A generation ago scientists worried about overpopulation.Today in developed countries there is the opposite fear ฺBirthrate measures the number of children a woman has during her life.The replacement birthrate is the rate required to maintain the population. indeveloped counties this number is 2.1 but in most of these contries birthrates are less than this.spain and south korea for example share a birthrate of 1.1; Italy and japan are at 1.2.internationally more than 40 percent of people live in countries where the birthrate is lower than the replacement level.