On May 21, 2009, the California Supreme Court closed another chapter in the state’s long-running fight over same-sex marriage when it upheld a 2008 voter-approved ballot initiative, known as Proposition 8, which amended the California state constitution to ban gay marriage. A month earlier, on April 27, 2009, the Iowa Supreme Court had unanimously ruled that a state law defining marriage solely as a union between a man and a woman violated the Iowa Constitution’s guarantee of equal protection. (See A Contentious Debate: Same-Sex Marriage in the U.S..)

As the California and Iowa rulings suggest, while the gay marriage controversy has many elements, including disagreements over religious and social norms, much of the debate is a legal one. Indeed, it was a 2003 Massachusetts high court decision legalizing same-sex marriage that elevated the issue onto the national stage, where it has remained ever since. Since 2003, courts in a number of states have handed victories to both opponents and supporters of gay marriage.

So far, court cases around the country have been based on state, rather than federal, constitutional provisions and thus have not been subject to review by the U.S. Supreme Court. However, a suit alleging that California’s Proposition 8 violates the U.S. Constitution was recently filed in a federal district court, giving the nation’s high court a potential opportunity to determine whether the U.S Constitution guarantees the right of gay and lesbian couples to wed. If the court takes such a case, its decision will likely stem, at least in part, from its prior rulings on privacy and related issues.

The Right to Privacy and the Griswold Revolution

Although the legal battle over same-sex marriage is rooted, in part, in the question of whether state and federal constitutions protect a right to privacy, the word “privacy” never actually appears in the U.S. Constitution. However, the Constitution does recognize several rights relating to privacy. For example, the Fourth Amendment recognizes the importance of privacy interests when it stipulates that because citizens need to feel “secure in their persons, houses, papers and effects,” the government may not carry out “unreasonable searches and seizures.” In addition, the Ninth Amendment leaves open the possibility of a broader privacy right when it declares that there are rights “retained by the people” that do not expressly appear in the Bill of Rights.

The Supreme Court first laid the foundation for an expanded right to privacy early in the 20th century in Lochner v. New York (1905). In this case, the court relied on the reference to “liberty” in the 14th Amendment’s Due Process Clause, which states that no person “shall be deprived of life, liberty or property, without due process of law,” to justify striking down a New York state law limiting the number of hours bakers could work each week. According to the court majority, the Due Process Clause implicitly guarantees citizens the “fundamental” right, free from state intrusion, to enter into employment arrangements.

The court’s reasoning in Lochner animated many subsequent decisions that form the foundation of what today is known as the constitutional right to privacy. In one such decision, Pierce v. Society of Sisters (1925), the court ruled that an Oregon law banning all private education violated the Due Process Clause because it directed how parents may educate their children, infringing upon parents’ fundamental right to rear their children as they see fit. In his majority opinion, Justice James Clark McReynolds went on to list other rights guaranteed by the Due Process Clause, including “the right of the individual … to marry, establish a home and bring up children … and generally to enjoy those privileges long recognized at common law as essential to the orderly pursuit of happiness by free men.”

Four decades later, in Griswold v. Connecticut (1965), the Supreme Court again turned its attention to whether the Constitution implicitly contains fundamental privacy guarantees. In this case, the court held by a vote of 7-2 that a Connecticut law prohibiting the sale and use of birth control was unconstitutional because it intruded on the right of marital privacy. Writing for the majority, Justice William O. Douglas asserted that a right to privacy exists not because of a specific constitutional provision but rather because it flows from several provisions relating to privacy, such as the Third Amendment right to refuse to quarter soldiers during peacetime and the Fourth Amendment prohibition of unreasonable searches and seizures. These and other privacy-related provisions, Justice Douglas wrote, create “penumbras” (or shadows) in which “zones of privacy” exist. Within these zones, he declared, are other rights, including the right of married couples to determine whether or not to have children.

