Think the sharp uptick in subway graffiti is just an eyesore? Think again: As Nathan Glazer wrote back in 1979, it can fuel serious disorder — and crime.

Graffiti vandalism in the subway tripled from 200 incidents five years ago to 619 last year, and 537 so far this year, The Post reported last week. Complaints jumped from 13,560 in 2015 to 21,006 in 2018, even as graffiti arrests have dropped.

The “art” is taxing an already debt-ridden MTA. Clean-up costs ran $131,549 in 2016 but a whopping $610,956 last year.

Far worse, though, is whether the surge signals a return to the “bad old days” of out-of-control crime, chaos and disorder.

Back in 1979, Glazer warned that criminals come to think governments that don’t curb minor crimes, like graffiti, won’t control major ones either. His warnings presaged the “broken windows” policing that Mayor Rudy Giuliani and Police Commissioner William Bratton used successfully to target low-level crimes. Their crackdown on vandalism, fare-beating and drug use helped bring an end to the city’s “bad old days,” the period of runaway crime Glazer was writing about.

And it’s not only graffiti on the rise: Fare evasion is up, too, and now expected to cost the MTA $300 million this year. Roving gangs of teenagers harass locals in city parks. The homeless litter the streets.

“Subway graffiti reminds us all of the decay our city once faced,” warns City Council member Joe Borelli.

Indeed, research suggests a strong link between graffiti, in particular, and increased crime: When researchers, for example, placed an envelope with cash in a pristine mailbox, 13 percent of passersby took the money and ran; in one covered with graffiti, 27 percent did.

Defaced trains aren’t just ugly, they’re an invitation. And a warning that the “bad old days” may be just around the corner.