There were not a lot of good employment options for women pre-1970 and many of the jobs that did hire women (nurse, teacher) would NOT employ a single mother....or even married women! Single mothers were viewed as bad moral examples so they were very discriminated against and only low paying types of employment were usually available to most. Even colleges would immediately kick out a unmarried pregnant student or refuse to admit a single mother as a student.
Families pressured unmarried women to give up children for adoption because the family might struggle to care/feed a new baby and it was believed the baby would ruin the woman’s prospects for a good marriage. Marriage was considered the primary goal for a woman, since there were not great employment options. On top of this a illegitimate child faced terrible stigmas - so both child and mom were punished.
Families who were well-off and faced with a single, pregnant daughter sometimes sent her away to have the baby (pretending she was visiting relatives) and then would quietly adopt the baby but pretend it was a foundling. Jack Nicholson is actually an example of this. He grew up thinking his grandparents were his parents and his mother his sister. He only found out the truth as a grown man in his 30’s. Ted Bundy was also another example, but his mother brought him into her new family when he was ten and she got married. I’m picking two wildly different examples of well know men from the same generation to show it wasn’t an uncommon phenomena.
Shame was certainly a big factor, but it was the harsh realities and economic survival that underpinned the stigma. Shame was a tool to deter premarital sex. Parents viewed single motherhood as destroying their daughters chances for a marriage, respectable employment and stable happy life. If the daughter kept the baby the support of both the mother and the child would usually fall upon the grandparents for many years, unless the father could be pressured into marriage. (Make no mistake in most cases a young pregnant woman meant a shotgun wedding though they would try to fudge dates to make it appear the pregnancy occurred after the wedding when possible) Caring for an unwed mother and infant just wasn’t feasible for many families who already struggled to survive.
In the pre-social safety net era even married couples would put up babies for adoption due to inability to feed and care for another mouth. During the Great Depression many of the children in orphanages were from married parents who simply couldn’t afford to care for them.
If an unwed mother did keep her baby she usually had to leave it in the care of a baby farm, or an older woman who would care for a infants/toddlers in exchange for payment, so the mother could work to survive and pay for the care of the child. The mother then would be able to visit on weekends. In these cases the mothers were able to hide the fact they had a child from employers, but lived in fear of it being discovered and losing their jobs. (Depending on what the job was, some fields were better than others for looking the other way)
Marylin Monroe was an example of a baby in this situation. Given that her mother was a troubled schizophrenic Ida, the woman her mother paid to care for her, was a better option. Her mom worked various jobs in the Hollywood studio system before being institutionalized. Marylin was raised from age two weeks until she was seven years old by Ida, but was returned to her bio-mom because she refused to let Ida adopt her.
In and out of foster care, the Hollywood icon struggled to connect with her mom.
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The govt didn’t help matter with the rules of their AFDC welfare program which would cancel meager benefits if a man was found living in the home. This tended to particularly hurt black families. If the unmarried mother tried to have the father, or any man, in the home she would lose her benefits. This ensured the women stayed unmarried and children without a father figure, because a guaranteed check was better than a husband that might struggled to earn a paycheck or who drank it away.
The stigma around unwed mothers had as much to do with how difficult survival was as one, even more than moral qualms, for most people. (Extremely religious ppl, who felt death was better than a daughter engaging in premarital sex, are another matter.)