The infant-rearing investigation the Ezzos conducted was performed by GFI and not published or subject to peer review. In training the infant to follow the recommended eating and sleeping schedule, their method expected that at certain times the infant would be left alone to cry when hungry or wakeful. The book justified the act of leaving a baby to cry alone by comparing that choice to the crucifixion of Jesus: "Praise God that the Father did not intervene when His Son cried out on the cross." The Ezzos wrote that leaving the infant "crying for 15, 20, even 30 minutes is not going to hurt your baby physically or emotionally." To counter the book's conclusions, Laura Bassi Zaff, PhD, an expert in childhood cognitive development, wrote that careful research has shown that leaving a baby crying may result in emotional harm, perhaps manifesting as "attachment disorder, or anxiety disorder, or crippling problems with self esteem and interpersonal relationships".