The tragic life of Rosemary Kennedy, the intellectually disabled member of the Kennedy clan, has been well documented in many histories of this famous family. But she has often been treated as an afterthought, a secondary character kept out of sight during the pivotal 1960s. Now the third child of Joseph and Rose Kennedy takes center stage in “Rosemary: The Hidden Kennedy Daughter,” by Kate Clifford Larson, a biography that chronicles her life with fresh details and tells how her famous siblings were affected by — and reacted to — Rosemary’s struggles.

Setting her story against the backdrop of the stigma attached to mental illness in the first half of the 20th century, Larson describes the hubris of ambitious and conflicted parents who cared for their daughter but feared that her limitations, if publicly known, would damage their other children’s brilliant careers. Unwilling to accept that anything could be truly wrong with his own flesh and blood, Joe Kennedy, with his wife’s complicity, subjected 23-year-old Rosemary to an experimental treatment that left her severely debilitated and institutionalized for the remaining six decades of her life.

What makes this story especially haunting are the might-have-beens. Rosemary’s problems began at her birth, on Sept. 13, 1918. Her mother’s first two children, Joe Jr. and Jack, had been safely delivered at home by the same obstetrician. But when Rose went into labor with Rosemary, the doctor was not immediately available. Although the nurse was trained to deliver babies, she nonetheless tried to halt the birth to await the doctor’s arrival. By ordering Rose to keep her legs closed and forcing the baby’s head to stay in the birth canal for two hours, the nurse took actions that resulted in a harmful loss of oxygen.

As a child, Rosemary suffered development delays, yet had enough mental acuity to be frustrated when she was unable to keep up with her bright and athletic siblings. Even with private tutors, she had difficulty mastering the basics of reading and writing. At age 11, she was sent to a Pennsylvania boarding school for intellectually challenged students. From then on, Rosemary changed schools every few years, either because the educators were unable to deal with her disabilities and mood swings or because her parents hoped a change of scene might prove beneficial.