Understanding Population Dynamics: Births, Deaths, and Growth

Natural population movements refer to the growth or decline in the number of people based solely on births and deaths. To understand this, we calculate the birth rate, mortality rate, and vegetative growth.

Birth Rate

The birth rate indicates how many people are born per thousand inhabitants in a given population.

Crude Birth Rate = (Births / Population) x 1000

It is considered high if above 30‰, moderate between 15 and 30‰, and low if below 15‰. Currently, in Spain, the rate is below 15‰.

Mortality Rate

The mortality rate indicates the number of deaths per thousand in a population.

Crude Mortality Rate = (Deaths / Population) x 1000

It is considered high if above 30‰, moderate between 15 and 30‰, and low if below 15‰. In Spain, the mortality rate is about 9‰.

Specific Rates vs. Gross Rates

There are also specific rates in addition to gross rates. Age-specific rates are more detailed.

Fertility Rate

More relevant than the birth rate is the fertility rate. The fertility rate only considers women of childbearing age.

Fertility Rate = (Births / Number of Women between 15 and 49 years) x 1000

Since women have dependent children, they are crucial for population regeneration. If there are few women, the possibility of replacement is very small. This is calculated using the reproduction rate.

Reproduction Rate = (Number of Girls Born / Number of Women aged 15 to 49 years) x 1000

Net Rates and Generational Replacement

To determine whether a population will have generational replacement, we use net rates. To obtain net fertility, a table similar to the mortality table is needed. This table includes a column for births at each mother's age, another for the probability of death for each infant, and another for survivors. This allows us to calculate how many children a woman would have throughout her life, assuming that birth and death rates remain constant as in the year of study. The result is given in children per woman. Usually, when we talk about fertility (without further specification), we refer to this rate (also called the Total Fertility Rate or TFR). The minimum fertility rate that ensures generational replacement is 2.1 children per woman. It is 2.1 (and not exactly 2) because, even if each woman has two children, a small excess mortality means not all will reach reproductive age. The net reproduction rate is calculated similarly, but only considering daughters, and the minimum value that ensures replacement is 1.

Final Descent

Another easier way to calculate this index is the final descent. The final descent is the total number of children of women who are past childbearing age.

Final Descent = Number of children that women between 15 and 49 had.

If we divide the number of children by the number of women, we also get the number of children per woman in a given population. In calculating the number of children a woman has, dead children are not supposed to be counted; therefore, the specific mortality rate is given. In any case, we know that both rates are equivalent.

Specific Mortality Rates

We are also interested in specific mortality rates, i.e., how many people die at each age.

Specific Mortality Rate = (Deaths at a specific age / Population of that age) x 1000

A specific case of a specific mortality rate is the infant mortality rate.

Infant Mortality Rate = (Deaths of infants under one year / Live births) x 1000

The infant mortality rate indicates a country's degree of development. The lower the rate, the higher the level of development, or at least better health benefits for the general population.

Life Expectancy

With specific mortality rates for all ages, life expectancy is developed. Life expectancy is the mathematical average obtained by dividing all the years lived by the entire population of children born within a year.

Life Expectancy = Total Years Lived / Births

Life expectancy indicates a country's degree of development. Developed countries have a life expectancy above 73 years. However, differences of up to seven years between developed countries are not necessarily due to further development but rather to having a large young population.

Natural or Vegetative Growth

Natural or vegetative growth is simply births minus deaths. If there are more deaths than births, the result will be a negative number, meaning the population is decreasing instead of increasing.

Natural Growth = Births - Deaths