75% of the births in the US in the late 19th and early 20th centuries went unregistered. Though the US pioneered the institutionalization of a decennial census, it lagged behind European countries in recording births, deaths, and marriages. Such records were not considered a federal matter, and the various states were all over the map with what they collected, if anything.
Progressive Era reformers got state and federal governments to adopt official records of birth during the first 40 years of the 20th century. These replaced affidavits, which were sworn and notarized statements by a child’s parent(s). Affidavits proved untrustworthy when it came to child labor laws: parents cheated, and notaries lied.
“The solution to the problem of enforcing child labor laws was, ironically, to shift the authority to authenticate a child’s birth away from the people who had actually witnessed it—parents. Professionally produced, birth certificates objectified a child’s age and identity, making a truth that existed apart from the personal relations that created the child.”
This was all part of the “bureaucratization, standardization, and quantification of information that accompanied modernization” and the birth of the “information state.” Documentary written evidence was privileged over the “oral and transient.”
See also the history of the passport.