Voiceover: Demographic transition is a model that changes in a country's population. It states that the population will eventually stop growing when the country transitions from high birth rates and high death rates to low birth rates and death rates, stabilizing the population. This stabilization often occurs in industrialized countries, because less developed countries tend to rely on and follow the more developed countries for their advancements. Right now, most countries have a positive growth rate, which means their population keeps getting bigger. First, let's pin down what the growth rate is. Growth rate measures how much the population of a country grows or shrinks over some time period. For example, let's take a look at this country. I'm going to call it "Zed". Zed, here, had one million people at the beginning of the year. If we want to know the growth rate of the population of Zed for the year, we count how many people were added to the population and how many people were removed. The number of people added includes the number of births and the number of people who immigrated into the country during that year. Let's say 20,000 babies were born this year and 50,000 people moved to Zed from other countries. Then you have to subtract from this number how many people were removed from the population, so the number of deaths and the number of people who emigrated from the country during that year. Let's say, during the year, 15,000 people died and 5000 people moved out of Zed. From here, it's pretty easy to figure out the population of Zed at the end of the year. Started with one million, add 20,000 births and 50,000 immigrants and subtract 15,000 deaths and 5000 emigrants, which gives us 1,050,000 people at the end of the year. If we want the growth rate over this year, all you need to do is take that total current population, subtract the total number of people in the country at the beginning of the year, and then divide by that number again. Multiply it by 100 and you turn it into a percentage. Now you have your growth rate. So now you can see why, when we say there's a positive growth rate, that means that the population is now bigger than the population in the past. But why do most countries currently have a positive growth rate? There are economic benefits, because children can work to help support the family. Sometimes, the government even provides incentives to families for each child, like in Japan, where birth rates are very low. Religion also influences population growth, because it often promotes large families, which increase the number of people in their faith and encourages stronger community. Some religions will even forbid the use of contraceptives by their followers, pretty much ensuring large families. And there are cultural influences that promote large families, too. Having children means that a person is passing down their own family's traits and values. There's a kind of prestige that goes along with having children. Okay, now let's dive into the demographic transition model. There are five stages to the demographic transition model. In Stage 1, a country has high birth rates, often due to limited birth control and the economic benefit of having more people to work. They also have high death rates, due to poor nutrition or high rates of disease. It is believed that most countries were at Stage 1 until the 18th Century, when death rates in western Europe began to fall. You can see this type of population modeled by a high stationary population pyramid, with a high birth rate. This pyramid shows the number of people alive in a population, depending on age and gender. As you can see, the Stage 1 stationary population has many births, creating a large young population, as well as many deaths, creating a small older population and keeping the over all population fairly stable. The second stage is seen in the beginnings of the developing country. The population begins to rise as death rates drop, because of improvements in health and sanitation and the availability of food. This trend can be seen in western Europe in the 19th Century, after the Industrial Revolution. The birth rates are about the same as they were in Stage 1, though, so the over all population begins to grow. This is an early expanding population pyramid. You have high birth rates still, see, lots of young people, but the death rate is declining, so you have more older people, making this nice pyramid shape. In Stage 3, the death rates continue to drop, but at the same time, birth rates also begin to fall because of access to contraception and a changing social trend toward smaller families. The society has better health care and is becoming more industrialized by this point, meaning there are fewer childhood deaths and also the kids don't need to work, or aren't allowed to work by law any more. Having lots of children isn't economically beneficial any more, as the kids are sent to school, rather than working to support the family. Many countries in South America and the Middle East have such declining birth rates. This population is still expanding, but at a slower rate. You can see in this late expanding population pyramid, that, as birth rates decline, there are fewer young people and, with the already-declining death rates, people are living longer lives. The population finally stabilizes in Stage 4 of the demographic transition model, where both birth rates and death rates are low and balance each other out. By this point, the population is rather large, because it had been growing up until this point. The low birth rates are due to a combination of improvements in contraception as well as the high percentage of women in the workforce and the fact that many couples choose to focus on careers over having children. Countries like the United States or Australia are in Stage 4 right now. The population can be modeled by a low stationary pyramid, with low birth rates and low death rates, as well as a longer life expectancy. The fifth and final stage is only a speculation. There are few theories as to what happens next. Some believe that the world population will be forced to stabilize as the Malthusian Theorem suggests. Perhaps we will run out of resources, and there will be a global food shortage. Already, of the more than seven billion people on our planet, there are about one billion world-wide who suffer from hunger and malnutrition. The world population continues to increase but, perhaps, we won't be able to maintain the natural resources at the rate we are going for how many people live on this planet, which Malthusians believe will lead to a major public health disaster and force the population to remain stable. Or, perhaps, the population will begin to decrease after it stabilizes, continuing the trend of decreasing birth rates until it drops below the death rate. With more people dying than being born, there would be a negative growth rate. This results in a constrictive population pyramid, where there are fewer young people than old. Perhaps this will be because of a rise in individualism or, perhaps, as the anti-Malthusian Theorem states, this will be because couples only want to have one child or they have children later in life. Some evidence shows that a better standard of living promotes smaller families, as children become an economic burden, rather than a source of financial support. Industrialized nations often have better education and access to health care, which contribute to more reproductive choices. Some governments, like in China, are even adopting policies that encourage small families to slow their population growth and save resources. Or, on the other hand, perhaps the population will begin to grow again after the stabilization of the fourth stage. Some evidence shows that high standards of living actually promote fertility and a higher birth rate. There's only one real way for us to find out what will happen next and we'll have to wait it out and see where the world is in a century or two. So, I'll see you there, right? To sum it up, demographic transition is a shift from high birth and death rates to low birth and death rates as a country becomes industrialized, but what will happen after that is impossible to tell. Will the population stabilize? Will it decrease, will increase? Will we move off-planet and colonize a world around a distant star? We could only guess for now.