From NPR, this is Hidden Brain, I'm Shankar Vedanta.

In the summer of 1968, a scientist created a small metal room at the National Institute

of Mental Health in Maryland.

It was known as Universe 25, and if you got close enough, you could hear its 8 tiny

inhabitants.

Universe 25 was a mouse utopia.

There were tunnels to play in and strips of paper to nest in.

Food and water were plentiful.

There were no diseases to get sick from, no cats to fear, no rain, no snow, no wind.

All they was to do was eat, play and reproduce.

And reproducing was key because the scientist, John Calhoun, had a question.

What would happen as the population of mice grew larger and the dimensions of the room felt

smaller?

Would a change in numbers set off a change in behavior?

Soon the mice began to have babies.

When those babies grew up and had their own babies, and those babies grew up and had

their own babies, and on and on and on, ballooning from dozens of mice to hundreds of mice to

thousands of mice, all crowded together in a 9 foot by 9 foot metal room, twitching,

scampering, screeching.

Soon it didn't feel like a utopia anymore.

The details of what happened are disturbing.

There were too many young mice and too little space.

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