account for differences in percentage population growth rates between more economically developed countries and less economically developed countries

less economically developed countries have higher population growth cuz' their economies are usually based on farming and raising animals which are labor intensive. so they need a large population to provide the labor. african countries are good examples of this.

more economically developed countries do not need a large labor force n tend to have low population growth.

In more economically developed countries, the cost of living is higher. So it costs more money to have kids and raise them. Therefore the family size is smaller and the population growth rate is lower than that of less economically developed countries.