This is Hidden Brain. I'm Shankar Vedantam.

In April 2017, a woman hurried to step off a subway train in the Bronx just as the doors were closing.

New Yorkers pull off such near escapes every day, but this passenger was not so lucky.

The subway doors pinned her in place, half in, half out of the car.

What happened next was deeply unnerving.

It wasn't that the subway train started moving.

With one of its doors partially open, the train was not going to leave the station.

No, the shocking thing, visible in a cell phone video recorded at the scene, was the sight of people walking past the trapped woman.

The woman was walking off the train when the doors closed on her.

And no one walking by seems to notice or care.

Instead of helping, one witness recorded it.

Once the video went viral, horrified reactions poured in. Like, how could someone just pass by that? People just caught in their own head. They want to do their own thing. Like, you know, I don't have time. What did that say about New Yorkers? I would be flailing and freaking out. In time, a subway employee got the doors open, released the woman, and allowed her to go on her way.

But for those who saw evidence of her lonely plight, a question lingered.

A person is suffering, clearly in need of help, and those who see her simply walk by.

Is this what we can expect from human nature, callous indifference and heartlessness?

Or is there more to the story?

This week on Hidden Brain, the science of altruism,

why some human beings perform acts of extraordinary selflessness,

and how the rest of us can need the help of others at some point.

In the hour of crisis, will someone

extend a hand? Abigail Marsh is a psychologist and neuroscientist at Georgetown University.

She has long been interested in why people help one another and also why they don't. Abby Marsh,

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