And as a consequence, the population was going to outgrow the capacity of the world known at that time to feed itself. If you remember, there was the old Malthusian idea that had occurred in Europe and that had occurred a couple of hundred years earlier that where you have sex, you have babies, and because people wanted sex, they were always going to have babies.

HARPER: That was the time when we actually thought that maximum world population was going to hit about 24 billion by the end of the century. And this fear influenced not just, you know, people reading books, but it influenced policymakers and planners all over the world, right? VEDANTAM: I want you to talk about this idea for a moment, Sarah, which is that - the fear was that overpopulation could actually destroy us. If we do not, by humane means, limit our numbers, then numbers are going to be limited by more famines and shortages and consequent social conflicts. could be busting out at the seams by the end of this century. MASUD OLUFANI: In the 1960s, a new kind of fear began to spread across America. HOWARD K SMITH: Overpopulation, so long predicted, has stolen upon us. This piece of tape I'm going to play you is from a recent documentary. VEDANTAM: So I want to play you a little tape about the kinds of fears that books like 'The Population Bomb' spurred. HARPER: The central warning of that book was that we were going to have massive, out-of-control population growth - so much so that we were going to increase to about 24 billion people on the planet and that that was just going to be totally unsustainable. And it was titled 'The Population Bomb.' What was its central message? VEDANTAM: In 1968, Sarah, a Stanford University biologist named Paul Ehrlich published a book. They also have enormous implications for our economic lives and the well-being of the planet. In her book, 'How Population Change Will Transform Our World,' she shows how large-scale demographic shifts shape intimate aspects of our lives from the number of children we choose to have to how we care for aging parents. Gerontologist Sarah Harper looks at the connections between these changes and what's happening on a large scale with the planet's population. And their choices about whether to get married and have kids are often very different from those of earlier generations. They work at jobs that didn't exist in the 1960s. VEDANTAM: In the decades since John Calhoun created his mouse utopia, the world has changed dramatically. And we find a twist in the doomsday story far stranger than anything John Calhoun imagined. VEDANTAM: This week on HIDDEN BRAIN, we revisit that dire prediction made half a century ago. With too many people competing for partners, shelter and social standing, we would eventually destroy ourselves just like those mice. It was every city, suburb and village projected into the future all around the world. And then he reached a controversial conclusion. VEDANTAM: John Calhoun watched closely as his rodent heaven became a rodent hell. Eventually, the entire population was dead. After a while, the mice stopped reproducing. VEDANTAM: At one point, nearly every mouse in the enclosure had a raw, injured tail from being bitten by other mice. With so many males withdrawn and distracted, nursing females had no one to defend their nests, and they became aggressive. Eventually, a horde of lower status male mice turned into outcasts, spending all day huddled in the center of the room vacant and listless. There were few places to nest and not enough room at the top of the mouse social hierarchy. There were too many young mice and too little space. The details of what happened are disturbing. VEDANTAM: Soon, it didn't feel like a utopia anymore.

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. And those babies grew up and had their own babies. VEDANTAM: Then those babies grew up and had their own babies. 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. And reproducing was key because the scientist, John Calhoun, had a question. All there was to do was eat, play and reproduce. There were no diseases to get sick from, no cats to fear, no rain, no snow, no wind. There were tunnels to play in and strips of paper to nest in. VEDANTAM: Universe 25 was a mouse utopia. And if you got close enough, you could hear its eight tiny inhabitants. In the summer of 1968, a scientist created a small metal room at the National Institute of Mental Health in Maryland.

Mouse utopia experiment full report