IN A quiet suburb of San José, Costa Rica’s capital, is a building that looks like the White House in miniature. Costa Rica’s government gave it in 1993 to the Inter-American Court of Human Rights (IACHR), established 14 years earlier, to show its commitment to human rights. Until 2008, when the court built a second floor above the garage with money from Norway, its seven judges deliberated in a repurposed dining room. The setting is modest. The decisions emanating from it, increasingly, are not.

On January 9th the court told Costa Rica to legalise same-sex marriage. That provoked a furore in the country, which is scheduled to hold the first round of presidential and legislative elections on February 4th (see article). A fringe candidate for the presidency who vows to ignore the ruling is suddenly leading in the polls. The judgment will also cause consternation in many of the 20 other Latin American and Caribbean countries that fall under the court’s jurisdiction. In the court’s view, its opinion is binding there, too.

It derives its power from its role as the final arbiter of the meaning of the American Convention on Human Rights, which took effect in 1978 and gained influence as dictatorships became democracies in the 1980s and 1990s. The court rules on cases referred to it by the region’s human-rights commission, a watchdog organisation that is based in Washington. The commission can issue preliminary judgments of its own. At first countries saw the IACHR, which operates under the aegis of the Organisation of American States (OAS), as a complement to their own judicial systems. But over the past decade it has become a supranational supreme court for human rights.

That shift has happened in part because in the mid-2000s the court invented the doctrine of “conventionality control”. This obliges national states and judiciaries to make their constitutions and laws compatible with human-rights treaties their countries have ratified. This principle gives the court, whose budget is a puny $5m, the power, in theory at least, to compel signatories of the human-rights convention to change their constitutions and laws. The European Court of Human Rights, which spends nearly 20 times that amount, has no such power.

Court in the crossfire

In its first decades the IACHR ruled mainly on cases stemming from violations of human rights by dictatorships in the 1970s and 1980s. It ordered governments to investigate atrocities and compensate victims. Its ban on amnesties for such crimes gave legal grounds for courts in Argentina, El Salvador and Guatemala to try alleged perpetrators. It provided justification for jailing Alberto Fujimori, Peru’s president in the 1990s, on charges of violating human rights. (On February 2nd the IACHR is due to hear a complaint against the recent decision by Peru’s current president, Pedro Pablo Kuczynski, to pardon him.)

Recently, the court and commission have moved into more contentious territory. In 2001 the IACHR struck down a clause in Chile’s constitution that stifled freedom of expression. In the same year it ruled that governments must consult indigenous communities before approving projects on their lands. In 2006 the court obliged Peru’s government to add the names of 41 members of Shining Path, a leftist guerrilla group, to a memorial for victims of terrorism. In the early 2010s the human-rights commission rebuked left-wing governments in Ecuador (over press freedom) and Venezuela (over political prisoners).

Such rulings have provoked a backlash against both the court and the commission. Brazil suspended its contribution to the OAS after the commission issued a “precautionary measure” that temporarily blocked construction of the Belo Monte hydroelectric dam in the Amazon. In 2012 Venezuela began the process of leaving the court’s jurisdiction. While the court’s word is supposed to be final, its mechanisms for enforcing its rulings are weak.

That will matter more as it wades into Latin America’s culture wars. In 2012 it issued a liberal ruling on in-vitro fertilisation, saying that life begins gradually, not at the moment of conception. It may invoke that principle when it comes to hear cases on the touchy issue of abortion.

The same-sex marriage ruling has pepped up activists. If it does not prompt governments to rewrite laws, it will provide a basis for challenging them in national courts. Iván Chanis, a lawyer in Panama, is preparing a brief on same-sex marriage for his country’s supreme court and helping in a case in Guatemala. Herman Duarte, the founder of Igualitos, a gay-rights NGO in Costa Rica, has filed a case in El Salvador against the country’s family law, which defines marriage as the union between a man and a woman.

Advocates of other rights hope to ride a wave of liberal jurisprudence. Several women’s-rights groups are “rethinking their strategies” after the same-sex marriage ruling, says Viviana Krsticevic of the Centre for Justice and International Law, an NGO in Washington. The ruling “has energised a lot of people”.

That includes critics, who will probe more avidly the court’s vulnerabilities. One is the doctrine of conventionality control, which is contested by some scholars. The court’s use of the doctrine “will weaken rather than enhance” its authority, says Jorge Contesse, a professor at Rutgers Law School. Another weak spot is the way judges are appointed by member countries of the OAS without public hearings. As the court’s influence expands, critics may demand more scrutiny. Governments that dislike the court’s socially liberal rulings could join forces to appoint like-minded judges. In a worst-case scenario, they might, like Venezuela, start the process of withdrawing from the court’s jurisdiction.

In Costa Rica, which has always complied cheerfully with the court’s decisions, a public outcry has begun. The same-sex marriage ruling has dominated the election campaign in its closing stages. The next president may try to defy the court. Officials at the suburban White House are shocked by their new notoriety. “My grandma called me up and asked why I’m working here,” says one. He may have to get used to such scolding.