Revisiting the Overpopulation Question

One of the strange things about my profession is that I can never predict what the public will be fascinated by. Some years ago, I wrote a paper predicting that the sperm quality of men in less industrialized populations would not be as compromised as that of us men in the developed world. But what went viral was the paper’s introduction, which repeated what pretty much every gamete researcher already knows, that measured sperm quality has been declining for decades.

Similarly, as a global health epidemiologist, one of the basic concepts that is quickly glossed over in the early chapters of our textbooks, yet one that I find I revisit often, is the Demographic Transition. The model kick-starts the conversation about overall population growth. Despite it being an old and fairly well trodden bit of theory, I carted it out for a Huffington Post article two years ago, which also achieved some level of “virality”, leading to an appearance on Al Jazeera‘s AJ+ platform in a segment titled, “Honey I Shrunk the Population”.

I’ve received hate mail from both ends of the political spectrum as a result of my position on overpopulation. The rabid Right accuses me of being an apologist for “the Muslim horde” beating down the border walls of Europe. And the rabid Left accuses me of being a traitor to the environment. You know what? Both poles are actually quite racist. When critics talk about the need to have fewer children, they’re almost always talking about “those people over there” and never about themselves or their own families.

The public thirst for positions on overpopulation has surprised me, to be honest. I was asked to contribute a position statement for this “Earther” segment on the Gizmodo website. And I now field regular media and student questions about the demographic and epidemiological considerations of population growth —something that is really not the core of my research focus.

Most recently, I had a lovely chat with Jocelyn Timperley, who is doing an article on overpopulation for BBC Science Focus Magazine.

Our skype call took place at 9:30AM my time (Jocelyn was calling from England). I was all set to go, dog by my side (as he always is), headphones charged and Skype online. Then the fire alarm went off. The dog starting howling and going crazy, jumping all over me, and sending me falling backwards toward the patio doors, whereupon the curtain rod separated and dropped the curtains over me and the howling dog. That’s when the Skype call started.

Whether or not I was wearing pants is for you to speculate about. I will offer no guidance on that front.

As noted, it was a good conversation, and Jocelyn asked me some challenging questions. I tried to write down the answers that I offered, as accurately as I could. So I have decided to share with you the responses that I gave, since I’m sure only a fragment of my perspectives will appear in the actual article. (That’s not a knock on Jocelyn. There’s only so much space in print articles and she has a lot of ground to cover.)

So below are some of the questions she asked me, followed by what I think were my answers. We began by going over the stages of the Demographic Transition. To recap, these are the four stages and a controversial 5th stage:

Stage 1: a largely agrarian lifestyle, high birthrate, high death rate. Population size is small and stable. It’s in your best interests to have many children, since you need them to work the fields, and most will die anyway. The cost of a child is the cost of its food.

Stage 2: with the advent of public health interventions (clean water, vaccination, etc), the child death rate starts to plummet. But the birth rate is still high. So population size grows.

Stage 3: with the advent of social interventions (education for girls) and a move away from agrarian lifestyles (living in cities rather than farms), the birthrate starts to plummet. The cost of a child is the cost of its shelter and education, which is much higher than just its food. There are now strong economic disincentives against procreation. The population is still growing, but slower.

Stage 4: in a service-based and mostly urban economy, birthrate and death rate are both low and roughly equivalent. The population has peaked and is not growing anymore.

Stage 5: the death rate now exceeds the birthrate, and the population size starts to shrink.

And now here’s what I can remember from my chat with Jocelyn…


How would you define overpopulation? How many people can Earth sustain?

I retreat to the standard demographer’s definition: the point at which a population exceeds the land’s ability to sustain it. At a global level, that means that the land is now the full biosphere. And we need to ask, if sustaining is sufficient, or do we mean sustaining at a certain level of production, comfort and technology.

What are we really talking about when we talk about overpopulation – food? Is it not also about emissions and climate impact?

We could be talking about many things: food, resources, standard of living, even aesthetics. Traditionally, we have talked about food, but of course we need to include some measure of ecological impact.

When do we expect population growth to tail off? Do populations naturally have fewer children as the standard of living improves?

The replacement rate is 2.1 children per woman. (One child to replace the mother, one to replace the father, and 0.1 to account for childhood deaths). Presently, the global fertility rate is about 2.5 children per woman.

But based on the Demographic transition, the global fertility rate is expected to drop to 1.9 kids/woman by 2100. A new UN report suggests that we will peak at under 11 billion by then, and either stay there or decline thereafter.

The current world median age is 31 (it was 24 in 1950), and is expected to be 42 by 2100. The world is getting older, this means probably an economic slowdown and definitely fertility deceleration. It might mean rethinking how we define economic wealth, as some countries face the “demographic cliff”.

