Her bio father was a sensible, stick-in-the-mud, too decent for her and her social climbing mother. The mother had departed her spouse to become "an actress" in NY, which in the early 1900s meant "prostitute." She left "Nancy" with aunt/cousins. Some years later, the mother had divorced and had latched on to a DOCTOR!1 One
Honor Loyal DAVIS. When the 2nd marriage took place, they sent for "Nancy," who really really wanted to be adopted by the doctor. So in her (teens?) she traveled to visit her bio dad and browbeat him into signing away his birth status, and the birth certificate was amended.
I'll look up her real name. The surname was ROBBIINS. She became "Nancy DAVIS."
On Edit: Couldn't find my paperback. Google shows her birth name as "Anne Frances ROBBINS." This first link is biased in her favor, saying that her bio dad "left the family." Apparently she was six (not in her teens) when the marriage to Loyal DAVIS took place.
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http://freepages.genealogy.rootsweb.com/~shannonk1973/928.htmAnne Francis (Nancy Davis) ROBBINS , First Lady
ACTRESS
Nancy was the daughter of an actress and a salesman. Her mother nicknamed her Nancy. Her father left the family shortly after she was born. Her mother remarried a Chicago surgeon named Loyal Davis. Nancy took her step-father's name.
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