u/yyyyellow

I think Charlie and Bella's gift to shield their minds was developed as a response to Renee's mental thoughts. And as a response to their shields, Renee's mental projections got louder which made their shields tougher. The dynamic would look very much like the [Red Queen theory] in biology.

From Midnight Sun:

> Renée's physical voice was perfectly normal, similar to but a little higher pitched than Bella's own. Her mental voice, however, was piercing. It was fascinating to watch how the other minds responded. No one seemed to notice the ringing mental broadcast, yet everyone was compelled to help her. Somehow, they were picking up on her need, and unable to ignore it. I listened, mesmerized by the interplay between her mind and theirs. An orderly and a nurse led her through the halls, towing her small bag for her, anxious to help.

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Charlie developed his shield as a response to his environment and then genetically passed it to Bella. Or maybe it's like learning a new language and Charlie's shield is weaker because he learned it as an adult while Bella learned it as a first language.

From Wikipedia:

> Predator-prey relationship between rabbits and foxes following the principle of the Red Queen hypothesis. The rabbit evolves increasing speed to escape the attack of the fox, and the fox evolves increasing speed to reach the rabbit. This evolution is constant; were one of the two to stop evolving, it would go extinct.

[Red Queen theory]: https://en.wikipedia.org/wiki/Red\_Queen\_hypothesis