In their book "The Population Explosion", Paul and Anne Ehrlich explain that the overpopulation problem could be solved without any change in the number of people, but with the advancement of production industrial and changes in consumption, however difficult they may be (Avilés). According to the US Census Bureau, a total of 83 countries and territories are believed to have a below-replacement fertility rate (Eberstadt). Below replacement fertility mainly means that the birth rate is lower than the death rate. These places have almost 2.7 billion people, or about 37% of the world's population (Eberstadt). Global population growth is expected to slow over the next generation. In fact, slightly fewer people will be born worldwide in 2025 than at any time in the past four decades (Eberstadt). A quantity-quality correlation was discovered by Willis and Becker Lewis in 1973 discussing how the quantity and quality of children are related (Shah). They explained how the marginal cost of a child is higher the higher the cost per child (Shah). On the other hand, the marginal cost of the quality of childhood is higher the greater the number of children. This explains why most poor people living in developed countries have much higher birth rates than higher income people in more Western countries, because poor parents tend to lean towards the quantitative approach (Shah). Figure 2 shows the quantity-quality trade-off curve. The curve illustrates how as quantity increases beyond the maximum utility curve, quality declines rapidly. This means that when populations in developing countries start to have an increase in income, the quantity of children should decrease and so should birth rates