In the modern world, medical science has advanced quickly and the benefits of that science have extended life spans even in the developing world. With increased life spans and reduced infant mortality and other advances, populations have grown in less developed countries. This is good in that it means that fewer people are dying before their time. The problem, however, is that sometimes the population growth outstrips the ability of the countries to support their populations. This is why reductions in standards of living can occur.
With more people, there is more demand for food and shelter and consumer goods. This can be problematic when the country is not able to produce enough to satisfy its people’s needs. This lowers standards of living because there is less available for each person. Often, many poor people crowd into cities, hoping to find jobs. This causes reductions in standards of living as well. People often live in improvised shanty towns. This brings about disease and crime and other problems because the places where they live are essentially without government services such as sanitation, clean water, and policing. The urbanization of the population can reduce the quality of life for those already living in the cities as more workers push down wages, as crime increases, and as people have to see others living in squalid conditions.
For all of these reasons, population growth can reduce standards of living in less developed countries.