Adrian Vermeule is the Ralph S. Tyler, Jr. Professor of Constitutional Law at Harvard Law School.

The end of a Supreme Court term almost always sees one or more conservative justices vote to hand the liberal justices a narrow but important victory. In case after case, conservative swing justices appear irresistibly drawn to join the liberals.

So it went this year. Chief Justice John G. Roberts Jr. defected to strike down admitting-privileges regulations for abortion providers and keep in place protection for immigrants brought to this country as children. The chief justice and Justice Neil M. Gorsuch joined their liberal colleagues to create new anti-discrimination prohibitions for sexual orientation and gender identity.

This is a familiar pattern. Last term, the chief justice dealt the administration a significant loss by frustrating its plans to add a citizenship question to the census. In the longer run, a string of 10 GOP appointments of new justices since 1973, compared to a mere four Democratic appointees, has produced little progress toward the central conservative goal of overturning Roe v. Wade, thanks in part to a series of dramatic defections from supposedly solid conservatives.

Why do these defections occur? One theory is that there is nothing to explain; justices simply follow their best understanding of the Constitution, and let the chips fall where they may. No doubt that is that is what the justices themselves think: that they are earnestly seeking to get the law right.

But this explanation fails to account for a basic asymmetry: While conservative justices often break ranks to give liberals a 5-to-4 majority, liberal justices rarely do the same in reverse. If the legal merits cut across political divides, there should be no such persistent imbalance. Some conservatives then claim that liberal justices are, despite their protestations, systematically less principled — a suspiciously partisan view that credits the reported experience of only some justices and discounts that of others.

A second view is that conservative defections occur because swing justices are not following the written law but their own personal preferences or individual self-interest. In this interpretation, conservative swing justices depart from their best judgment about what the Constitution requires in controversial cases because they are overwhelmed by the political, social and cultural pressures of the left-elite milieu, especially the praise or censure of the mainstream media.

Yet this account fails to explain why defections have persisted or even increased as the influence of the mainstream media has declined. It also posits either conscious infidelity to law or an implausible lack of self-awareness.

There is a third theory that has more explanatory force: The justices are indeed acting faithfully to law, but not by enforcing the written Constitution and laws. Rather what they enforce is our real, unwritten constitution, a set of understandings that underlies and shapes our interpretation of the law. Justices interpret open-ended provisions (“due process of law”) in light of this unwritten constitution. And because the background small-c constitution embodies a liberal order, it is unsurprising that their decisions do as well.

What are the principles underlying our unwritten constitution? It is best understood as a sociopolitical order that privileges a particular set of commitments held passionately by educated urban professionals and what Joel Kotkin and Fred Siegel have termed “gentry liberals.” Among these, and relevant to the current term, are ensuring a high rate of immigration, encouraged by policies against full enforcement of the law, and protecting sexual expression and liberty, including contraception, abortion and unconstrained expression of sexual preference and gender identity. These beliefs are not spelled out explicitly in law, yet they exert a gravitational force that powerfully influences the justices’ interpretations.

These unwritten norms allow the expression of dissenting views, but only as dissenting views. They thus exert the most force on justices who are otherwise least convinced of the conservative position in a given case. Hence the most conservative justices rarely defect. But in critical cases, involving central commitments of the unwritten constitution, it is highly likely that one or more of the middling conservative justices will do so.

Liberal justices, by contrast, are more likely to agree with the concrete order’s commitments in any event. Hence they end up forming a solid bloc. The resulting asymmetry in defections between liberal and conservative justices arises not from bad-faith, results-oriented liberal judging, but because the underlying norms and principles of the sociopolitical order of which they are a part skew systematically in a liberal direction.

The influence of these norms explains the conservative defections in this term’s gay rights cases, the abortion ruling and the immigration regulation. It offers a framework for understanding similar rulings by the previous swing justice, Anthony M. Kennedy, on abortion, affirmative action and, above all, gay rights.

All these results flow directly from the commitments of the liberal order. Conversely, the shape of the unwritten constitution also explains recent cases in which no conservative justices defected. The conservatives have largely held together in a string of recent religious liberty cases, for example, because gentry liberalism accepts negative liberties that allow religious groups freedom of internal organization, participation in benefits programs on the terms available to any other group and regulatory exemptions.

Understanding the influence of the unwritten constitution does not dictate what justices should do, but it has implications for what claims it is fair to make about their choices. Before accusing liberals of deliberate bloc voting or conservative swing justices of unprincipled opportunism, we should consider that faithful judges appear to act in these ways because they are interpreting a background sociopolitical order, our unwritten constitution, that is itself skewed in liberal directions.

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