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Voiceover: Population dynamics looks at how the population of a country or a region or even the world changes. It takes into account the factors that increase a population and the factors that decrease population to create a total growth rate. There are three factors that contribute to this total growth rate. These are fertility, migration, and mortality. Fertility is the natural ability of human beings to have babies. And babies, obviously, add to the population. Migration looks at the number of people who are moving into and out of countries. It doesn't change the total number of people living on the planet. But it does change the number of people living in a specific country or region. This doesn't mean that when you go on vacation to a different country, that you are migrating there. Migration means that you are moving somewhere permanently to live and work and eventually die. Which brings us to our third factor, mortality. Which is the fact that everyone eventually will die. This obviously decreases the population. In order to measure these three factors, we use rates to measure the number of people who are born. The number of people that move out of or into a different country, and the number of people who die in a certain period of time. Usually, we measure birth migration and death rates over a year's worth of population change. Because it is enough time that an obvious change is visible, but not so much time that we miss trends in how the population changes. And as human beings, we tend to like nice, round measurements like one year. We also like to compare rates that are actually comparable. So all these rates are measured per 1,000 people. This way, we can compare the different rates equally because they are all scaled to that same value. A worldwide birth rate of about 18 point nine births per 1,000 people is much easier to grasp than looking at the around 134 million total births worldwide. And it's easier to compare country birth rates when they are scaled, rather than looking at the total births. Take Mali, for example, where about 700 thousand people will be born this year. This doesn't seem like a lot compared to the 134 million births in the world. But when scaled, the birth rate in Mali is about 46 births per 1,000 people. Which is more than twice the world average. That's a pretty big difference. All right, so now that we've got some of the background and terminology out of the way, we can look at what it is that affects population changes. For this, I'm going to just look at the population growth within a single country. There are two factors that will increase a population of a country, births and immigrations. Immigration is the movement of a person into a country. Conversely, movement out of the country's called emigration. But that decreases the population, so I'll get to that in a minute. As we've already seen, birth rate is the number of births per 1,000 people. But another way to look at it is the total fertility rate, which is the number of kids a woman is predicted to give birth to in her childbearing years. That's why you hear of the typical family with two point one kids. On average, a woman in the U.S. is predicted to give birth to two point one children in her life. This means that the total population of the U.S. will slowly increase. A total fertility rate of exactly two neither adds nor subtracts from the population. This is because the woman is giving birth to the number of people that created the children. This rate only looks at the number of children a woman will bear. So the two children exactly replace the biological parents. And finally, a fertility rate of less than two will decrease the population. Immigration, as I mentioned before, is the movement of people into a country. This number is also scaled and, again, is measured by the number of people immigrating per 1,000 people in the country. If you want to look at the total population increase, take the number of births plus the number of immigrations per 1,000 people, and there you go! That is the rate at which people were added to the population. Now, we can look at the factors that decrease a country's population And you guessed it, death and emigration. When we look at the number of people who have died, we often refer to mortality rates, which are the number of deaths per 1,000 people. You have to be kind of careful when looking at mortality rates, especially when comparing the mortality rates of different countries. Just because the country has a high mortality rate, doesn't mean there are a lot of young or unnatural deaths. For this, we can take a look at population pyramids. A population pyramid graphs the age and sex distribution of a population. On either side, you have males and females. And along the vertical axis, you have increasing age. If there are a lot of elderly people living in one country, who are living a long time and dying of old age, you get either a stationary or constrictive pyramid. These kinds of population pyramids usually indicate low birth rates and low death rates throughout the population. A constrictive pyramid indicates that there are fewer young people than old and is generally seen for very developed countries. So the death rate of this country with lots of old people doesn't compare well to a country where, perhaps, people are dying young from disease. This country's population can be modeled by an expansive population pyramid, which indicates high birth rates and high death rates. A better comparison is to look at age-specific mortality rates, so you can see how many people are dying within a specific age range. Now, you can compare just elderly mortality rates between different countries or just the mortality rates for any specific age or age range. Say, 20 to 24-year-olds, or 45 to 49-year-olds. There's a lot more information you can glean from age-specific comparisons. When you split up the mortality rates by age, you can get what's called a life table or a mortality table. This table tells you the probability that someone will die given their age, which can vary from country to country. But when looking at the population of a single country, an all-encompassing mortality rate is sufficient. The second factor that decreases a population is the emigration of people to other countries. Just like all the other rates I've mentioned, emigration rate is measured by the number of people emigrating per 1,000 people in the country. So if you take the number of deaths plus the number of emigrations per 1,000 people, you can get the rate at which people are removed from the country. Sometimes, it's interesting to look at just the migration statistics. The net migration is the difference between the number of people entering the country and the number of people leaving the country. There are many reasons for people to migrate from the country of their birth to a new country. Worldwide, the trend is generally that people are moving to the more industrialized countries because of the potential for a better life. Some people leave their homes for political reasons and become refugees in a new country. Some people migrate for their jobs, or because they just want to live somewhere foreign. For many of these same reasons, people will often move within their own country, which is called internal migration. While this doesn't change the total population of the country, it can affect the economics or cultures of a country. Often, internal migration is a large factor in urbanization, as people move from rural areas to urban areas. Fertility, migration, and mortality all contribute to the growth rate of a country. But remember that growth rate is not always a positive number. If we want to look at this as an equation, you have the initial population, plus the number of births, minus the number of deaths, plus the number of people emigrating into the country, minus the number of people emigrating out of the country. So now, you have your current population. And if this current population is less than the initial population, then you get a negative growth rate for that country. So while our world population continues to grow, the growth rate of some countries is, in fact, negative, as more people die and leave that country than are born and move into it. So as you can see, there's a lot to consider when looking at how a population changes. You can't just look at births or deaths, alone. You have to take into account that people move to different countries. And even if you take into account every factor you can think of, it is still only an estimate. We can't go around to get a headcount of all seven billion-plus people in the world. We can try with surveys and records. But in some countries, even that just isn't feasible. So we extrapolate and estimate as well as we can. So we can study how our populations are changing.