In their dissent, Justices Potter Stewart and Hugo Black, while agreeing with the majority that the Connecticut statute was “an uncommonly silly law,” explained that they would have upheld the law because the Constitution does not speak to a general right to privacy. Both justices expressed a fear that the majority’s focus on constitutional penumbras would lead to judicial overreaching. “The adoption of such a loose, flexible, uncontrolled standard for holding laws unconstitutional, if ever it is finally achieved, will amount to a great unconstitutional shift of power to the courts, which I believe … will be bad for the courts and worse for the country,” Justice Black wrote.

Echoes of Griswold could be heard two years later in Loving v. Virginia (1967), a famous case involving a challenge to a Virginia law banning interracial marriage. In a unanimous decision, the court ruled that this law violated the 14th Amendment’s Equal Protection Clause, which guarantees all citizens equal protection under the law and thus prohibits the government from discriminating on the basis of race. But the court moved beyond the issue of racial discrimination to assert that the right to marry is itself protected by the Constitution. “The freedom to marry has long been recognized as one of the vital personal rights essential to the orderly pursuit of happiness by free men,” wrote Chief Justice Earl Warren for the majority. “Marriage is one of the ‘basic civil rights of man,’ fundamental to our very existence and survival,” the chief justice added.

In Eisenstadt v. Baird (1972), the court broadened the right to privacy enunciated in Griswold to include unmarried people. This case involved a Massachusetts law prohibiting the distribution of birth control to single people. By a vote of 6-1 (there were two vacancies on the court at the time), the court struck down the law. “If the right of privacy means anything,” Justice William Brennan wrote, “it is the right of the individual, married or single, to be free from unwarranted governmental intrusion into matters so fundamentally affecting a person as the decision whether to bear or beget a child.”

The following year the court extended privacy rights even further when, in Roe v. Wade (1973), it established a constitutional right to abortion. Writing for the majority, Justice Harry Blackmun explained that the “right of privacy … founded in the 14th Amendment’s concept of personal liberty and restrictions upon state action is broad enough to encompass a woman’s decision whether or not to terminate her pregnancy.” The court thus returned to locating the right to privacy in the Due Process Clause.

Starting in the 1970s, some politicians and legal thinkers began arguing that the court’s privacy jurisprudence, particularly as articulated in Griswold and Roe, amounted to judicial activism by creating rights that were never envisioned in the Constitution. Although this critique was most popular in conservative circles, it also won over some liberals and moderates who feared a significant expansion of the Supreme Court’s power. In response to this criticism, some legal scholars countered that the Griswold and Roe decisions were logical outgrowths of a long line of the court’s decisions and were therefore harmonious with the court’s tradition of enforcing fundamental rights that are not directly specified in the Constitution.

The Gay Rights Movement and the Road to Lawrence

During the 1970s, a gay rights movement, patterned in many ways after the civil rights and women’s rights movements, developed momentum as the sexual revolution spurred new social and sexual mores, which in turn prompted legislatures to repeal many state laws regulating sexuality. For instance, some 20 states, including California and Ohio, struck from the books their anti-sodomy laws. Still, by the mid-1980s, laws that prohibited certain acts between people of the same sex, and in some cases between those of the opposite sex, remained in force in 25 states.

One of these state laws, a Georgia anti-sodomy statute, became the subject of a landmark high court ruling. The case, Bowers v. Hardwick (1986), arose after Atlanta police arrested Michael Hardwick for having consensual sex in his own bedroom with another man. Georgia, like most states, rarely enforced its anti-sodomy law and, indeed, the state eventually dropped its charges against Hardwick. Nevertheless, Hardwick sued the state, alleging that the criminalization of private and consensual sex between people of the same gender violated his constitutional right to privacy.

