WHY ARE FAMILIES GETTING SMALLER ACCORDING TO ECONOMICS?IS THIS ABOUT BENEFITS AND COSTS?
Yes, it is definitely about cost-benefit analysis. Here's the usual explanation:
In the old days (or in poorer countries) families survived by farming. They needed more people to help with the farm so more kids were useful. They also didn't have social security or the ability to save much money so they needed kids to take care of them in their old age.
In modern societies, kids don't really help a family economically. If anything they hurt it because A) you have to buy them stuff, including a college education and B) having lots of kids means that the woman (usually) doesn't have as much of a career as she might otherwise have had.
So nowadays in modern countries, kids cost more and have fewer benefits than they used to.
This is a great question. If you look at places like Japan and Italy the population is decreasing. They are not developing countries, but fairly wealthy ones. So, why is this taking place? I think part of the reason is due to rising costs. It is expensive to have a family. Imagine paying for college for three kids! Also since these societies are materialistic, they want other things like better housing and newer cars. Economies are built on consumer spending. So, there is an economic basis for smaller families. On the flip side, what might be interesting to study is whether families of poorer countries are increasing in number.