* These figures are drawn from a number of sources—medical and psychological periodicals, policy reports and budgets, books published within the past seven years (most listed in the bibliography), and other sources. The numbers I've used are those most consistently cited across the literature; there is some variation between sources, and different studies come up with a range of conclusions. However, these numbers are representative of common conclusions in the research. By the time this book is published, the precise percentages may have changed, but they all indicate consistent trends.