In a 5-4 ruling, a bitterly divided Supreme Court ruled that the constitutional right to privacy did not protect the right to have private, consensual sex with a person of the same gender. Writing for the majority, Justice Byron White declared that earlier privacy cases, such as Griswold and Loving, concerned “family, marriage or procreation.” It would be an untenable stretch, White reasoned, to extend privacy rights to “any kind of private sexual conduct between consenting adults.” Furthermore, he wrote, while existing privacy protections concerning marriage or child-rearing “are deeply rooted in this nation’s history and tradition,” the opposite was true of sodomy, which all states had at one time banned and which half the states still banned at the time of the decision.

In a strongly worded dissent, Justice Blackmun dismissed the majority’s contention that the case ultimately involved a “fundamental right to engage in homosexual sodomy.” Indeed, he accused the majority of “distort[ing]” the ultimate question before the court by ignoring the fact that the Georgia statute outlawed sodomy between heterosexuals as well as homosexuals. What the case actually concerned, Justice Blackmun wrote, was “the most comprehensive of rights and the right most valued by civilized men, namely, the right to be let alone.” More specifically, he argued, the Constitution guarantees each person, regardless of sexual orientation, the liberty to have consensual intimate relations in his or her own home.

Many conservatives hailed the Bowers decision as a much-needed restraint on a court that they said had become untethered from its constitutional mandate. But a hint of change in course came just four years after the ruling, when Justice Lewis Powell, who had provided the crucial fifth vote for the Bowers majority, stated publicly that he subsequently regretted his decision. “I think I probably made a mistake,” he admitted, three years after retiring from the bench.

Meanwhile, in 1996 the high court decided Romer v. Evans, an important gay rights ruling that, while addressing a different question than the Bowers case, ultimately proved to be a strong indicator of how the court might rule if it revisited the sodomy question. In the Romer case, the court considered a challenge to an amendment to the Colorado Constitution (approved by the state’s voters in 1992) that nullified local anti-discrimination protections for homosexuals and prohibited passage of any such anti-discrimination laws in the future. By a 6-3 vote, the high court held that the Colorado amendment violated the 14th Amendment guarantee of equal protection. “A state cannot so deem a class of persons a stranger to its laws,” wrote Justice Anthony Kennedy in the majority opinion. In particular, he found, “the [Colorado] amendment imposes a special disability upon [homosexuals],” who are “forbidden the safeguards that others enjoy or may seek without restraint.”

While Justice Kennedy did not specifically mention Bowers, Justice Antonin Scalia, in a forceful dissent, reasoned that the Bowers decision should have led the court to uphold the Colorado law. “If it is rational to criminalize the conduct,” he wrote, referring to the Georgia sodomy statute upheld in Bowers, “surely it is rational to deny special favor and protection to those with a self-avowed tendency or desire to engage in the conduct.”

The Lawrence Case and Scalia’s Dissent

As if in response to Justice Scalia’s dissent in Romer, the court soon revisited the issue it had decided in Bowers, taking up another challenge to a sodomy statute, this one from Texas. The case, Lawrence v. Texas (2003), is remarkably similar to Bowers in many of its facts. Once again, police discovered two men having consensual sex in a private residence and arrested them under a state anti-sodomy law. And, once again, the defendants challenged the sodomy statute’s constitutionality, taking the case all the way to the Supreme Court.

Despite these factual similarities, the court in Lawrence overruled its earlier decision in Bowers, thereby invalidating not only the Texas statute but all anti-sodomy laws. Writing for the majority, Justice Kennedy stated that the court in Bowers had been mistaken in concluding that the government had historically restricted private and consensual intimate relations between people of the same sex. Moreover, Kennedy explained, sexual mores had changed since Bowers, as evidenced by the fact that in the 17 years between the Bowers and Lawrence cases, 12 states had repealed their anti-sodomy statutes and nine stopped enforcing these laws, leaving only four states that continued to enforce them.

