That's very interesting about the amygdala and the prefrontal cortex, and how they play into the development of the brain. It goes a long way to explaining many aspects of typical teenage behaviours. Marianne certainly strikes me as very 'teenagerish' and immature.

As for Mrs Dashwood, the fashion for 'Sensibility' coincided with her natural inclinations. It was culturally accepted or approved of, particularly in females, and perhaps was one of her attractions for he late husband.

I have the impression that she had never been forced by circumstances to develop her 'Sense'. Though she brought little money to her marriage she has been comfortably provided for ever since, and has 'never saved in her life'because she didn't need to. Secure in an affectionate marriage, I doubt she has ever needed to engage in any meaningful way with materialistic or ill-disposed people who don't share her outlook.