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Joel E. Cohen is a mathematical biologist and Professor of Populations at Rockefeller and Columbia Universities. His research deals with the demography, ecology, epidemiology and social organization of human and[…]

A Conversation with the mathematical biologist at Rockefeller University.

Question: Why is the world’s population growing so rapidly

Joel Cohen: Well, I've learned to think about population growth, not as an isolated phenomenon that runs itself, but as part of a system that really has four major components. Population is one, economics is two, the environment is three, and culture is four. And each of those four sphere's of understanding interacts with all three of the others. So I think of these as a triangle, economics, environment, culture, and population on top. Now, if you're an economist, rotate it and put the economics on top, or if you're an ecologist, put the ecology on top, but they all interact and although people think the growth of the population is surging, actually there are at least two different planets on the earth. There's the world in which the population growth rate has fallen very dramatically and there's the world in which the population growth rate has fallen less dramatically. But overall, the global population growth rate has fallen from about 2.1% per year, around 1965, to 1.1% per year. So the population growth rate, that's the interest rate on the bank account, is about half of what it used to be. 

On the other hand, the annual increase, the numbers of people we add each year, it's about 80 million more or less, whereas in 1900, the annual increase was 10 million per year. So rapid population growth on the historic scale continues, but it's slower than it used to be. So you got to keep that in mind. There are two different worlds. There's the world of really low growth, that's Europe and China. China's average number of children per woman per lifetime is about 1.6, 1.7. Europe's is about 1.7. The whole rich world is about 1.7. In some European countries, it's much less than that, it's like 1.1, 1.2, especially Catholic countries, where it's exceptionally low, Spain and Italy. And then there are the countries and Sub-Saharan Africa and the Middle East, where the number of children per woman is very large. And the factors responsible for that is probably what you're having in mind, and we believe that it's a low status for women, lack of job opportunities, lack of education, lack of independence for women. 

Question: What areas should we be focusing on to make population growth more sustainable

Joel Cohen: I'd say there are three areas, well, there are at least three areas that need attention and could be, could be dealt with rather inexpensively. So education, nutrition, contraception. I think we need to assure that every child in the world gets a good, not only primary education, but a secondary, a good secondary education. And what good means should really be decided within a lot of different cultures, but I think it should include recognition of the diversity of human values, recognizing that other people have different values and that in itself is a value. But that, I think, is a universal value, recognizing that other people have other values. And people should have a scientific understanding of their own bodies. How does my body work and how does that person's body work and what does it take for health? So that's education and how does that relate to my earlier interest in population? We know that on the average, more educated women have fewer children. It is not true in every society, but it is true statistically across all societies. And I think educating both women and men, because having ignorant men and educated women is not a solution to the problem. The men tend not to behave very well. So educating both boys and girls gives them options that are alternatives to childbearing. It means people can have children if they want to have children and they should if they want to have children. But it means that childbearing is not the only route to status, the only asset that a woman can have for herself. If she is uneducated, the only thing she can do is have children and hope that they'll take care of her in her old age and having other capacities gives her other options. 

So education is one. I've spoken about nutrition. I can't understand why there is no news about the fact that a billion people are hungry every night when they go to sleep and that we have the food to feed them and we don't get it to them. It just drives me nuts. And so this is a second area, we should fee the people we have, we have the means to do it and we've got to get them the capacity to eat. 

And then the third is contraception so that every child that's born is a wanted child. According to a recent survey, in the United States alone, about 57% of births are unintended, and that's just way too high. So those are my three. 

Question: Do you have any specific examples of the four factors of population growth affecting a material good

Joel Cohen: Meat is a beautiful example of the interaction of population, economics, environment, and culture. And I can give some specifics. First of all, let's deal with the culture part, because that's interesting. The word for home in Chinese is **** and the character has two parts. It has a roof, a horizontal line with a little thing on top, that's the roof, and it has a pig underneath. So in China, the very character for, in classical Chinese, written Chinese, the very character is the roof you put over your pig. And if you go to Yunnan today, you can still see homes that are built on stilts, the pigs live underneath, the warmth rises and keeps the home warm during the winter season, and the waste from the humans goes down and feeds the pigs. So home and pig are intimate, and so, for a billion people. 