Finally, Kennedy wrote, gay people have a “liberty under the Due Process Clause [that] gives them the full right to engage in [intimate] conduct without intervention of the government.” No matter how unpopular a group’s sexual norms, he explained, the government may not “demean their existence or control their destiny by making their private sexual conduct a crime.” In making this claim, though, Kennedy was quick to note the limited breadth of the decision. This case, he assured, did not address the regulation of prostitution or other public sexual acts, nor did it require the government to extend marriage or civil unions to same-sex couples.

Just as he did in Romer, Justice Scalia dissented in Lawrence, arguing once again that the majority’s reassurances did not agree with their logic. Scalia asserted that by rejecting moral-based legislation, Kennedy and the other justices on the majority were paving the way for a future ruling requiring states to recognize same-sex unions.

The Massachusetts Decision

Ironically, Justice Scalia’s interpretation of the Lawrence majority opinion would prove to have a profound effect on the gay rights movement. Many lawyers subsequently arguing for a right to same-sex marriage pointed to Scalia’s dissent in Lawrence as evidence that the majority opinion in that case generated a constitutional right to marriage for people of the same gender.

Previously, in the 1990s, supreme courts in Hawaii and Vermont interpreted their respective state constitutions to require that their state governments offer same-sex couples the same rights and benefits of marriage afforded to opposite-sex couples, even if the state chose not to define these rights and benefits as marriage. In response, many states, fearing that their courts would issue similar decisions, passed what are known collectively as Defense of Marriage Acts (DOMAs), which specifically define marriage as a union between an opposite-sex couple. In addition, several states amended their constitutions to prohibit same-sex marriage, hoping that such an explicit ban would prevent their courts from reading other constitutional provisions so broadly as to guarantee same-sex couples the right to marry.

Despite the creation of DOMAs and other state legislative actions preceding Lawrence, the Lawrence decision dramatically changed the same-sex marriage landscape by articulating a constitutional framework that could provide robust rights for gay and lesbian couples. Indeed, echoes of Lawrence could be heard in Goodridge v. Department of Public Health (2003), a decision by the Massachusetts high court that ignited a national debate over the meaning of marriage.

In Goodridge, the Massachusetts Supreme Judicial Court held by a vote of 4-3 that the state’s constitution requires the government to offer “the protections, benefits and obligations conferred by civil marriage to two individuals of the same sex who wish to marry.” The case arose after Julie and Hillary Goodridge, a lesbian couple, sought a marriage license from the Massachusetts Department of Health. The department denied the request, claiming that Massachusetts did not recognize same-sex marriage. The Goodridges then sued the department, alleging that this denial violated their right to individual liberty and legal equality as guaranteed by the Massachusetts Constitution.

Writing for the majority, Chief Justice Margaret Marshall held that denying marriage benefits to same-sex couples violated the Massachusetts Constitution because it did not accomplish a legitimate government goal. Indeed, the court explained, the reasons the government offered for banning same-sex marriage – promoting procreation, ensuring a good child-rearing environment and preserving state financial resources – would not be promoted by prohibiting same-sex couples from marrying. Thus, according to the court, the only basis for the state’s decision to exclude same-sex couples from the institution of marriage was a disapproval of their lifestyle. Because the court concluded that condemning a lifestyle is not a “constitutionally adequate reason” for denying marriage benefits, it held that the state must permit same-sex couples to marry.

In contrast, the three dissenting judges in Goodridge argued that they would not have required the state to recognize such unions because the state legislature enjoys broad discretion when regulating nonfundamental rights, such as the right to same-sex marriage. Given this discretion, the dissenting judges argued, the court should be very deferential in determining whether there is a connection between the ban on same-sex marriage and the legislature’s asserted interests. Applying this level of deference, the dissenting judges concluded that the legislature had a rational basis for two of its three stated purposes in banning same-sex marriage: to communicate to its citizens the view that marriage is about procreation and to promote the optimal setting for rearing children.

