This refers to any fertility rate below the replacement rate that would cause the population of a certain group of people to decrease over time. A population’s fertility rate falls below the replacement rate when adults fail to produce enough offspring to even replace themselves. The sub-replacement fertility rate is generally considered to be anything below the replacement fertility rate of 2.1 children per woman in developed countries. The replacement rate, however, differs across the globe. For instance, it is much higher in developing countries where the infant mortality rate is much higher than in the developed world due to various reasons.