If you’ve heard about Molly Jong-Fast’s new memoir, “How to Lose Your Mother,” it won’t surprise you that a great many readers are coming for the juicy matricidal takedown of the feminist icon Erica Jong. I came for the dementia.

Ms. Jong-Fast bracingly challenges the sentimental conventions of so many family stories, refusing to sacrifice her story to preserve appearances. The telling of her mother’s cognitive decline, however, follows a pattern that shapes how we write and even talk about dementia, a condition that affects 57 million people worldwide. It’s been described as the tragedy narrative. Perhaps you’ve noticed it, too.

In its various iterations across books and films, the dementia tragedy narrative tells a story of inexorable decline and universal diminishment, in which the afflicted person steadily vacates her body until she becomes essentially absent. While this process may include moments of lucidity or levity, nothing substantially positive, life-giving or new can emerge for the person or her family and friends — because the person as person is disappearing. “My mother is just a body now,” Ms. Jong-Fast writes. “She has dementia. She has breath and hair and pretty blue eyes but Erica Jong the person has left the planet.” She is “dissolving,” “slipping away,” “a faint fragment,” “an echo,” “a zombie.”

The trouble with this well established approach is not that the tragedy narrative is completely false. There are indeed losses and suffering associated with dementia, experiences that confound and aggrieve, and these descriptions resonate with many people’s own experiences. The problem is that narrating them in this manner, turning a multidimensional phenomenon into a story of unidirectional decline and disappearance, reinforces stigma around cognitive disability. And the notion that people are gone before they are dead directly harms the care they receive, exactly when they need it most.

People with dementia are extremely vulnerable to being abused and neglected, financially drained (even just to get necessary care), improperly medicated and restrained, infantilized, ignored and socially abandoned. The tragedy narrative obscures the reality that a significant measure of their suffering emerges not from the condition alone, but from the social response to it — the part of the situation we are most able to fix.

These assumptions are easy to find. They’re sprinkled liberally throughout everyday language and metaphors, appearing anytime dementia is imagined as ceaseless suffering that turns people into shells, husks, the living dead or at least less “real” versions of themselves.