A new study led by UCLA law professor Máximo Langer has found that recent criminal justice reforms in the most populous state in Mexico have led to swift adjudication of cases, but show failings in protecting the rights of defendants, providing police oversight and investigating crimes.

The study is the first part of a comparison of criminal justice systems in four Latin American countries — Mexico, Argentina, Brazil and Colombia — by the UCLA Transnational Program on Criminal Justice.

In June, Mexico replaced a criminal justice system that relied entirely on written testimony and did not allow the accused to address judges directly or confront their accusers. The new adversarial system allows for public hearings and trials, oral testimony, plea bargaining, mechanisms for diverting cases, and appointment of pretrial judges to oversee work by police and prosecutors.

The national shift was preceded by legal changes in various Mexican states, including reforms implemented in 2009 in the state of Mexico — with 17 million people the most populous state in the country. Researchers from UCLA, the Mexico City-based Centro de Investigación y Docencia Económicas and Universidad Tres de Febrero in Argentina examined a representative sample of 1,145 cases adjudicated in the state of Mexico from 2010 through 2014.

They found that the adversarial system disposes of most cases expeditiously: 76.9 percent of cases studied were adjudicated with a guilty plea one month or less after charges were filed. Only 3.2 percent of cases had a pretrial phase that lasted more than six months, and only 3.6 percent of cases went to trial.

But the new system is hard on defendants and weak on its investigative capacity, the study found.

In an overwhelming majority of cases, arrests were made on the spot and without a warrant (91.7 percent). Warrants were issued in only 8.3 percent of cases.

According to Mexican law, a doctor must examine a defendant to document any evidence of torture or inhumane treatment by police. In 18 percent of the cases studied, defendants showed signs of torture or cruel or degrading treatment by the police. In nearly all of these cases — 97.4 percent — courts neither excluded evidence nor ordered an investigation when there were signs that defendants had been subjected to police abuse.

Further, prosecutors and judges do not provide oversight of police investigations. In 61.5 percent of the cases, neither the prosecutor nor the judge gathered any elements of proof in the pretrial phase other than evidence provided by the complainant or the police. Defense attorneys did not provide any element of proof in favor of their client in the pretrial phase in 76.9 percent of cases.

In 86.2 percent of cases, no appeal was filed on behalf of convicted defendants.

“While the adversarial system provides an expeditious response to cases, it presents weaknesses in the quality and oversight of police investigations, and the extent to which it respects defendants’ rights,” said Langer, who is also director of the UCLA Transnational Program on Criminal Justice. “Our goal with this study is to illuminate several different criminal justice systems in order to encourage reforms that promote due process, transparency and public safety.”

The report, “How Are Cases Adjudicated in the State of Mexico: A Report on the Operation of the Adversarial Criminal Justice System,” is written in Spanish and is a collaboration led by Langer; Marcelo Bergman of Universidad Nacional de Tres de Febrero; and Gustavo Fondevila, Carlos Vilalta and Alberto Mejía of Centro de Investigación y Docencia Económicas.

The study was funded in part by the University of California Institute for Mexico and the United States Centro de Investigación y Docencia Económicas and the UCLA Transnational Program on Criminal Justice.

Mexico Evalúa, a think tank in Mexico, published the report on Oct. 18 and disseminated it through an event in Mexico City and Spanish-language media. The report has been widely publicized in Mexican media.