For another billion people, a pig is forbidden meat. The word is Arabic is **** it means not kosher, and Muslims do not eat pig at all. But I was recently in the Atlas mountains in Berber villages and the homes are built up high and down beneath, immediately underneath, are cattle, cows, and goats. So they have, the culture has changed the animal, but the relationship between people and the animals they raise for meat is still very intimate, okay? So animals play a major role in human culture and they're different in different cultures. So culture influences which animals you have. 

Now, what difference does it make whether it's a pig or a cow? Pigs have one stomach, they are mono-gastric. Cows have multiple stomachs and they are called ruminants, pigs are not. The ruminants eat grass and they have bacteria in their stomachs that digest the grass for them to a point where the animal can use it. That digestion generates methane, which is a greenhouse gas that is much more potent than carbon dioxide, although people don't talk about methane, it is a major forcing factor. And the cattle of the world generate a lot of methane. The estimates vary from 3% to 18% of all of global greenhouse gas emissions come out of our domestic animals, depends, and I don't know what's the right number, because the calculations are done in many different ways, but we do know that it matters. So the cultural choice of which animals you choose to raise has an influence on our human greenhouse—the animal greenhouse gas emissions that we put out. 

So that's an example, interaction between culture and the environment. Over the last 40 years, the tons of meat that we grow, per year, has increased a factor of fourfold from about 70 million metric tons to about 280 million metric tons. That's between 1961 and 2008. In that period, the number of people on earth has gone from 3.1 billion to about 6.7 billion, 6.7 or 8 now, but 6.7, so it's more than doubled, so the meat production per person has doubled, okay? And so the meat production is driven about half by population growth, what drives the other half? Growing wealth. The first thing that poor people want when their income rises, is more meat in their diet. And so the interaction of population growth and economic growth has meant a huge change in the composition of diets, especially in Asia, where economic growth has been fastest. Not only in China, but in the Little Tigers around China. 

That has had health consequences for you and me. What's the health consequence for you and me? As the South Asians have gotten richer, the easiest kind of meat to grow is poultry, chicken and ducks and geese. Where do they like to go? They like to live in wetlands, ducks. Quack, quack, in the water. So many wetlands used by wild migratory birds were populated with domestic ducks and geese, okay? And the wild fowl have an influenza virus called H5N1, that avian influenza. Doesn't bother them, they're used to it, because it's evolved with those animals. They poop in the water and the virus goes into the poop and into the water, that infects the domestic water fowl. They get the avian influenza, H5N1, then the caretakers bring them home and they hang around with their ducks, and I can show you people with their ducks right next to the house. And that increases the risk to the humans of getting the H5N1, the avian influenza concern is a result of growing wealth, growing meat consumption, and invasion of new habitats. 

Let me just, one more step. The same lake, the biggest lake in China is in the center of China, it's a salt lake, and that same lake is visited by wild fowl that migrate north across the Bering Straits and into North America. And so the US Geological Survey has found the genome of avian influenza from China in the viruses being shed by water fowl in the United States. The only reason we don't have to worry as much as the Chinese about getting avian influenza from our wild fowl, is that we keep our chickens and ducks separated from the natural wetlands, where the migratory wild fowl from Asia go. But if we did the same thing that the Chinese do, which is put our ducks out into the wetlands, we would have the same problem. And they've also suggested a solution for them, which is to separate their domestic fowl from the wetlands, but that costs money 

Question: What is a “food web?” 