Then-governor of Massachusetts Mitt Romney responded to the Goodridge decision by proposing to amend the Massachusetts Constitution to define marriage as a union between one man and one woman. Under Massachusetts law, the legislature must approve a constitutional amendment in two consecutive sessions before the people can vote on it. After fighting a long and contentious battle over Romney’s proposal, the legislature approved a compromise amendment in 2004 that prohibited gay marriage but created civil unions for same-sex couples. In the following session, however, the legislature changed course and rejected this proposed amendment, thus denying voters the opportunity to consider it.

The Growing Battle Over Same-Sex Marriage in the States

Meanwhile, a debate over same-sex marriage was heating up at both the federal and state levels. Many states became concerned that the U.S. Constitution’s Full Faith and Credit Clause, which generally requires states to enforce judicial decisions issued in other states, would require each state to recognize a marriage between same-sex partners that took place in Massachusetts. Many scholars have noted that this is probably an unfounded concern since the Supreme Court held many years ago that states need not violate their own policy interests in enforcing other states’ policies on marriage. In addition, the federal Defense of Marriage Act, which was signed into law by President Bill Clinton in 1996, explicitly exempts states from having to recognize a same-sex marriage legally performed in another state.

Nevertheless, in 2003, 2004 and 2006, opponents of gay marriage in the U.S. Congress found this concern serious enough to declare it as the principal basis for proposing an amendment to the U.S. Constitution banning same-sex marriage in every state. This effort, however, failed to receive the two-thirds majority of both the U.S. House of Representatives and the U.S. Senate required to send a proposed federal constitutional amendment to the states for ratification.

At the state level, the success of the plaintiffs in Goodridge inspired other gay and lesbian couples to file similar claims around the country. Until May 2008, however, only one of these cases, Lewis v. Harris (2006), was even partly successful. In that case, the New Jersey Supreme Court found that the state constitutional guarantee of legal equality required the state legislature to grant same-sex couples the same rights and benefits of marriage that opposite-sex couples have traditionally enjoyed. Although important, the Lewis decision did not match the breadth of Goodridge because the court permitted the state lawmakers to decide how to grant these rights – either by marriage or civil union. Soon after the ruling, the New Jersey Legislature passed a measure allowing gay and lesbian couples to enter into civil unions but not to marry.

Besides the New Jersey decision, all of the suits on this subject at the state supreme court level were unsuccessful until May 2008. Indeed, in 2006 and 2007, the highest courts in New York, Washington and Maryland found that their state constitutions do not guarantee same-sex couples the right to marry. Each of these decisions held that recognizing same-sex marriage is a policy matter, not a constitutional matter, and that the decision must therefore rest with the people’s representatives in the legislative and executive branches.

Proposition 8 and the Battle in California

In May 2008, the California Supreme Court held 4-3 that state laws limiting marriage to opposite-sex couples violated the state constitution. The California decision, which consolidated six individual cases, is much like the Goodridge decision in that it found that same-sex couples are constitutionally entitled to the identical marital rights and privileges as opposite-sex couples. The reasoning in the Goodridge and California decisions, however, differed in one fundamental respect: Whereas the Massachusetts court in Goodridge found a right to same-sex marriage on the ground that there is no rational basis for denying marital rights to same-sex couples, the California court went significantly further, elevating gays and lesbians to have the same protected legal status as racial minorities and women. A law that discriminates against one of these protected groups is constitutional only if it meets a compelling government need. So the upshot of this ruling was that the California Constitution generally forbids any distinction whatsoever between same-sex and opposite-sex unions, whether in benefits or merely in name.

Seeking to overrule this decision, same-sex marriage opponents placed Proposition 8, a measure to alter the California Constitution to ban gay marriage, on the November 2008 state ballot. Following expensive and contentious political campaigns waged by both supporters and opponents of the initiative, California voters passed the proposition by a narrow margin – effectively outlawing gay marriage in California.

