This is not to say that numbers speak for themselves. Interpretations must be made, and, given the imperfections of theory and data, some scholarly disagreement about them is inevitable. For most questions about family, work, health, and education, I try to alert the reader to alternative interpretations and to indicate the degree of confidence warranted for the explanation under discussion. Thus the book offers less certainty than some would like, but does not shy away from making judgments, even when the basis for judgment is incomplete.
Despite the scope of the book, certain questions have been excluded. For instance, the causes of economic growth are not considered, although it is the basic cause of most of the phenomena under discussion. Neither does the book attempt to explain cultural differences in values, important though such differences may be in shaping behavior. Finally, no effort is made to explain individual differences. The contribution that economics makes to the understanding of human behavior is at the level of populations, not individuals. Its principal object is to explain or predict the behavior of aggregates or averages and to draw conclusions that are true in general, even though it is always possible to think of individual exceptions.