In his review, Dr. Steven Levine of Case Western Reserve School of Medicine notes that the most carefully done surveys find that between 23 percent and 25 percent of men and 12 percent and 15 percent of women have had extramarital sex. He classifies extramarital affairs into four categories: 1) "Affairs"--in which a couple becomes intimately close and psychologically attached; 2) "Just-sex"--basically one-night stands or encounters with prostitutes; 3) "Making-do sex"--in which a couple has only limited attachment to one another but, for whatever reason, makes do with sex anyway; and 4) "Imaginary-partner sex"--which is typified by phone sex relationships.