procedure Act. I think this decision was correct, for the reasons Chief Justice John Roberts explains in his majority opinion. But the Court would have done better to simply rule that the inclusion of the question on the census was outside the scope of federal power under the Enumeration Clause of Article I of the Constitution. The majority's reasons for rejecting that argument strike me as weak.

Article I Section 2 of the Constitution requires Congress to make "an actual enumeration" of the population every "ten years," for purposes of apportioning representation in the House of Representatives. The enumeration is to be done "in such Manner as [Congress] shall by Law direct." This latter phrase undoubted gives Congress broad power to determine exactly how the census shall be done. But broad power is not unlimited power.

Congress clearly has the authority to include almost any questions on the census that would increase the accuracy of the resulting count. It also has the power to ask a wide range of questions that would not have a significant effect on accuracy either way (especially if those questions involve information-gathering of a type authorized by Congress' other enumerated powers, which would provide additional authorization for them).

But matters are different if the addition of a given question is likely to reduce the accuracy of the count. Some of the powers the Constitution grants to the federal government are powers to regulate a particular type of activity, largely without regard to the purposes for which the regulation is undertaken. For example, Congress' power to regulate interstate commerce can be used for a wide range of different goals, some of which may even conflict with each other.

By contrast, the Enumeration Clause is a power to achieve a particular type of objective: in this case an accurate head count of the population of the United States. Indeed the Enumeration Clause is a rare constitutional provision that not only grants authority to pursue a particular objective, but actually mandates it. The Clause requires the government to conduct a new enumeration every ten years. The power in question therefore cannot be used to authorize policies that actually impede its mandated purpose. And that is exactly what the addition of the citizenship question would do.

Survey research experts, including the federal government's own experts at the Census Bureau, agree that including the citizenship question would substantially reduce the accuracy of the count. The reason is that a significant number of immigrant and other households that include non-citizens would likely choose not to participate, for fear that they or their relatives might end up getting targeted for deportation as a result. Even if people in question ultimately forestall deportation by prevailing in court, a legal battle of this type can be painful, costly, and disruptive. Many would prefer to avoid even a small chance of inviting it.

While the Enumeration Clause gives Congress the power to choose the "manner" of conducting an enumeration, that is not the same thing as using that power to undermine the very goal it is supposed to achieve. The power to do X is not also a power to impede X. If Congress and the president had unlimited authority to conduct the census in any way they want, unconstrained by the requirement that the methods must further enumeration rather than impede it, they could, for example, adopt a law under which anyone counted by the census would have to pay a tax in order to be included. This would clearly be a regulation of the "manner" of enumeration. But, just as clearly, it would be unconstitutional, because it would lead to widespread undercounting by creating an incentive to avoid being counted.

The Trump administration's proposed citizenship question is just a less extreme example of the same problem. Indeed, the administration likely added the question precisely because it would reduce the accuracy of the count in states with large immigrant populations, thereby reducing the number of congressional seats allocated to those states. But even if the administration's motives were pure, the addition of this question still undermines enumeration rather than furthers it, and thereby falls outside the scope of the federal government's Enumeration Clause powers.

A ruling against the question based on the Enumeration Clause would have ended the case immediately (as opposed to the current remand for further consideration of possible alternative rationales for including the citizenship question). It also would have obviated the need for consideration of the administration's motives and the procedure by which it decided to include the question.

Chief Justice Roberts' best argument for ruling that the citizenship question falls within Congress' authority is that a similar question was included in all but one census between 1820 and 1950, and the question was also given to sub-samples in several censuses since then. But this evidence is less impressive than it might initially seem to be.

During the 1820-1950 period, we did not have social science evidence showing that inclusion of a citizenship question would reduce accuracy. While the meaning of the enumeration power was the same, whether the inclusion of a given question furthers the authorized purposes of that power or impedes it is a factual question. This is one of a number of situations where originalists and textualists can and should take account of new factual evidence. The resolution of any legal case depends on a combination of textual meaning (which, at least under originalist assumptions, does not change) and factual evidence (which can indeed change, and often does).

Moreover, during most of the time from 1820 to 1950 there were few or no federal immigration restrictions, and even less in the way of systematic federal efforts to deport undocumented immigrants from the interior of the country. Thus, there was little incentive to avoid participation in the census for fear of deportation. The situation today is, for obvious reasons, very different.

Roberts also notes that the census has always been used to ask a variety of other demographic questions, such as ones about respondents' age and sex. But including these questions creates little or no risk of creating a significant undercount.

Assessing whether the inclusion of a census question undermines the goal of enumeration would require courts to consider social science evidence. But no more so than is routinely done in a wide range of other routinely heard by federal courts, such as antitrust cases and redistricting cases where plaintiffs allege racial discrimination. It might be reasonable to defer to the federal government's judgment in close cases, where the evidence is ambiguous. But not so in cases like the citizenship question, where it is overwhelmingly on one side.

As both Chief Justice Roberts and Justice Stephen Breyer (in his partial concurring opinion), note, if the federal government cannot include a question on citizenship in the 2020 census, it can still do so in other surveys. Census Bureau experts recommended exactly that course of action. It is likely that a survey of citizens can be authorized under one of Congress' other enumerated powers (especially under the very broad modern interpretations of some of them), even if not under the Enumeration Clause. But the federal government cannot include such a question in the census mandated by the Enumeration Clause.

This flawed part of yesterday's decision probably will have little effect on cases outside the context of the census. In that sense, the harm it causes will be limited. But it is still unfortunate that the five conservative justices (all of whom joined this part of the the Chief Justice's opinion) signed on to such a seriously flawed ruling which allows a federal power to be used for the opposite of its textually required purpose. That goes double for those, like Clarence Thomas, who are generally very careful to enforce structural constraints on federal power in other contexts.

The four liberal justices also deserve some criticism here. They all refused to sign on to this part of Roberts' opinion, which suggests they likely disagree with it. Yet none bothered to explain the grounds of that disagreement. It would have been helpful if they had done so.

UPDATE: It is worth noting that non-citizens (and other non-voters) have always been included in the "enumeration" required by the Enumeration Clause for apportionment purposes. Indeed, such inclusion is mandated by the text of the Constitution, which mandates a count of all "free persons," except for "Indians not taxed."