The category that sent women to Sprogø was "moralsk åndssvage" — morally feeble-minded. The official definition turned on sex rather than intellect: it covered women whose eroticism was held to present a substantial danger, in free society, of spreading venereal disease. In practice the label caught poor girls. The intelligence tests used to sort them measured familiarity with a bourgeois world many of them had never entered, and a young woman who had been pregnant, or promiscuous, or merely unmanageable in the eyes of a parish or an employer, could be found defective on that basis. There was no charge, no hearing and no sentence with an end date. Danish law of the period gave institutions wide discretion over people it deemed incapable of managing themselves, and the discretion belonged to the doctors.

Life on the island was organised around work — farming, housekeeping, needlework and craft. The women were called "the girls" regardless of age, and many were in their twenties, thirties and forties; the diminutive did steady work, reinforcing the idea that these were people who could not be trusted with their own lives. Discipline was left largely to the institution, since no legislation governed how such places should treat their residents. Wages could be docked. Women could be put in solitary confinement, and physical restraint was used. Early residents typically spent about seven years on Sprogø. During the German occupation the home was moved to a facility at Brejning between May 1940 and the summer of 1945, then returned to the island.

Denmark passed a trial sterilisation law in 1929, a compulsory law in 1934 and a voluntary law in 1935, and roughly 13,000 Danish citizens were sterilised between 1929 and 1967. On Sprogø the connection between the operation and the door was direct. Nearly every woman was sterilised before she was let go, and after the 1934 law widened the grounds, most left within about two years of the procedure. The reasoning was frankly stated at the time: the state's concern was unwanted pregnancies, and once a woman could not conceive, the danger she was said to represent had been surgically removed. Contemporaries considered the arrangement humane, on the argument that confinement without sterilisation would simply have lasted longer.

The home closed on 1 April 1961. Denmark abolished forced sterilisation in 1967. For decades afterwards the women of Sprogø were a half-remembered story, surfacing in Danish culture mostly as material — the island's history shaped the plot of the 2018 thriller Journal 64. Official acknowledgement took much longer. On 11 September 2023, Social and Housing Minister Pernille Rosenkrantz-Theil delivered a formal state apology. A memorial to the women confined there was dedicated on 8 April 2025, more than six decades after the last of them left.

Everything else about Sprogø is a story of watching the strait. The name was recorded in the twelfth century as Sproøe, scout's island, from the old Danish verb spro, to scout — modern Danish borrowed sprog for language later. Around 1160 Valdemar the Great ordered a fortress here, a square castle with a courtyard 26 by 32 metres, its walls built of the new monastery brick. From 1801 the island carried a station of the optical telegraph line between Korsør and Nyborg, a mast with ten wings capable of more than a thousand distinct signs, until electricity made it obsolete. The lighthouse of 1868 stands on the foundations of Valdemar's tower. Archaeology done during the Great Belt works found traces of people here 8,000 years ago, and enlarged the island itself from about 38 hectares to roughly 150. Nobody lives on Sprogø now. It belongs to the bridge company, it is a nature reserve, and you can visit only on an organised tour.