Population structure is the number or % of males and females in different age groups within a population. Population pyramids show population structure. You can learn a lot about the demographics of a place from its population pyramid.
Variation in Population Pyramids
Population pyramids for different countries vary because of different demographic factors.
A country's population structure changes through time as it moves through the stages of the DTM
Migration Changing Population Structure
Internal migration from rural to urban often affects the number of young adults. They move away from the countryside into the cities to find jobs. This is rural-urban migration. This can affect birth rate as the migrants are of reproductive age.
Emigration away from countries at later DTM stages
Emigration away from countries at later DTM stages into other countries decreases the population of the source country and increases the number in the host countries.
Immigration into countries at later DTM stages
Immigration into countries at later DTM stages from those at earlier DTM stages increases the population of people of working and reproductive age. This increases the population of young people and increases birth rate.
Push and Pull Factors, Obstacles and Opportunities
Things that make people want to move out of the place they're in. They're negative factors about the place they're leaving, e.g. lack of jobs, poor living conditions, poor services, fear of political persecution.
Things that attract people to a new place. They're positive factors about the place they're moving to, e.g. better jobs, more job opportunites, better living conditions and services.
Things that make migration more difficult e.g. cost of moving
Opportunities individuals encounter that mean they stop before they reach their intended destination