One reason is economic. In a traditional agricultural economy, large families are helpful. Having more children means having more workers in the fields and someone to take care of the parents in old age. In an industrial economy, the situation is different. Many children do not help a family; instead, they are an expense. Thus, industrialization has generally brought down the birth rate. This was the case in Italy, which was industrialized quite recently and rapidly. In the early part of the twentieth century, Italy was a poor, largely agricultural country with a high birth rate. After World War II, Italy’s economy was rapidly modernized and industrialized. By the end of the century, the birth rate had dropped to 1.3 children per woman, the world’s lowest. 

      However, the economy is not the only important factor that influences birth rate. Saudi Arabia, for example, does not have an agriculture-based economy, and it has one of the highest per capita incomes in the world. Nevertheless, it also has a very high birth rate (7.0). Mexico and Indonesia, on the other hand, are poor countries, with largely agricultural economies, but they have recently reduced their population growth.