We’ve been growing at 1-2% since 1950, but this will drop to 0.1% or lower by 2100.

There are confounding factors, of course. Global economic forces might compel some populations to stay in stage 2. For example, if you’re a quinoa farmer who lives an agrarian stage 2 lifestyle, the globalized economy might require you to stay in that role, so that the quinoa continues to flow. Or religious forces might compel you to value reproduction beyond your personal and community better self interests. When this happens –a population’s hesitation to move from stage 2 to stage 3– this is what is called the “Demographic Trap”.

You mentioned in the Earther article that it’s not the number of people that’s the issue but how much those people consume. Could you explain why?

Let’s talk about food first. The original objections to population growth (Malthus) were all concerned with how we would outgrow our food supply. However, due in large part to various agricultural revolutions, we actually produce more food than we need at the moment.

However, we waste or lose about 1/3 of that food. That’s 1.3 billion tonnes of lost food annually.

In low income countries, most of it is lost in the production and transportation stage. In rich countries, the lion’s share is lost at the retail or consumption phase. Clearly, with better food management, we could conceivably triple the amount of food available to people.

We’re all aware of places in the world where people are not getting enough food. Famines are actually rare, with only 2 true famines declared in the past 15 years (south Sudan, Somalia). Those cases are the result of improper global distribution of food, which is largely a political and economic problem.

And as is well known, switching to a largely plant based diet requires fewer natural resources for production than a meat based diet. There’s a reason we don’t eat carnivores. If you raise carnivores, you also have to raise herbivores to feed those carnivores. And if you eat herbivores, you need to raise crops to feed those herbivores. So from a resource efficiency standpoint, just cut to the chase and eat the crops yourself.

The kind of economy also matters. Manufacturing economies are more polluting, but service based economies seem to be more energy dependent. This is confusing, because manufacturing economies have more poor people who are likely consuming less individually. But service economies have more middle class people with high consumption lives, even though the actual means of production is less damaging. So as China and Indian transition from a manufacturing to service based economy, we might see less pollution, but more demands typical of a middle class lifestyle, like travel, technology and meat. But those are cultural issues that, I feel, can be more easily addressed than economic issues.

Is deciding not to have a children/have less children (assuming you live in a wealthy, developed country with high per capita emissions) in your view a credible means to address your personal climate impact?

In my personal view, no. At the micro level, if you have the resources to support and care for a child, then I don’t see a problem with having a child. I’m not a fan of downloading the burden of global care to the choices of individuals, who must often make personal choices against their personal self interest.

Rather, it makes more sense to create economic incentives and disincentives to guide populations into making more sustainable choices. The demographic transition is just that: as public health and economic guarantees heighten the chances that your few children will survive, you are much less likely to *want* more children. The data bear this out again and again.

It really bothers me when crusading environmentalists shame a young couple for simply having a baby, which is the single greatest biological drive we know. There’s usually a bit of racism involved, too. It’s always THOSE PEOPLE are having too many kids.

Is there an argument that the more people there are, the less resources there are available to each person? Because even in countries with low per capita emissions, people should fairly be able to increase them? In short – if the pie is divided up fairly between more people we will all have a smaller slice, and thus this could impact our ability to stay within the constraints imposed by dealing with climate change, or mean we have to give up other things like meat/flying etc.

It’s an interesting argument that ignores an important economic consideration: wealth is tied to population size. Labour is capital. There is a point at which there aren’t sufficient people to sustain the great wealth that gives us the toys and resources to be divided up.

Our technological civilization requires a minimum number of people, multiple industries, and a great deal of energy to sustain it. I think your argument only holds water if we’re willing to retreat to a Victorian era technological state.

Project Drawdown ranks family planning and improved education for girls high on its list of the best ways of reducing emission. Do you agree with this/see any issues?

Educating girls is one of the most impactful methods, statistically speaking, for pushing the transition to stage 4. Absolutely.

New innovations now need to be considered: the ways in which we power our economies (sustainable power), the dietary choices nations make, how we measure wealth (including the inclusion of all externalities, especially environmental ones, seeking a steady state rather than growth), and valuing services more than manufacturing. This will likely require a large scale cultural shift away from overvaluing consumption and material goods, and more toward the valuation of intangible goods, like information, knowledge and services.


And because I hate you all, I leave you with this lasting image of the Blonde Girl and I, just doing our thang.


UPDATE: J.J. pointed out a flaw in my math. If 1/3 of food is currently wasted, recovering it doesn’t “triple” the available food, as I erroneously stated. Rather, it would increase the available food by only 33%. Baaaad Ray.

UPDATE #2: J.J. pointed out that recovering the 1/3 lost food would not increase the available food by 33%, as I erroneously stated, but by 50%. Zod damn it!