Overpopulation is considered to be the result of improved medicine, sanitation and agriculture worldwide. In the early days of human history, the death rate was relatively close to the birth rate, ensuring a very slow population growth. As life spans improved due to advancements in technology, the birth rate began to grow much faster than the death rate, creating a larger surplus of births every year.
The earliest factor that increased the population was the development of agriculture. Planting crops allowed communities to harvest food much more efficiently than hunting and gathering, providing a bulwark against starvation. With an abundance of food, people lived longer, and children were more likely to live to adulthood to reproduce. As technology has improved, the nutrition provided by the food supply has improved, as well, further increasing the lifespan and the reproductive potential of the population.
Modern medicine is another major factor in overpopulation. Major disease outbreaks are less common, and when they do occur, they tend to be much milder than in the past. Medical treatment allows people to survive illnesses that once would have killed them. Improved sanitation and access to clean water are also major factors that allow people to survive longer and reproduce.