This is Invisibilia, stories about the invisible forces that shape human behavior.

I'm Lulu Miller and I'm Elise Spiegel.

And today we are talking about fear.

And like many stories that involve fear, this one begins in the woods.

This is taped from a film which shows two little children, ages four and six,

together in a clearing in the forest.

They're alone two tiny bodies dwarfed by tall, dark trees.

Close by in the brush, a man is watching them.

By his side, there's a camera.

But really, the children don't even seem to notice the man.

They're too busy, absorbed in one of the most central, secret activities of human childhood.

The production of fart noises.

Now, this film is all about the fart noises in a way.

The man filming them from the trees was an environmental psychologist who was interested in

what children do when they're alone.

Because at that time, this film was taken in the 1970s that work had literally never been done

before. They just hadn't been studying children in natural settings.

This is Roger Hart, the environmental psychologist in the trees.

Almost nothing was known about how children even explored the world.

And then I came across a book on baboons.

And I realized that when you more about baboons everyday behavior,

then we did about children's behavior outside of school.

...