In New York, a 78-year-old woman strolling in her neighborhood was punched in the head by a stranger and tumbled to the ground. In Washington, a 32-year-old woman was swarmed by teenagers on bikes, and one clocked her in the face. In Jersey City, a 46-year-old man died after someone sucker-punched him and he struck his head on an iron fence.

In each case, police are investigating whether the attacks are part of a violent game called “knockout,” where the object is to target unsuspecting pedestrians with the intention of knocking them out cold with one punch. Authorities and psychologists say the concept has been around for decades — or longer — and it’s played mostly by impulsive teenage boys looking to impress their friends.

“It’s hard to excuse this behavior, there’s no purpose to this,” said Jeffrey Butts, a psychologist specializing in juvenile delinquency at the John Jay College of Criminal Justice. “When someone runs into a store and demands money, you can sort of understand why they’re doing it, desperation, whatever. But just hitting someone for the sheer thrill of seeing if you can knock someone out is just childish.”

At least two deaths have been linked to the game this year and police have seen a recent spike in similar attacks.

New York City police have deployed additional officers to city neighborhoods where at least seven attacks occurred in the past few weeks, including the assault on the 78-year-old woman. Police Commissioner Raymond Kelly said some are smacked, some are more seriously assaulted, and some harassed. The department’s hate crimes task force is investigating, because some attacks have been against Orthodox Jews in Brooklyn.

In Washington, D.C., police were investigating two assaults in the past week, both of which resulted in minor injuries but not unconsciousness.

One victim, Phoebe Connolly, of Brattleboro, Vt., said she was randomly punched in the face by a teenager while riding her bike during a work-related visit to Washington last Friday. Connolly, who is 32 and works with teenagers in her job, said the blow knocked her head to the side and bloodied her nose.

“I don’t know what the goal was,” she said. “There wasn’t any attempt to take anything from me.”

While some of those attacked have been white, and some suspected attackers black, experts said the incidents are more about preying on the seemingly helpless than race or religion.

“It’s about someone who is seemingly helpless, and choosing that person to target,” Butts said.

A recent media blitz about the game circulating on television stations and online isn’t helping, Connolly and experts said, especially because images are being repeatedly broadcast of victims in a dead fall, smacking the ground with a limp thud. The viral footage comes from older incidents: In one instance from 2012, 50-year-old Pittsburgh English teacher James Addlespurger was punched in the face and falls to the curb. The image was caught on surveillance cameras, and a 15-year-old was arrested.

“The behavior of the sudden assault of someone who seems helpless has appealed to the idiotic impulsive quality of adolescence forever,” said Butts. “But there are now bragging rights beyond your immediate circle, when this is on television and online.”

Paul Boxer, a psychology professor at Rutgers University who studies aggressive behavior, said Thursday the media stories may perpetuate the assaults, but most teens clearly aren’t unfeeling sociopaths.