My opinion

I want to choose the topic, No,5 or No, 6.
Because I am interested in them. Both of them are very good for me, but now, I will write about No,5.

Exactly, families in developed countries are very small. What is called "nuclear family" is the mainstream in countries such as America, Japan, England, France, German and so on.
I think there are main reason why we can see the state of affairs like this.

Parents want their children to take special education for gifted children in developed country. For example, playing instrumental, dancing classic ballet, "Jyuku" for taking entrance examination to a private school. In developed country, environment for education is very substantial. So, parents in these country tend to spend much money for education. But, they can not spend money for many children. So, parents in these country want to bring up few children.

Ofcourse, there are many other reasons. I want to write them in my essey.