Joel Cohen: A food web is a description of which species living in a place eat which other species. You can think of it like a roadmap with one-way streets. It shows you, if there's an arrow from A to B, it means the energy flows from A to B, or in other words, B eats A, okay? So it's usually drawn with a bunch of circles, you put the name of a species in the circle and then you draw an arrow showing which way the food is flowing. Okay? That's what a food web is. 

Now, we have just been talking about a food web. We've been talking about the food web in which people eat ducks, geese, swine, cattle, okay? And, what most people don't realize is, the things that eat us are the infectious diseases, like the viruses and the bacteria and the worms and the other parasites, much more important than the lions and the tigers. 

And what I've been studying is how the animals we eat put us at risk of being eaten by the infectious agents that eat those animals. So when we eat the duck, it puts us at risk of being eaten by the virus that eats the duck, the H5N1. But let me give you some other examples, okay? The monkeys that live in the forests of west Africa, have long been infected by a virus called the simian influenza—sorry, simian immunodeficiency virus. Okay? SIV. We now believe that people went hunting for those monkeys and either got the blood in their hands or ate them without cooking them fully, and the simian immunodeficiency virus infected the people who were dealing with the blood from those animals and evolved very slightly, because we can compare the genes, and gave us the human immunodeficiency virus, HIV. So by going into a new habitat, eating the monkeys and getting their blood, the things that were eating the monkeys turned to eating us. That's the current understanding of the origin of HIV. It was going after food in the forest. 

Another, okay, do you follow that? We've talked about avian influenza, that's from the expansion into new habitats. Another example is swine flu and trichinosis. We eat the pigs, the worms of trichinosis, if they're not cooked to death and we eat uncooked ham or pork, they start infecting the people. The influenza that swine have infects the people who live with the swine. A last example, mad cow disease. Mad cow disease is a prion that causes bovine spongiform encephalitis, BES. If you eat the flesh of a cow that has been eaten by one of those prions, it will eat you, and then you get Jackob Creutzfeldt disease. 

And, so there's a World Health Organization for animals, did you know that? Probably not. It's World Organization for Animal Health, it's called, and they have a long list of what are called zoonotic diseases. And a zoonotic disease is a disease that regularly infects vertebrates and will also infect people if they are exposed to it. And many, many, many of those zoonotic diseases are diseases that arise because we raise domestic animals for meat. So there's a connection with the meat and our health that's very close. 

Question: How much of our available land do we use in creating food

Joel Cohen: Well, the ice-free land of the earth, you know, the world is mostly water, like about 71% water, and 29% is land. Now, some of that land is under ice, Antarctica and Greenland, so let's put that aside. If you're only dealing with the ice-free land, it's about 130 million square kilometers, okay? Of that, humans use 38% for agriculture. So almost two-fifths of the entire surface of the earth is now devoted to making food for you and me, and 6.8 billion other human beings. That's a huge chunk of the earth's surface for humans to be manipulating. 

Of that 38%, I'm going to divide it into 2 pieces, 8% and 30%, okay? The 8% grows cereal grains for humans to eat, that go into human mouths—rice, wheat, corn, and then other grains also, okay? The 30% feeds our domestic animals. So the way we use the land, we're using 30% of the entire land area of the earth free of ice to feed our domestic animals. And of the cereals we grow, one-third goes into the mouths of animals and about half goes into human mouths, and about one-sixth goes into the mouths of the machines, our pets, for bio-fuels, and for starches and for other industrial uses. 

So we can say that the production of food by humans has had a huge impact on the use of land on the earth, on the extinction of species due to burning of forests and change of grasslands by animals. And if you look at the amount of energy that the domestic animal populations consume, it's about twice that that all humans consume. So even though we consume a lot, our domestic animals consume about twice more. 

