Only in Chicago, where murder is a macabre civic pathology and an almost daily habit, would the slaughter of more than 400 people in 2013 be cause for relief. But with the 503 killings of 2012 still haunting the city, this year’s 410 through Dec. 26 is an improvement. Context, though, counts: These two years flank by nearly equal margins what’s now the typical murder toll here: When 2013 ends Tuesday, Chicago will have averaged some 458 killings in each of the last 10 years.

That constancy is remarkable, given that murder is not one crime but a constellation of crimes — from tavern brawls to child abuse to gang shootings — in which someone winds up dead. Note in the accompanying graphic how gradually the trend line typically changes through the decades. Looking at that graphic, you’d think murder has a mind of its own, diverting from a reliable average one year and, the next year, stubbornly regressing toward the mean.

No wonder, then, that criminal justice researcher James Alan Fox of Northeastern University in Boston hazards a disturbing guess of what happens now if Chicago doesn’t keep finding ways to buck the averages. Fox, who over the years has examined Chicago murders as a consultant to the Tribune Editorial Board, has a unique view of homicide stats: his database of America’s more than 600,000 murders over the last 36 years, a trove that even the FBI envies. That unmatched wealth of numbers tells him:

“The good news for Chicago is that your murder toll is way down. But the odds say it’ll probably go up next year.”