If you knew you had a 50/50 chance of having a genetic mutation that would lead to frontotemporal dementia (FTD), a condition that would wipe your affect like a chalkboard, taking your personality, your language abilities, and your judgement, eventually rendering you unable even to manage toileting on your own, would you want to know? What would you do with the information? For The New York Times Magazine, Robert Kolker unpacks the terrifying dilemma faced by one family of nine siblings.

Barb decided to start a search of her own. She contacted the National Institutes of Health and learned about a condition that bore a striking resemblance to Christy and Mary’s symptoms: frontotemporal dementia, or FTD, which emerges in the prime of adult life — as young as 40, in some cases — and relentlessly attacks the part of the brain responsible for planning, organizing, expressing language, understanding social cues and exercising judgment. Unlike more common brain disorders like Alzheimer’s, FTD is confoundingly rare; researchers suggest that perhaps 60,000 Americans have it, though that estimate is complicated by how difficult it is to diagnose.

Christy wasn’t sad or delusional; she wasn’t even upset. It was more as if she were reverting to a childlike state, losing her knack for self-regulation. Her personality was diluting — on its way out, with seemingly nothing to replace it.