Baddiel's book, "The Secret Purposes", is based in part on the internment of his grandfather on the Isle of Man during the Second World War. His father is from Swansea and his mother was born in Nazi Germany, a swastika appearing on her birth certificate. An episode of the BBC's genealogy series "Who Do You Think You Are?" investigated his heritage in some detail, but failed to prove his theory that his mother had been secretly adopted from another Jewish family who had no hope of escaping. Despite his upbringing, he has described himself as a "10 out of 10 atheist" and as a "fundamentalist" "Jewish atheist". Baddiel suffers from insomnia.