Poverty creates population.1893829Population growth and economic development have proved the truth in this axiom put forward by Karl Marx more than 100 years ago. Demoeconomists often attribute this phenomenon to the low marginal cost of children，especially low support cost，under poverty conditions. While this approach is correct and significant in providing guidance for reproductive behavior，I consider it inadequate since technological advances are not taken into account in the analysis of the cost of children and the characteristic differences under different technological conditions are usually not examined in the analysis of the marginal benefit of children. This inadequacy prevents further study on the cost and benefit of children，especially the study on the transformation of the fixed or quantitative cost of children to the variable or qualitative cost of children.