Javier Corrales, a professor of political science at Amherst College, is the author, most recently, of “Fixing Democracy: Why Constitutional Change Often Fails to Enhance Democracy in Latin America.”

On May 26, Costa Rica legalized same-sex marriage. At a time when so many democracies are experiencing conservative backlashes, Costa Rica’s achievement seems exceptional. What happened in Costa Rica is the type of civil rights victory one should expect when a country adopts a model of governance merging social inclusion with liberal institutions, and rejects populism in favor of the rule of law.

Same-sex marriage has become the standard in largely wealthy democracies, but remains rare almost everywhere elsewhere. There is a reason for it.

Opposition to same-sex marriage tends to be widespread and virulent, at least initially. Costa Rica was no exception. A 2014 poll revealed that only 28.2 percent of the population approved of gay marriage. But the country’s social democrats were uncomfortable with this conservatism. Costa Rica has a tradition of using state institutions to promote the health, education and labor rights of the poor. Activists came together to demand that these institutions be updated to also protect stigmatized sexual minorities.

The problem was how to effectuate change in light of widespread opposition. In 2016, President Luis Guillermo Solís, who at least publicly had not been a strong supporter of equal marriage, accepted a recommendation from members of his cabinet to officially consult the Inter-American Court of Human Rights, or I.A.C.H.R., on whether Costa Rica was obligated to offer L.G.B.T. people property and estate rights, and allow trans people to change their gender identity in national documents. The I.A.C.H.R. not only said yes, but went further by arguing that states should offer all rights, including marriage rights.