A growing population shares a country's output among more people

Growth is really about the living standards of the average person, so economists track GDP per capita: GDP divided by the population. With population in the denominator, a rise in population on its own lowers income per head. A growing population helps the average person only if the economy's income grows faster than the population does. A country whose output and population both grow quickly can produce far more each year and still leave each person no better off, because the extra output is shared among extra people. Comparing the two growth rates is the first test of whether the average person is better off.

The age of the population decides how many people each worker supports

Most low-income countries still have a high proportion of children and young adults, while in many high-income countries the elderly form a growing share. Children and most older people produce little but still consume, so income per head depends on how many people each worker's output must support. Where there are many children for every worker, much of each family's income goes on feeding and schooling them, and little is left to save, so there is less to invest in machines, roads and skills.