When Barb’s father all but left, her mother turned inward, sitting quietly in front of the television, always smoking, often with a cocktail. Something had overtaken her, though it wasn’t clear what.

Six years later, Barb was 20 and in college when someone else in the family needed help. Her sister Christy was the second-born, 24 years older than Barb and the star of the family in many ways. But where once Christy was capable and professionally ambitious and socially conscious, now, at 44, she was alone, her clothes unkempt and ripped, her hair unwashed, her marriage over.

Depression was the first suspected diagnosis, then schizophrenia, though neither seemed quite right. 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.

What was left of Christy was chaotic and unpredictable. She refused to bathe and stopped bothering to make meals. She crashed a neighbor’s party and made odd conversation with strangers. She clogged a toilet with tampons and flooded the house. She was gleefully impulsive, spending thousands of dollars a year on magazine subscriptions. That strange, reckless profligacy made Barb think of their mother, who in her final years sat at home, saying yes to every sales phone call. How heartbreaking but also interesting, Barb thought, that Christy shared the same peculiar tendencies — a family trait of sorts.

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Living on Earth: March 30, 2012