There’s not THE reason, but it emerged as a consequence of a few tendencies:
* Mothers die. Childbirth wasn’t safe until, like, yesterday.
* Stepparents are, statistically, more dangerous to children than birth parents. Before you get mad (and I realize it’s too late), I’m not saying stepparents are dangerous. They are not. The vast majority are harmless; they are just less harmless than biological parents. This is actually called the Cinderella Effect. The wiki article about it explains it and has some evidence, but I’ll post one here: *Based on data gathered from the Australia National Coroners’ Information System, stepchildren under five years of age are two to fifteen times more likely to experience an unintentional fatal injury, especially drowning, than genetic children*
* The sense of transgression is much worse when “the call is coming from inside the house.” It’s just more heinous to harm someone you’re supposed to help than random violence (and in the past, random violence was a given).
The Grimm brothers. When they (re)wrote many of the classic fairy tales, they changed mothers and mothers-in-law into step-mothers because they felt it was an insult to birth mothers.
Also, given the dangers of child birth back then, the prevalence of step-mothers was a much more common thing back then, so it looks more out of place now.
The evil stepmother is an important figure in one of the most famous and adapted fairy tales – Cinderella. Many times, an evil stepmother is an explicit reference to Cinderella, either because the work is adapting Cinderella or just as a nod towards it.
More generally, “evil” stepparents (stepfathers are also frequently painted as villains, though they tend to be more directly violent) tap into a fundamental anxiety about family, especially for children. One of your parents is gone and has been replaced by someone who lacks a real connection to you or even actively wants to harm you. This could relate to the literal loss of a parent through death or abandonment or the metaphorical loss through disinterest, strife, or mental health problems.
Aside from the fairy tales, human babies require A LOT of hard work and resources to raise into a healthy adult. Because of this evolution selected for mothers to release a cocktail of hormones at birth to create an intense bond and love for their helpless infant.
A step mother and step child have to create a social bond which like, any social bond, takes time and is often fraught with or impaired by complex family dynamics. It can be even more complex if the stepmother has her own children with the stepchild’s father. The stepmom typically has that chemical bond at birth with her own child and if her social bond isn’t strong with her stepchild/stepchildren it can appear or feel to the stepchildren that there’s favoritism. Sometimes there is, sometimes there’s not. Not to mention the step child feeling like an outsider looking in if they only visit their father every other weekend. It’s a lot of complex emotions and dynamics.
Of course there are plenty of loving stepmoms and terrible biological mothers out there.
As other posters have said, childbirth is dangerous and before modern medicine, it was very common for people to die having kids. So Mom croaks while having her fifth baby or whatever, and Dad remarries and has a couple kids with Wife #2. So it was also pretty common to have a strange woman, or at least a woman who’s not your recently deceased mom, roll up to your house and be in charge of you now, without your having any input.
That causes enough anxiety, right? Is she nice? Is she mean? What if she doesn’t like you? Nothing you can do about it! Then you also have the problem: say Dad’s rich and/or titled, who gets his stuff when he dies? The stereotype was that Wife #2 would favor her biological children to be the heirs, and wouldn’t want her stepkids to get anything—and might take some extreme measures to clear out the line of succession.
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