During the 1930s, the U.S. Supreme Court struck down as unconstitutional a number of state and federal economic regulations such as minimum wage laws. In doing so, it stopped relying on a line of cases known as the Lochner precedents.

The court’s approach contained in the Lochner cases was, somewhat ironically, referred to as “substantive due process.” The phrase “due process” comes from the Fifth Amendment (which applies to actions of the federal government) and 14th Amendment (which applies to state actions). The due process clause in these amendments provides (from the 14th Amendment, though the key language is basically identical): “nor shall any State deprive any person of life, liberty, or property, without due process of law.”

On its face, the language seems to provide a straightforward protection of persons accused of a crime. Specifically, that they would not be subject to the death penalty (deprived of life), jail terms (deprived of liberty), or fined (deprived of property) unless proper (“due”) legal procedures (“process of law”) are observed. That is consistent with the context of the Fifth Amendment, which also specifies grand jury requirements, prohibitions on multiple trials for the same offence, and protection from self-incrimination.

But could this clause have a broader reach?

The concept of “substantive due process” assumes the answer is yes. During the early 20th century, the typical application of the idea would be a court invalidating a restriction on the number of hours that a baker could work as a violation of a right to contract, a right inferred to be part of the “liberty” protected by the due process clause.

The irony of the phrase substantive due process was in the joining of apparent opposites: process and substance. The court interpreted the due process clause not only to ensure proper procedures were being followed in criminal cases, but also to protect substantive rights inferred by the term “liberty.” If liberty could be interpreted to include a right, like the right of contract, the court could invalidate laws that limited that right without proper justification.

Today, a court might say that if something is a fundamental right, the government cannot limit that right unless it has a compelling reason to do so.

When the court abandoned the Lochner line of cases, the use of substantive due process appeared to be at an end and few, if any, judges would accept it as a characterization of their approach.

That reversal, though, did not mean that the court would leave the business of interpreting the Constitution’s protection of “liberty.” It only meant that the court would shift from economic rights to other arenas.

In 1965, the court struck down a Connecticut law prohibiting the possession of contraceptives by married couples as a violation of a right to marital privacy. That right would expand in 1972 into a right “to be free from unwarranted governmental intrusion into matters so fundamentally affecting a person as the decision whether to bear or beget a child.” Then, in 1973, it would expand to include the right to abortion.

Other cases would recognize in 1972 a right of unwed fathers to raise their children, in 1977 a right of extended family members to live together despite zoning restrictions, in 1987 a right to marry despite being in prison, etc.

In the mid-1990s, the court seemed to settle on a more cautious approach to recognizing new rights. In rejecting the claim that the Constitution created a right to physician-assisted suicide, the majority of justices held that to assert a right not specifically mentioned in the Constitution, someone challenging a government action would have to demonstrate that the right was consistent with the history and tradition of the nation.

In the same-sex marriage decision of 2015, however, the court appears to have stepped away from the more circumspect analysis of unwritten rights needing to be grounded in history and tradition and embraced a more expansive role for the courts. It remains to be seen how the court might approach novel rights claims in future cases.

While the scope of the interpretation of the term liberty in the Constitution has been and still is somewhat unsettled, it is clear that the court will still consider claims for new rights protected by the Constitution and is likely to remain at the center of the expansion or limitation of those rights.