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.