What caused the structure of the family to change?
Here is an example from the Western world.
In the early 1900s in the United States, the divorce rate (i.e., the percent of legal endings compared to the number of marriages)
began to decline; in other words, couples stayed married for fewer yeas, and they had fewer children.
Women often chose to get an education and take jobs outside the home.
Decades later, the same things began to occur in other industrialized countries--and then developing countries of the world as well.