I'm what they call an only child. Someone who has no brothers or sisters.
Do you know what percent of children are only children?
Well around the 20th century is about the time birthrates started dropping drastically in life. Maybe due to women joining the workforce more?
I believe many more people want to live their lives without children nowadays.  It was frowned upon in a way not to have children long ago.
It happened in the past during The Great Depression as well. There were more only children in those times.
Yes in that time they probably did not have the money to support multiple children.  Are only children more common in any countries?
Yes absolutely. In China they had a one child policy from 1979 to 2015. 
Wow they only ended the policy very recently.