Sitting on a crowded subway is a lesson in sharing. Each person is allotted 17.5 inches, the width of an average seat, but for some that's simply not enough.

"Manspreading is when men take up too much room on the subway by spreading their legs in a wide V, Like geese traveling," actress Kelley Rae O'Donnell said.

O'Donnell has become an anti-manspreading activist, making stopping the spread a personal mission, reports CBS News correspondent Vladimir Duthiers.

"I guess you would call it subway shaming? It's what my friends accuse me of doing," O'Donnell said.

Three years ago she started taking pictures of people -- mostly men -- spreading out, and posting them on Twitter.

"I spend a lot of time commuting back and forth into Manhattan from Brooklyn, and there was so much of it on these crowded trains that I just starting taking pictures," O'Donnell said. "Mostly because they wouldn't move for people or allow other people to sit down."

"It's an issue that our customers notice and, for lack of a better word, can be an annoyance," Metropolitan Transportation Authority spokesman Kevin Ortiz said.

For the first time the MTA, the organization tasked with keeping the country's largest public transportation system running, is asking men to mind the gap.

In a new courtesy campaign launching this month, they're putting up posters reminding riders to be more considerate of others, including "stopping the spread."

"If there's an opportunity for someone to sit down, especially on a crowded train, where all it takes is a guy to squeeze in a little bit more and let that other person sit down, that's something that we'd like to see happen," Ortiz said.

Subway ridership in New York has doubled in the last 30 years, with nearly 6 million people hopping aboard daily. That means more crowing and less space.

While some call the campaign anti-male, O'Donnell applauds the MTA for encouraging better etiquette.

"This is just a campaign asking people to be more considerate. And if you want to be anti-being-more-considerate of other people, I think there's a bigger issue there that maybe you should look at as a human being," O'Donnell said.

O'Donnell said manspreading comes in many different forms: including the "hard day at work manspread," and the "multi-manspreaders."

Entire blogs are dedicated to pointing out egregious seating, including one where cats are superimposed between men's wide open stances.

So why do men spread?

"It gets hot on the train. You need a little bit more room," one man said.

"It's more comfortable. The other way's a little restraining," another said.

Some were a little shy after getting caught in the act.

"Is this 42nd Street? I've gotta get off!" a man said, avoiding the question.

O'Donnell's favorite reason has to do with anatomy.

"The anatomy you guys have ... you need extra room down there," she said.

The movement against manspreading is going global, with Chicago, London and Toronto each thinking about taking on the spread. And it's not just a male problem. The female equivalent? Bag spreading.