But just one day after the passage of the initiative, same-sex marriage advocates filed a lawsuit in the California Supreme Court claiming that Proposition 8 was not constitutionally valid and thus did not trump the California high court’s earlier ruling that the state must recognize the right to same-sex marriage. The legal basis for the lawsuit was that California law provides for two types of changes to the state constitution: revisions and amendments. A revision is a “substantial” change to the constitution; it requires a vote by at least two-thirds of both houses of the California Legislature to submit the proposed revision to a popular vote or to a constitutional convention. An amendment is a less substantial change to the constitution that only requires voters to place the proposed amendment on a state ballot and win a majority vote, as Californians did with Proposition 8. Gay-rights advocates argued that because the right of gays and lesbians to wed is a fundamental right, banning the practice required a constitutional revision, not just an amendment. Therefore, these advocates asserted, Proposition 8 was invalid.

On May 26, 2009, the California Supreme Court ruled 6-1 that Proposition 8 was an amendment, not a revision, and thus was valid. In its decision, the court noted that constitutional revision generally entails a significant change to the state’s governmental framework, something Proposition 8 does not do. In addition, the majority noted, earlier ballot initiatives that had altered fundamental constitutional rights – such as the restoration of the state’s death penalty in 1972 – had also been deemed to be amendments, not revisions.

After upholding Proposition 8, the court turned its attention to the status of the 18,000 gay and lesbian couples who had legally wed during the roughly five-month period between the May 2008 high court decision legalizing same-sex marriage and the early November passage of the proposition banning it. Here the court sided with gay marriage advocates, ruling that the marriages would remain valid because the language of Proposition 8 does not explicitly apply the ban retroactively.

Finally, the court noted that those same-sex couples could still achieve the material benefits and obligations of marriage under California law thanks to the court’s May 2008 decision legalizing the practice. Proposition 8 did not “repeal or abrogate” these rights, wrote Chief Justice Ronald M. George for the majority. Instead, George added, Proposition 8 merely carved “out a narrow and limited exception to these state constitutional rights, reserving the official designation of the term ‘marriage’ for the union of opposite-sex couples.” Furthermore, the other major piece of the May decision – making gays and lesbians a protected class in future discrimination cases – also continues to stand.

Connecticut and Iowa Legalize Gay Marriage

Gay marriage advocates counted the Proposition 8 decision as a major defeat, but they did score important victories around the same time in two other state high courts. The first of these wins occurred in October 2008, in the Connecticut Supreme Court. In Kerrigan v. Connecticut Department of Health (2008), the state’s high court ruled by a vote of 4-3 that a state law banning same-sex marriage violated the state constitution’s guarantee of equal protection under the law. The ruling overturned an earlier decision by a Connecticut trial court that had found no violation of the state’s Equal Protection Clause because the state had enacted a civil union law giving gay and lesbian couples the same legal rights as married couples. In this earlier decision, the trial court judge had written that the state constitution requires equal protection under the law but “not that there be equivalent nomenclature for such protection.”

But the majority of the Connecticut Supreme Court disagreed, ruling that offering homosexual couples civil unions in lieu of marriage amounts to unequal treatment “because the institution of marriage carries with it a status and significance that the newly created classification of civil unions does not embody.” In addition, the Connecticut Supreme Court elevated the legal status of gays and lesbians, giving them greater protection against discrimination than was granted in the Massachusetts decision, but not the highest level of protection that was established in the California ruling. In Kerrigan, the Connecticut high court ruled that laws that discriminate against gays and lesbians must be subjected to what is known as “intermediate strict scrutiny.” This means that a law that discriminates against homosexuals will be struck down unless the state can show that it substantially furthers an important government interest.

Five months after the Connecticut decision, the Iowa Supreme Court followed suit, unanimously affirming a lower court ruling that struck down the state’s 1998 DOMA banning same-sex marriage. The decision shocked many observers because Iowa is a Midwestern state with a substantial population of Christian social conservatives. However, the state also has long been known for its populist tradition and its willingness to embrace socially liberal causes.

