Consider the following:
Organize the population into generations, that is, for instance, consider all 0 to 25 years as 25 years old, all 25 to 50 years olds as 50 years old... Then you have 3 or 4 generations with each of the same age: 25, 50, 75...
Now, you will see under the one-child policy, the population of each yo8nger generation halves, because each couple (2 persons) produces only 1 person to replace them when they die. In two generations, the population will be 1/4 of their grandparents' generation.
The so-called "complexity" of the population dynamics is only about spreading out the age-oranginzed generations into a continuum, but it is still centered around the simple argument I gave above.
You figure out the rest.
