If memory holds, Kosinski has confessed to inventing much of the putative horrific autobiography that he depicts in The Painted Bird. Source
Authorship controversy
According to Eliot Weinberger, contemporary American writer, essayist, editor, and translator, Kosiński was not the author of the book. Weinberger alleged in his collection Karmic Traces that Kosiński had very little fluent knowledge of English at the time of its writing.
M.A. Orthofer addressed Weinberger’s assertion by saying: “Kosinski was, in many respects, a fake – possibly near as genuine a one as Weinberger could want…
– as is the case with Kosinski.) Kosinski famously liked to pretend he was someone he wasn’t…and, apparently, he plagiarized and forged left and right.”
In June 1982, a Village Voice article accused Kosiński of plagiarism, claiming much of his work was derivative of Polish sources unfamiliar to English readers. The article also claimed that Kosiński’s books had actually been ghost-written by his “assistant editors,” pointing to striking stylistic differences among Kosiński’s novels. The New York poet, publisher and translator, George Reavey, who in Kosiński’s American biographer James Sloan’s opinion was embittered by his own lack of literary success, claimed to have written The Painted Bird. Reavey’s assertions were ignored by the press.
The Village Voice article presented a different picture of Kosiński’s life during the Holocaust – a view which was later supported by a Polish biographer, Joanna Siedlecka, and Sloan. The article revealed that The Painted Bird, assumed by reviewers to be semi-autobiographical, was a work of fiction. The article maintained that rather than wandering the Polish countryside, Kosiński had spent the war years in hiding with a Polish Catholic family and had never been appreciably mistreated.