Moreover, Goring's estimates regarding the British convicts enable us to judge that only about 25% of the criminals of this generation inherit a predisposition to crime from parents who were the criminals of the last generation (_20_, p. 336). Nobody has suggested isolating all persistent delinquents. We could not expect that the isolation of both the deficients and delinquents would completely remove the diathesis of delinquency from society. The predisposition is received not only from the deficients and delinquents, but also to some extent from those above the borderlines. We could not raise the borderlines of deficiency without isolating many whose social deficiency or delinquency it would be presumptuous to predict. We should not look forward, therefore, to the sudden elimination of the problem of delinquency even when it is attacked at its most vital spot. On the other hand Dr. Hart, in a bulletin of the Russell Sage Foundation, has worked out a practical plan which would isolate the lowest 0.3% of the girls and women of child-bearing age in this country within five to ten years. Some similar plan for isolating all deficient delinquents would materially lessen the cost of recidivism in the present generation. The most hopeful sign is that we are no longer content merely to guess at the relative importance of the sources of delinquency and deficiency, but our efforts to promote social welfare are directed by scientific investigations which are utilizing new and more efficient methods of research. PART TWO THEORETICAL CONSIDERATIONS CHAPTER XIII. THE THEORY OF THE MEASUREMENT OF MENTAL DEVELOPMENT In defining the borderline of feeble-mindedness it will be found that certain assumptions are usually tacitly made as to the form of the curves of normal and retarded development. These assumptions which are often based on vague conceptions of mental measurements should be brought clearly to mind if we are to compare the relative merits of different scales of mental tests or different ways of stating the borderlines of deficiency. With this in view it is proposed to take up in this second part of the monograph a brief technical discussion of the units of mental measurement, the equivalent individual differences at different ages, and the curves of mental development. The bearing of these conceptions on the various quantitative definitions of tested deficiency, including the percentage definition, will then be discussed in the following chapter. Practical advice as to individual diagnosis or group comparisons has been confined to Part One, so that those who are not concerned with the theoretical assumptions on which the conception of mental development and the interpretations of tested deficiency are based should omit Part Two. [Illustration: FIG. 3. _Hypothetical Development Curves (Normal Distribution)_]