Question: How much energy do humans consume

Joel Cohen: Well, the average number of calories that we eat per day is about 2,100 kilocalories per day. Okay? And that's energy per unit time, kilocalories per day. So energy per unit time is what we call power in physics. Not the Washington DC kind of power, but power for physicist is how much energy are you burning up per unit time? And if you calculate how much that works out to, it's the same thing as a 100 watt bulb. So a watt is a joule per second and if you calculate how many joules your burn up in a day at 100 watts, that works out to be 2100 kilocalories per day. So if you have, like if there are three people in the room and it gets warm, it's because they're generating 300 watts of heat, it's like having a 300 watt bulb going on. So if you multiply 100 watts by 6.8 billion, which is the number of people on the planet, you find out that the whole power generation of the human species is about .68 terawatts, okay? A terawatt is a lot of watts. Let's see, there's kilowatts, megawatts, gigabytes, gigawatts, and then terawatts, so it's a lot. 

So our domestic, just to finish the thought, our domestic animals, if we are .68 terawatts, our domestic animals are about twice that. So that makes it about 1.4 terawatts. And then if you look at all of the domestic, all of the inanimate energy that people produce, it's about 15 terawatts. So you should think of each person you see as a parade of 25 people. First there's the person and that person's energetic consumption, immediately behind are two people who represent the domestic animals on average that that person is responsible for. And after that, there are 22 more people who represent the inanimate energy, okay? So 1 plus 2 plus 22 is 25, so why are we transforming the earth? Because we have 7 billion people, but each of them has 25 shadows, energetic shadows, so it's the same as if were covered with 175 billion people. That's how we are transforming the earth. And the animals are a part of that, they're the equivalent of two of those people 

Question: Is the Earth close to reaching maximum capacity

Joel Cohen: I wrote a book in 1995 called How Many People Can The Earth Support, and I made a real effort to find every scientifically, allegedly scientifically-based estimate of how many people the earth can support and I found 66. After I published the book, naturally I found a few more, but they didn't change the conclusions. In the last half century, the estimates ranged of how many people the earth can support, ranged from less than 1 billion to more than a trillion, which is a thousand billions. They can't all be right. What's going on? They make different assumptions. They make different assumptions about the average level of well being that people will want to have, and the distribution of well being. And about how we will settle our differences, whether we'll settle them by peaceful negotiation, or by violent methods and what kinds of economies will we have? Will we trade or will we not trade? And what kinds of families will we have? Who will take care of young people and who will take care of old people? And what kind of an environment do we want? They make assumptions about whether we want parking lots or parks. Jaguars with a small J or Jaguars with a big J. You know, what do we want? Tin cans with wheels or are we going to walk, are we going to take a bike? So they make very different assumptions and I don't think is a question that's completely specified, it's not a science question. And the numbers that are given are too often political numbers, they're numbers intended to persuade you that we have too many people or we could have a lot more people and we don't have enough people. 

I really don't, having spent four years writing that book, I now think that we should pay much more attention to the quality of life of the people who we have now and the quality of life for the children that we're going to have in the next year and 5 years and 20 years. And so I've shifted my emphasis from trying to answer a question that's not well formulated in the first place, to what can we do to improve life for ourselves and for the children, and especially for the billion people who are chronically hungry now. It's amazing to me—so you ask about how many people can the earth support? Last year, we grew enough grain to feed decently between 9 and 11 billion people, okay? We have under 7 billion, and of those, 1 billion are chronically hungry, they're not getting enough. How is it possible? We're growing 9 to 11, we've got a billion hungry people. We're only 7 billion altogether, how does that work? For every 3 kilograms of grain that goes into a human mouth, we put 2 kilograms of grain into an animal mouth. And we have put animals in the queue for food ahead of a billion people. And the reason is, the billion people are poor. They got no money and they're economically invisible because the price of food doesn't take account of hunger. Especially children. They've got no, they don't have it. And so they are hungry, we've got a billion hungry people and we're feeding 40% of the consumed grain to animals, not to mention the one-sixth that goes into the machines. So I think we've got a problem, when we put machines and animals ahead of people. 

