But it’s not as simple as high population - bad, low population - good. In countries where a reduction in the fertility rate has led to a decline in the population – in Europe, Australia, and the United States – the structure of the population may change too quickly, or too severely, for existing infrastructure.
A rapidly aging population and a shortage of workers can lead to social and economic turmoil. And while fertility rates are slowly starting to fall in traditionally high rate areas, such as Africa, there is no guarantee that cultural, political or economic attitudes will continue to support the decline.
Other social challenges arise from changes in mortality rates. Mortality rates may increase, or decrease, based on the transmission of disease, development of successful treatment, and accessibility to health care. In politically unstable areas with high populations and no food security, mortality rates may prove a more powerful predictor of future population trends than fertility rates. What may turn out to be the most significant factor to consider in the over-population debate is whether a society is prepared to deal with whatever the future size, growth rates or composition of the population turn out to be.
As if to prove what some researchers have suggested – that any predictions past 2030 are little more than guesses – the United Nations estimates “the number of humans populating the planet in 2100 will range from as few as 6.2 billion (almost a billion less than today) to as many as 15.8 billion on the high end."
Another guess? The International Institute for Applied Systems Analysis stated “if the world stabilizes at a fertility rate comparable to that of many European nations today (roughly 1.5), the global human population will be only half of what it is today by the year 2200, and only one-seventh by 2300."