Population dynamics | Society and Culture | MCAT | Khan Academy


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.

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