By building up a simple model, the author found that the difference of the ability of supporting the elderly between the families with more children and the families with fewer children depends on the local economic condition. In the less developed areas, the families with more children take some advantages; inversely, in the more developed areas, the families with less children hold stronger ability to economically support the elderly. The influence of family planning is based on the economic condition. During the period of rapid economy growth, family planning strengthens the ability to support the elderly within families.
