Overall, crime is at historic lows, including violent crime and murders. Some estimates put the figures at about half their peak in the early 1990s. That’s not to minimize the fact that some cities have, in fact, gone through crime spikes in recent years. Chicago is among them, even if Brennan’s researchers consider it “an outlier” and believe “no other large city is expected to see a comparable increase in violence.”

On paper, Chicago’s 9.1 percent increase in overall crime and 16.2 jump in violent crime seem to pale in comparison to, say, San Antonio’s experience: 23.3 percent increase in crime, 52.2 spike in violent crime, and 52.9 percent climb in murders. The key difference is ratios versus actual numbers. Far fewer crimes and murders have to happen in a less populated and less violent city for it to see a significant jump in its per capita rates.

The Brennan report singles out Chicago for its “higher concentrations of poverty, increased gang activity, and fewer police officers.” In general, the authors posit that in cities, conditions like persistent high poverty, racial segregation, and unemployment can create conditions that lead to “short-term spikes in crime.” But Brennan analysts concluded that no real pattern emerges across the biggest 30 cities, and that “spikes are created by as-of-yet unidentified local factors.”

Ames Grawert, one of the authors of the Brennan report, pointed to some potential local factors. “We found in cities that had a pronounced increase in the murder rate both this year and last year, those were cities that had less healthy institutions, higher-than-average poverty and a higher-than-average unemployment rate going back 10 years or so,” he said.

With figures provided by the San Antonio Police Department, the Bexar County Sheriff’s Office, and Brennan, I put some of those theories to the test in San Antonio, one of the fastest-growing cities in the country and one that faces similar socioeconomic challenges as Chicago, though on a more moderate scale.

Bexar County, which encompasses San Antonio, had a 10 percent population increase between 2010 and 2015, with a projected 20 percent increase expected by 2021. Today, 1.8 million call it home, according to the Census, with 20 percent of them living in poverty. Its unemployment rate is 4.2, compared to the national average of 5.6, according to the Bureau of Labor Statistics figures from August. The poverty rate has oscillated between 13 and 21 percent in the last decade.

Grawert also believes it’s worth exploring whether cities that are growing very rapidly might have a harder time coping with increases in violence. “They might not be able to scale their police force and their law enforcement as rapidly to match the growing population,” he said. Might there be a tipping point? Would single-digit or double-digit growth accelerate the incidences of crime?