As in Connecticut, the justices in the Iowa case struck down the law on the grounds that it violated the state constitution’s equal protection clause. And, as in Connecticut, the court in Iowa established the “intermediate strict scrutiny” test when assessing laws – including the DOMA – that discriminate against gays and lesbians, meaning that these laws can only be upheld if they substantially further an important government interest. In this case, the court ruled, Iowa’s DOMA did “not substantially further any government objective” and was thus unconstitutional.

Looking Ahead

Gay rights advocates will almost certainly continue to file lawsuits at the state level. In addition, two prominent lawyers from opposite sides of the ideological fence have moved the issue into the federal court system. Former Solicitor General Ted Olson and high-profile litigator David Boies, the two lawyers who faced off against each other in the famed Supreme Court case Bush v. Gore (2000), have joined forces and filed a suit in federal district court in San Francisco arguing that Proposition 8 violates the equal protection and due process clauses of the U.S. Constitution. Olson and Boies have publicly predicted that their case will end up before the U.S. Supreme Court, although such a result is by no means assured.

Still, filing such a claim at the federal level poses some risks for the gay marriage movement. An unfavorable outcome for same-sex marriage supporters in the U.S. Supreme Court would prevent gay couples from subsequently arguing for a right to same-sex marriage under the U.S. Constitution; gay couples would be able to secure a right to marriage only under state constitutions. Moreover, an unfavorable high court ruling might influence the judgment of state courts in future gay marriage decisions. Because of such concerns, a number of prominent gay rights groups, such as the Human Rights Campaign and Lambda Legal, have publicly questioned the decision by Olson and Boies to file the suit, arguing that such a case may be premature or even dangerous to the success of the same-sex marriage movement.

While it is impossible to predict how the high court might rule in a same-sex marriage case, several of the justices’ past decisions offer some clues. Of all the justices, Justices Scalia and Clarence Thomas probably offer the most solid indicators as to how they might rule. Indeed, given their dissenting opinions in Lawrence, it would be surprising if either justice voted for a constitutional right to same-sex marriage. At the same time, it is more difficult to predict the votes of their two conservative colleagues, Chief Justice John Roberts and Justice Samuel Alito, because Roberts and Alito were not on the court when earlier gay rights cases, such as Lawrence, were decided. However, many constitutional scholars predict that Roberts and Alito would most likely join Scalia and Thomas on this issue.

The court’s more liberal justices, Stephen Breyer, Ruth Bader Ginsburg and John Paul Stevens, have voted in favor of gay rights claimants in every gay rights case they have considered. However, these prior decisions, including Lawrence, never explicitly addressed whether the Constitution guarantees the right to gay marriage, making it difficult to know exactly how these justices might ultimately vote in a same-sex marriage case.

Like Roberts and Alito, federal Judge Sonia Sotomayor – recently nominated by President Barack Obama to replace retired Justice David Souter – has had no role in any major gay rights cases, and thus leaves few clues as to her likely vote in a case involving the constitutionality of same-sex marriage. However, many Supreme Court watchers predict that, if confirmed, Sotomayor would probably join the more liberal wing of the court on most, if not all, issues.

If conservatives and liberals on the court were to split 4-4 on the issue of same-sex marriage, the outcome in such a case would likely hinge on Justice Kennedy, whose vote can be difficult to predict. While Kennedy’s opinion in Lawrence showed great sympathy for the gay rights movement, Kennedy also emphasized that, from a constitutional perspective, the case did “not involve whether the government must give formal recognition to any relationship that homosexual persons seek to enter.” Kennedy thus explicitly refused to commit one way or the other on whether the constitution requires recognition of same-sex marriages.

This report was written by David Masci, Senior Research Fellow, and Jesse Merriam, Research Associate, Pew Research Center’s Forum on Religion & Public Life.