In Michigan v. EPA, all nine justices of the Supreme Court agreed that an administrative agency must – unless Congress provides otherwise – consider the costs of regulation before regulating. Five justices, led by Justice Scalia, thought this principle so robust that it required an agency to consider costs even before starting down the path toward regulation. Four justices, led by Justice Kagan, would have allowed an agency to take steps toward regulation without considering costs, so long as it brings costs into the journey at some point. No justice explained exactly what a proper consideration of costs would entail. But all agreed that such an accounting – whatever it was – was, as a matter of law and basic rationality, required.

This is a first. The justices had never before formed a united front, in this substantive direction, on this fundamental issue. Indeed, in the prior case closest on point, the Court had unanimously ruled in the opposite direction, deciding that an agency could not consider costs in setting national standards that were expected to cost billions of dollars. In other cases raising similar issues, the justices had divided over the role of costs in the regulatory process even when the question was whether the relevant agency was allowed – rather than required – to consider costs in regulating. One may quibble over differences among the cases in terms of statutory language and context, but there is no denying the philosophical shift reflected in the Court's newfound unanimity in Michigan v. EPA.

As a matter of law, logic, and experience, however, the justices' new outlook is deeply mistaken.

No legal principle – constitutional, statutory, or otherwise – requires that an agency consider costs before regulating. Indeed, short of an express statutory command, precious few prior cases have required that an agency consider any particular factor in a regulatory decision. The absence of legal authority for the interpretive principle the justices have now announced is reflected in the paltriness of the legal precedent Justices Scalia and Kagan cite for the principle. Between them, Justices Scalia and Kagan could muster, for legal support, only two snippets from two solo separate opinions by two justices in prior cases, neither endorsed by any other justice and one explicitly rejected in a later case by a majority of the Court. These are not legal precedents; they are the ruminations of two justices speaking only for themselves.

Nor is the new interpretive principle embraced by the justices compelled by a basic understanding of rationality, or "reasoned decisionmaking," as Justice Scalia asserts. While it may seem merely rational to require that an agency consider costs before regulating, in fact the principle embraced by the justices introduces many logical incongruities. Endorsing this principle in this case meant sending a rule, decades in the making, back to EPA for reconsideration of a threshold judgment first made in the year 2000, in the face of EPA's representations that it would reaffirm that judgment if the matter were returned to it. Unless pointless delay of health- and life-saving regulation is what the Court means by rationality, it is hard to see how rationality is served by the analytical exercise the Court has foist upon EPA. If the Court, instead, believes that rationality consists of considering all potentially relevant factors in coming to a decision, then its own decision does not ensure or even endorse this result. An important issue raised in Michigan v. EPA was whether EPA was entitled, in assessing the benefits of its rule controlling hazardous air pollutants, to consider the "ancillary" benefits of reducing air pollutants not directly covered by the regulatory program for hazardous air pollutants. Justice Scalia mentioned these benefits but did not decide, one way or another, whether EPA would be allowed to consider them on remand. If the Court were truly interested in a rational weighing of all relevant factors, this should have been a no-brainer. But the Court demurred. Justice Kagan's dissent did no better. In high-fiving the cost-benefit analysis required by the White House for major rules, Justice Kagan aligned herself with a methodology notorious for its trivialization of important values.

Both the majority and dissent, moreover, ignored the backdrop of the law they were construing: the statutory provision at issue in Michigan v. EPA, regulating hazardous air pollutants, was amended by Congress in 1990 to address EPA's almost complete failure to regulate these pollutants. Congress designed the amended provision to prevent EPA's underreaching, not EPA's overreaching. The Court's unanimous embrace of a cost-sensitive interpretive default moved in the opposite direction, recoiling from overreaching, with an encouraging gesture to underreaching.

Imagining how regulatory history would have unfolded if this interpretive principle had been introduced years ago further exposes the lack of sense in the justices' new perspective. Some years ago, two co-authors and I examined three iconic decisions protecting the human health and the environment, decisions that many people would readily support today: EPA's ban on lead in gasoline, the federal government's refusal to approve hydroelectric dams in the Grand Canyon, and OSHA's strict regulation of the carcinogen vinyl chloride in the workplace. We found that if these decisions had been required to pass today's cost-benefit analysis, they would have failed; "[a] rigid insistence on making regulations pass cost-benefit tests would, in retrospect," we wrote, "have gotten the wrong answer time after time." Justice Scalia may have thought it "unreasonable" not to consider costs in regulating, and Justice Kagan may believe it is even "fundamentally silly" not to do so, but the fact is that many of our successful programs to protect human health and the environment would have shrunk if not disappeared entirely if the agencies creating them had been required to undertake the kind of analysis the justices appear to have endorsed.

It remains to be seen, of course, what will become of the cost-based interpretive principle the justices signed on to in Michigan v. EPA. Perhaps it will be limited to the Clean Air Act provision at issue there. Perhaps the courts will, under this provision, ultimately uphold a cost-based analysis far less unforgiving than the formal cost-benefit analysis that the Court pointed to as one possible cost-based regulatory framework. Perhaps the justices who silently joined the majority and dissenting opinions in that case have their own ideas about the reach and content of the new interpretive principle. For now, the justices' unanimous embrace of an interpretive principle that defies law, logic, and experience simply makes me anxious.