The mystery contained in this chart has puzzled health researchers for decades:

It’s called the Hispanic mortality paradox. In the United States, Hispanics die at slower rates than non-Hispanic whites. This is true even before accounting for differences in incomes and health-care access, which put Hispanics at a disadvantage.

One of the first people to point that out was Kyriacos Markides, a professor at the University of Maine who noted in a 1986 paper that Hispanics in the Southwest were remarkably healthy. Compared to whites, they had lower rates of death from cardiovascular disease and cancer, and lower rates of infant mortality. The “accumulating evidence,” he wrote, “suggests that the health of Hispanics is much closer to that of other whites than to the health of blacks, with whom Hispanics share socioeconomic conditions.”

Why? Markides speculated that Hispanic culture may have some protective effect. Tight-knit families and immigrant communities offer crucial support to people battling illnesses. Hispanics, especially recent immigrants, also tend to behave in more healthy ways, smoking and drinking less. This may explain why second- and third-generation Hispanics, who are more plugged into the mainstream culture, see less of a boost to their health.

Some, on the other hand, suggest that the Hispanic mortality paradox is mostly a quirk of migration and miscounting. Immigrants are in better physical condition, says the “healthy migrant” hypothesis. Others posit that sick Hispanic immigrants tend to return to their home countries, so they don’t show up in the statistics. This is the “salmon bias” hypothesis.

Researchers once proposed that the death-reporting process was undercounting Hispanics, but a 2010 paper in the American Journal of Public Health dismissed that concern. The authors compared Census Bureau data to death certificate records and concluded that the Hispanic paradox is real. By their calculations, even accounting for possible misreporting, the Hispanic death rate is nearly 20 percent lower than that of non-Hispanic whites.

Last year, another paper in the same journal used an entirely different methodology to confirm those results. Piecing together results from 56 medical studies that followed more than 4.6 million patients over time, the authors found that Hispanics were dying at a rate 17.5 percent slower than non-Hispanics. Because all the subjects were followed, this eased some of the concerns about migration.

“Researchers are now seeing that finding as less controversial and more fact,” said John M. Ruiz, one of the authors of the study. “Now the question is, ‘How does it vary within particular diseases?’” For instance, Ruiz said that recent research has focused on why Hispanics seem to be better at surviving cancer and cardiovascular disease.

Understanding the origins of the Hispanic paradox will tell us a lot about the future of health care in the United States. And if the Hispanic advantage does have something to do with culture — with strong, supportive communities and healthy behaviors — then that is something that the rest of the country can emulate.

Hispanic enrollment in the Affordable Care Act’s insurance exchanges has been disappointing. It’s an important demographic: According to a Kaiser report from 2013, nearly a third of non-elderly Hispanics lacked health insurance in 2011. The language barrier and poor outreach were partly to blame for the low signup numbers. But more than any other ethnic group, Hispanics say they’re healthy enough not to need insurance.