- Across the globe, a growing number of countries are facing a major demographic shift.

Too few babies are being born.

That's raising some worrisome questions about whether there will be enough workers to drive the global economy and to fund the safety net for a rapidly aging population.

Even China, once known for its one-child policy, is scrambling to reverse the trend.

But wasn't the world supposed to be on a path to way too many people?

- That's a notion that can be traced to a best-selling book written five decades ago.

The book's author made alarming predictions that overpopulation would soon tip the world into chaos, causing famine and conflicts in environmental disaster.

He called it "The Population Bomb."

- Overpopulation, so long predicted, has stolen upon us.

It's getting worse week by week.

- [Narrator] In the 1960s, a new kind of fear began to spread across America.

- [Reporter] The US could be busting out at the seams by the end of the century.

- 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.

- [Narrator] The idea that human population was outstripping the earth's ability to support mankind was a powerful one and it was one man, Stanford biologist Paul Ehrlich.

- Population growth will kill you stone-cold dead.

- [Narrator] Who pushed the dramatic message home.

- If we continue to let population grow, and if we continue to exploit the underdeveloped countries, if we continue to pollute the seas with a wide variety of compounds and so on, it's very difficult for me to picture things holding together for more than another decade or so.

The basic point is so simple.

We have a finite planet with finite resources, and in such a system, you can't have infinite population growth.

- [Narrator] Ehrlich, who had previously focused his scientific research on butterflies, laid out his hypothesis in a slim volume called "The Population Bomb."

It was a call to action for many, including a student Ehrlich advised, Stewart Brand.

- There's too many people and we'd like to see people have fewer children and better ones.

- The whole idea that people make more people who make more people until there's too many people, and by then it's too late, that's a very persuasive argument.

- [Narrator] Adrienne Germain, a women's health advocate, found herself drawn to Ehrlich as well, due to his support of birth control.

- The message was that we were already in a crisis, and if we didn't have urgent and immediate action, the world would simply destroy itself.

- Look at what the year 2000 will be.

Our cities are gonna be choked with people, they're going to be choked with traffic, they're gonna be choked with crime, they're going to be choked with pollution, and they will be impossible places in which to live.

- Paul's picture of doom and gloom looked real.

- Net world population is increasing by 23 people every 10 seconds.

It's clear that world population growth remains completely out of control.

- I bought it totally, many of my friends bought it totally.

I organized an event for 60 people to starve in public.

Maybe anybody who's thinking of having a third child oughta go hungry a week.

The mode became don't have kids.

There's nothing in the world.

And if you're friends have kids, it's fine if they feel uncomfortable about that.

- We had formed an organization called Zero Population Growth, and then Johnny took me on "The Tonight Show."

- Would you welcome Dr. Paul Ehrlich.

(audience clapping) - You have to get the death rate and birth rate in balance, and there's only two ways to do it.

One is to bring the birth rate down.

The other is to push the death rate up.

I did the show maybe 20 times, and we went from six chapters and 600 members to 600 chapters and 60,000 members.

We are starting in now, this is the first step.

- [Reporter] The Bagleys belong to a growing number of young marrieds who favor ZPG, zero population growth.

- [Woman] How many children do you have?

- Two, I have two children.

- [Narrator] Ehrlich's views on how to bring that birth rate down were concrete, compulsion if voluntary methods fail, creating a blacklist of people, companies and organizations impeding population control in the United States.

responsibility prizes for childless marriages, a tax on children, and a luxury tax on diapers and cribs.

- Concerns about population became misanthropic, and it was taken with so much seriousness that Paul Ehrlich could recommend things like putting stuff in public water that would make people not as fertile.

- Panic is not too strong a word to use for some of the advocates that I referred to as true believers.

- It appears that large families are on the way out and ZPG may be possible.

- [Narrator] The idea also took hold in the developing world, where governments like India's had already begun to embrace population control.

- The core message of the book, population growth outstripping food supply, resonated quite a bit with India's elites, with the middle classes.

They much preferred to believe that the poor were poorer because of too many children, rather than being poor because of an unfair and unequal economic system.

- If you start with that problem definition, then it's almost inevitable that there will be circumstances where governments and other actors will act in a way that is coercive.

- [Narrator] In the mid 1970s, the Indian government began a controversial program to encourage mass sterilization.

- We do want to create an atmosphere in which people realize the importance of this program.

- [Narrator] It led to abuses, access to food aid and housing were sometimes used as coercion.

Others weren't even given a choice.

- [Reporter] More than eight million sterilizations were performed, many forcibly.

The people, in the words of one family planning expert, were treated like cattle.

- Zero population growth is a tragic frame, in the sense that it was assumed that there was no way out, that people would just go on reproducing until there really was a desperate circumstance in the world.

- [Narrator] Ehrlich's message could be summed up in a dramatic prediction.

- Sometime in the next 15 years, the end will come, and by the end I mean an utter breakdown of the capacity of the planet to support humanity.

Predictions do not necessarily come true.

The critics go in and look at these little stories that won't come true, and when they didn't come true, say, "Ehrlich was wrong."

- [Narrator] But Ehrlich says it could still be just a matter of time.

- One of the things that people don't understand is that timing to an ecologist is very, very different from timing to an average person.

- How many years do you have to not have the world end to decide that whatever reason you thought the world was gonna end, it actually maybe didn't end because that reason was wrong?

- Ehrlich predicted that by the 1970s, India would be starving.

Quite to the contrary, the Green Revolution came to India with a big bang and a boom in such a rapid way that India has never looked back.

- [Narrator] Although an estimated three million children around the world still die of malnutrition every year, the Green Revolution's farming technology helped lessen rates of hunger in the developing world over the decades, even as the world's population skyrocketed.

- There's a tendency to apply to human beings the same sort of models that may apply for the insect world.

The difference, of course, is that human beings are conscious beings and we do all kinds of things to change our destiny.

- [Narrator] That story is playing out today in parts of India.

In growing cities like Shinay in the South, large families once needed for farming are no longer always seen as the key to success.

- Previously, my father used to have four children, and my grandfather used to have seven children.

But the things are changed, even myself I have only two children.

Even my sister is having only one kid because now education become the prosperity idea.

- "The Population Bomb" was diffused by urbanization and by people getting out of poverty all over the world, by having enough to eat, so you didn't have multiple children in the hopes that some of them would survive.

And it's somewhat ironic that what Paul Ehrlich saw as a horrible, hellish vision of the future is what turned "The Population Bomb" upside down.

- [Narrator] Brand says that Ehrlich did succeed in raising awareness about important issues, such as the destructive effect population growth can have on the environment, even if some of his predictions didn't come to pass.

- If you ask me the question, are there things that I have written in the past that I wouldn't write today, the answer is certainly yes.

I expressed more certainty because I was trying to bring people to get something done.

- [Narrator] But his core message remains the same today.

There are nearly four billion more people in the world, and they are consuming more resources than ever before.

- I do not think my language is too apocalyptic in "The Population Bomb."

My language would be even more apocalyptic today.

The idea that every woman should have as many babies as she wants is to me exactly the same kind of idea as everybody oughta be permitted to throw as much of their garbage into their neighbor's backyard as they want.

- [Narrator] But if the world were to succeed in its decades-old task to curb population growth, what then?

- What if large population is not bad, but is good?

- What many more countries are already trying to come to terms with is aging of the population.

- Japan needs more women to have children.

- America is in the midst of a baby bust.

- China is hoping for a new baby boom.

- The point at which population peaks, around nine billion in the 2040s or '50s, the story will not be, "Oh my god, we got nine billion people, how horrible."

It will be, "Oh my god, we're running out of people."