How do births, deaths, immigration, and emigration change a population?

Think of the population as a count within a defined place and period. Births add people, as does immigration—movement into the area being counted. Deaths remove people, as does emigration—movement out. [S1]

A simple diagram is:

Births ───────────────▶ Population ◀────────────── Immigration Deaths ◀────────────── Population ──────────────▶ Emigration

The arrows toward the population are additions; the arrows away are removals. To compare populations, use the same kind of area and time period. For example, “people living in a town during one year” is more precise than just “the town’s population.”

Use this equation:

Ending population = Starting population + births + immigration − deaths − emigration

Worked example (assumed numbers, for illustration only): suppose an area starts with 1,000 people. During the period, 25 births and 40 arrivals occur, while 10 people die and 15 move away.

1,000 + 25 + 40 − 10 − 15 = 1,040

So the ending count is 1,040, which is 40 higher than the starting count. You can also group the additions and removals: 65 people were added and 25 removed, for a net increase of 40. This is an accounting example, not a claim about any real place. If the result were below 1,000, the population would have decreased over the period.

No. Birth and death describe people being born or dying within the population count. Immigration and emigration describe movement across the boundary of the area you are counting: immigration is movement in, and emigration is movement out. [S1]

That boundary matters. A person moving from one town to another can be an emigrant from the first town and an immigrant to the second, while remaining part of the wider country’s population. So state the area before interpreting the movement.

Calculate the balance separately for each period, then compare the results. Birth, death, and migration rates can differ across populations, and population size and composition can fluctuate over time. [S1][S2] A single year’s count describes that period; by itself, it does not establish what will happen in later years or why the pattern changed.

For each year, record the starting count, births, immigration, deaths, emigration, and ending count. Check that the equation balances. If you compare rates rather than raw counts, identify the rate’s time period and population base; the example above uses counts only, not rates. Keep each period’s figures together so you can see which additions and removals contributed to its result. The cited material supports the basic additions-and-removals distinction and change over time, but it does not establish a forecast or explain the cause of a particular population’s changes.