Where did the King and Queen get the idea to lock Elsa away?

Question

After Elsa freezes Anna's head Grand Pabbie gives Elsa and her family some advice on how to deal with her growing powers.

Grand Pabbie: It's for the best. Listen to me, Elsa, your power will only grow. There is beauty in it. But also great danger. You must learn to control it. Fear will be your enemy.

King: No. We'll protect her. She can learn to control it. I'm sure. Until then, we'll lock the gates. We'll reduce the staff. We will limit her contact with people, and keep her powers hidden from everyone. Including Anna.

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So why lock the gates, reduce the staff, limit contact and provide her with gloves? Why did they think that was the best course of action?

Answer 1

This seems to boil down to four main elements;

1) Grand Pabbie's info was quite vague. His general advice was that she needs to control her powers. Her parents take this to mean that she should restrain them rather than learning to use them effectively.

2) Elsa is seen by her parents as being a danger to herself and others. Arguably, when she does take the gloves off (both metaphorically and literally) she almost kills the entire kingdom.

3) Elsa's magical abilities are growing and becoming more uncontrollable over time. Her parents seem deeply worried that if the Kingdom find out about her powers, that they'll react with fear and loathing, harming her chances of becoming the next Queen.

The king and queen acted just as cautiously inside the castle. As the princesses grew, their parents did everything they could to ensure that Elsa learned to control herself. That meant the girls were hardly ever together. Nor did Elsa seek Anna out, since she was afraid she might accidentally hurt her. Day after day, Elsa spent most of her time training to be the next ruler—and learning to keep her powers in check.

The training was difficult, and Elsa often felt unable to contain her magic. Ice seemed to form on her fingertips whenever she laughed or cried or became upset.
Worried, the king gave Elsa a pair of leather gloves. He advised her to keep them on at all times, and reminded her that she had to hide her icy magic in order to stay safe. “Conceal it,” he told her.
“Don’t feel it,” she answered.
“Don’t let it show,” he agreed.

_{^{Frozen: The Junior Novelisation}}

Elsa herself is unwilling to use her magic constructively. Her fear is that she will cause harm to others. She simply wants to repress her powers.

Without any memory of Elsa’s magic, Anna had no idea why she was always alone. Over time, she simply came to accept that her sister’s coldness was part of who she was. She didn’t know that Elsa was lonely too, and that she missed Anna as much as Anna missed her. Elsa longed to play with Anna but was fearful of the harm her magic might cause by mistake.

_{^{Frozen: The Junior Novelisation}}

Answer 2

One theory is that they (Elsa’s parents) got the Idea from an ancient book. The same one with the map of how to find the trolls. This book pops up whenever one of the royal family has great need of it, holding solutions to terrible problems to maps to finding the trolls.