system
A chat on subreddit /r/NoStupidQuestions
human
90% of the population used to be farmers.  Children worked on the farm.  And the children took care of their parents when their parents got old.  So children were both a source of labor and a retirement plan.  So having a lot of children was beneficial.

Now most people work in "regular" jobs.  Children do not work.  So children are an expense, not a source of labor.  And children do not take care of their elderly parents.  Parents save for retirement - which is easier if you don't have children.

So children went from being a source of wealth to being an expense.
gpt
&gt;Children do not work

Technically correct, but not practically correct, the new work for kids is school, a phase of their life where they fulfil part of the social contract to become productive adults later in life. Back when kids did work, a school education was not required to enter the workforce, now the minimum is almost at a college degree. So most kids now work hard in school to enter the workforce.