Edlund and Machado used Federal Communications Commission data on cellular-infrastructure deployment and matched it against the FBI’s (admittedly spotty) database on homicides across the country. They demonstrated a negative relationship that was even stronger for black and Latino populations. The title of their paper suggests that a crucial aspect of understanding declining crime has been hiding in plain sight for years: “It’s the Phone, Stupid: Mobiles and Murder.”

Their theory is the latest entry in a series of attempts to explain the components of the long-term decline in crime that began in the early 1990s. The rise and fall of crime in the late 20th century (and into the 21st) is one of the great mysteries of social science. No one has come up with an explanation that fully—and incontestably—accounts for the falling crime rates. Many have tried, and shown substantial initial results, only to have their findings disputed.

Edlund and Machado are not the first to suggest that phones could have played a role in the decline. Among others, the criminologists Erin Orrick and Alex Piquero were able to show that property crime fell as cellphone-ownership rates climbed. The first paper on the cellphone-crime link suggested that phones were an “underappreciated” crime deterrent, as mobile communications allow illegal behavior to be reported more easily and quickly.

But cellphones are far from the only possible explanation. Any measurement that was going up in the ’90s correlates with the decline of violence. Thus, there are probably too many theories out there, each with limited explanatory power. One commonsense argument that’s been made is that certain police tactics (say, stop-and-frisk or the “broken windows” approach) or the explosion of incarceration rates must have been responsible for the decline, but most careful reviews have found little evidence to suggest that they had more than a marginal impact.

Read: What caused the great crime decline in the U.S.?

The University of New Haven criminologist Maria Tcherni-Buzzeo published a review of the contending theories in 2018 that found no fewer than 24 different explanations for why crime began a multi-decade decline in the early 1990s, through economic times good and bad, in different countries and cities, under draconian policing regimes and more progressive ones.

Every theory has its proponents and detractors. For example, the economists Steven Levitt and John Donohue proposed (and doubled down on) the idea that legalizing abortion reduced crime rates by cutting down on the number of unwanted pregnancies and children born into situations that make them more likely to fall into criminal life. Tcherni-Buzzeo described the theory as “thoroughly debunked by empirical research” in a 2018 book chapter looking at the theories behind the crime decline. Yet Levitt and Donohue’s most recent research, published as a working paper this month, contends they were even more right all along than they’d thought, and that the “cumulative impact of legalized abortion on crime is roughly 45 percent, accounting for a very substantial portion of the roughly 50–55 percent overall decline from the peak of crime in the early 1990s.”