Question: How can we stop wasting so much food

Joel Cohen: Well, I don't know that it's wasted. The people who are getting rich are demanding the meat, they want, in the economic sense, they're buying meat, they want it. And the farmers are supplying where there's demand. What I think we need to do is take several steps to break the cycle of poverty so that the people who are poor can also get grain or meat or whatever they want. I mean, if they've vegetarian, fine, let them eat vegetables. If they want meat, let them buy meat. But part of the problem is that meat consumers in rich countries have no idea of the environmental impacts of eating meat. So the price of meat does not reflect the real costs in terms of well being for the earth. It does not reflect the land degradation due to over-grazing. It does not reflect the water pollution, it does not reflect the greenhouse gas emission that affects everybody whether you eat meat or not. So the prices do not incorporate the real cost. The price of meat doesn't incorporate the health risks. The infectious diseases, the e-coli infections, the salmonella. You know, there was just another food recall in the New York Times yesterday because of salmonella. It doesn't reflect the losses of species. 

So the first thing I would do is get the prices of meat to reflect all of the costs and I think that would be a signal of what's in, to the consumer, and the other thing is to provide the consumers in the west with information. What are the consequences of eating meat? But that's not enough, because there's a whole billion people who are outside of the price system and for them, I think we need three kinds of interventions. You ask what to do? Here are three things I think we should do. There are about 200 million women in the world who have an un-met need for contraception. That means if you ask them, do you want to have another child and they say no, and are you having intercourse and exposed to the risk of conception and they say yes, they have an un-met need for contraception. There are about 200 million. It would cost about $4 billion to fix that problem. That is not a big deal, we could fix that. 

The second thing, and we should be educating teenage boys and girls about contraception before they become parents. Secondly, we should have nutrition education for pregnant women, for parents, and for teenagers, how to make a whole protein out of whatever your local food is. Rice and beans, it’s a great combination, it gives you a complete protein, but in Liberia, a lot of people eat just the polished white rice because that’s the upper class food, and beans is cheap food and they consider that not dignity. This is where culture comes in, eating poor people’s food is not dignified, as a result, they get malnutrition. But if we could teach people to make whole protein from whatever is locally available, by nutrition education, that would be a big step. 

And the third thing is, as a temporary measure, we need to provide good diets to pregnant women, lactating women and their offspring, and infants up to the age of three, because that's when their brains are being laid down and give them a start to get out of this trap of poverty. The poverty makes them have bad diets and the bad diets stunts their mental and physical growth and that keeps them in poverty. So there are some specific things that we can do to get out of this bind. 

Question: What are some of your favorite science jokes

Joel Cohen: Let me tell you the joke that got me into this trouble in the first place. I went to the Institute for Advanced Study in Princeton and I was at a dinner party and so I told the following sexist joke, this is a sexist joke. There are three men in a bar, a doctor, a lawyer, and a mathematician. By the way, I get my just desserts at the end of this, so don't worry. A doctor, lawyer, and a mathematician, and they are discussing whether it's better to have a wife or a lover, a mistress, a wife or a mistress. The doctor says, "It's much better to have a wife because you work all day, you come home at the end of the day, you want to have a steady, settled family life." And the lawyer says, "No, no, no, no, no. It's much better to have a mistress because maybe for the first 5, 10, 20, 30 years you'd like to have a steady family life, but if you change your mind, it gets incredibly complicated, so it's much, much better to have a mistress." And they're going back and forth, they can't agree, so they turn to the mathematician. "What do you think?" And the mathematician says, "It's better to have a wife and a mistress. And the reason is, your wife will be afraid to ask if you are spending time with your mistress and your mistress will know that of course you have to spend some time with your wife. Which means you can spend your time doing mathematics!" 

