Demographic transition model| Human populations| AP Environmental science| Khan Academy

In this video, we're going to study something called the demographic transition model, which is something demographers use. The demographers are people who study the makeup of populations and how those transition over time and why that might happen. This model, in particular, is one to think about how a country goes from being pre-industrial to post-industrial, and what happens to the birth rate, the death rate, and the total population over time as they make that transition.

As we'll see, it happens over four stages. So let's just first understand what's going on in this graphic here. Our horizontal axis we see is our time axis, and depending on the country we're studying, this might occur, oh, in fact, it's definitely going to occur over many decades. But some countries still haven't even gotten through all of these phases, so it might have been a century already, and they might still be in, say, this second phase, which we'll describe in a little bit more detail.

The vertical axis, you can see, says "rates per thousand," and this is useful for looking at the birth rate and the death rate. The total population here, which is in this light blue color, really shouldn't be read as a rate; that is the total population. You could probably add another axis here to measure total population, but it's telling us indicatively where the total population is. Now we see the birth rate in this blue color, and we see the death rate in the salmon or this pink-orange-red color.

We can see that as a country is, I guess you could say, starting out, or really according to this model, we would call it the pre-industrial phase, which is pre-industrial. This is the phase right over here. You have relatively high birth rates and relatively high death rates. Why is that happening? Well, in most pre-industrial societies, you have a high death rate because health care is either non-existent or it isn't that good.

You could have high child mortality; a lot of people are dying from diseases that are easy to cure in an industrial society or in a post-industrial society. We'll talk about why that is. There is a high birth rate because, oftentimes, culturally, women still haven't entered the workforce in a major way. Education generally is at a low level, especially for women at this stage, because fundamentally, there is a lack of a health care system for most people. You might not have things like family planning, and so you could imagine that might lead to an overall higher birth rate.

Now, because both the birth rate and the death rate are high but they're about the same, you can see that you have a relatively stable, but we'll say in absolute terms low, population. Then countries might start to industrialize, and so we call this state right here the transitional state. We can see that at least on this graphic, it is elongated, and different countries go through this in different amounts of time. You might have a country like Japan that went through this in less than half a century, but you might have other countries that are still going through this even after a half a century, or even a century or more.

But in the transitional state, this country will start to industrialize, and this whole model is based on looking at what happened in Europe in the 19th century as much of Europe industrialized, and say, okay, maybe other countries will go through the same pattern of demographics as they industrialize. So, let's think about what is happening here. We see that the death rate is starting to come down pretty reasonably. Now, the argument why that would happen as this country is industrializing is that they would now have better health care.

They might have better nutrition. Now, in fact, that's another reason why the pre-industrial society might have a high death rate: people are even having illnesses or dying from malnutrition. But now, because, say, farm productivity is higher, because they're able to use more modern methods, the nutrition is better. Death rates start coming down and birth rates might start coming down a little bit, but not as quickly as the death rates.

Now, why might the birth rates start coming down? Well, once again, access to health care. More women might be able to enter the workforce; you're having a higher level of education as the society starts to develop and becomes wealthier. Now, with the death rate going down at a faster rate than the birth rate, all of a sudden, this will add to the population. If fewer people are dying than being born, the population is increasing over time, and in fact, the rate of population increase is increasing as this gap between birth rate and death rate increases.

Now, the next stage of our demographic transition model we would consider industrial, and the real marker of that is we see that in the industrial phase our birth rate is starting to catch up with our death rate. In this phase, women might be entering the workforce in a major way. Culturally, it is far more acceptable for things like family planning. Culturally, it is far more acceptable for people to delay giving birth, especially for women, so that they could get their education.

There's just more awareness that by having fewer children, the children you do have will be more prosperous and will get better nutrition. The family is overall likely to be wealthier. So as the birth rate and the death rate curve get closer and closer together, the rate of population growth starts to slow.

Then, this last phase right here, over here, the fourth phase in our demographic transition model, which we could call post-industrial, we see that not only has the birth rate caught up to the death rate that it actually can go below the death rate. In that situation, you have a stable population, but if the birth rate actually even goes below the death rate, you can even see the population begin to decline.

There are societies that are already there; for example, Japan has a declining population. Even places like the United States, if we did not have immigration, you would have a declining population because we are in this last phase right over here. This post-industrial declining population, because birth rates have gotten so low, has its own set of issues. You now have an aging population; if birth rates are much lower and people are living much longer, a higher and higher percentage of the population is older.

Older people, especially once they are out of the workforce, raise the question: who’s going to support them? Who is going to provide the health care for them? Where are those resources going to come from? If your population is shrinking, then your economy might not be growing as fast, which would make it even harder to support the cost of an aging population.

So, I'll leave you there. Once again, this is a model; it's a hypothesis. But it's a useful one to at least have a framework of how things might transition in society over time. People use this type of model to advise countries, especially ones that might be in this transitional state. The transitional state is sometimes the place where countries might get stuck because they're getting delayed economic development, so they're not quite able to get to industrial.

Demographers, economists, and others will think about, well, how can you make this transition as well as possible?