Okay. All right? Not sexist, okay. Okay, so I'm sitting at the table and this lady, Betsy Devine, is at the table, she's married to Frank Willcheck. At that time, she and Frank were living in Einstein's house, this is the truth, I'm not making this up. They were living in Einstein's house in Princeton. Then they moved to MIT and he goes and gets the Nobel Prize in physics. So anyway, two weeks later, she comes around, she is raising money for the Institute for Advanced Studies' softball team—you probably haven't heard of them in the Major Leagues. And she is selling this mimeographed handout. I opened it up, what is this? Three women are in a bar, a lawyer, an engineer, and a computer scientist. And they're debating whether it's better to have a husband or a lover. So I got my comeuppance, so I said to her, "Look, you stole my joke." She said, "Yeah, I stole your joke." So I said, "You should have stolen more of my jokes, I have a lot more." So we collaborated and that's how we did this book. Now, I will tell you a couple more jokes from that book. 

Here is a limerick which I made up myself and published in this book. There once was a mathematician who preferred an exotic position; twas the joy of his life to explore with his wife, topologically complex coition. So that was a very successful. And here's the last one and then I'm going to quit on this nonsense. Well, actually I have two more, but anyway, all right. 

An ecologist, an economist, and a statistician go on a deer hunt and they're creeping through the forest and they're hunting with bow and arrow and suddenly they see a deer right straight in front of them. And the ecologist says, "Me, first." Takes careful aim, fires, and the arrow goes 5 meters to the left of the deer, because ecologists are leftists. Then the economist takes careful aim and fires, and the arrow goes 5 meters to the right of the deer, because the economists are rightists. The statistician looks at the two arrows and looks at the deer and jumps and down and says, "We got it! We got it!" On the average.

Question: Why are population rates declining in wealthy nations?

Joel Cohen: You have asked the $64 million question. And demographers believe they have answers, but they are partial, Monday morning quarterbacking after the fact. So the truth is that nobody predicted ahead of time that the world's population growth rate would peak at 2.1% per year in 1965 and then fall by half by 2000. Nobody predicted that. On the other hand, fertility began to fall around 1750 in France, before the French revolution, before the invention of the condom, before there was literacy, before there was women's rights, why? Because the nobility didn't have enough lands to tax to feed all of their sons, forget the daughters, the sons got the land. So we know from the records that the nobles of France started reducing the number of children they have from natural, compared to natural levels of fertility. They spaced them and they stopped having them earlier because they couldn't, there weren't enough people to tax. The peasants saw what the nobles were doing and they didn't have enough land to divide up among their children, so they started reducing the number of their children. So if they didn't have condoms and they didn't have the pill, which hadn't been invented, and they didn't have diaphragms, they didn't have IUD's, what did they do? You tell me. What did they do? Withdrawal, coitus interruptus. Okay? It's perfectly effective if you really are serious about it, okay, I don't recommend it as a practice today, it's too risky, but statistically, it worked.

So fertility in France declined, began to decline around 1750 and has continued. When did fertility begin to decline in the United States? Do you know? Civil war. After the civil war, began to decline and has continued to decline. It dropped during the Depression in the '30's, spiked at the baby boom after the war, but has dropped back down. The only reason it's as high as it is now is due to immigrants, who have higher levels of fertility, but even within immigrant population, within a generation, it falls back to the level of the native born population. Okay?

This shows that any simple answer I could give to your question would be wrong. Because in some countries, education makes a huge difference, and in some countries, economics makes a huge difference, and in some countries, you know, technology makes a huge difference, having the pill and contraceptive devices available. So it's multi-factorial and we don't have a good theory to explain the past and we have even less understanding of what will happen in the future. Let's take countries that are in decline now, Russia is using a million people a year roughly. Japan is in decline, Russia is in decline, Germany is in decline. Will those countries ever have a level of fertility that picks up enough to keep them from going out of business? If you can answer that question? You get the gold star, because we don't know. And demographers make different scenarios about the future, but the honest fact is, we don't know what the future will bring in that dimension because we don't have a basic understanding of why people choose to have children.