XV. Percentage of pupils 12 and 13 years of age most seriously 193 retarded in school XVI. School position of delinquents at Glen Lake relative to 204 their intellectual development XVII. Goring's data as to the percentage of mental defectives 213 among men convicted of various offenses XVIII. Goring's data as to groups of crimes committed most 214 frequently by those mentally deficient XIX. Four-fold correlation table for juvenile delinquency and 222 deficiency in Minneapolis XX. Average Intelligence Quotients of children of different 296 ability XXI. Test records with random 15-year-olds 344 XXII. Test records with delinquents at the Glen Lake Farm School 349 FIGURES 1. Mortality among feeble-minded in institutions compared with 32 the general population 2. School retardation of Minneapolis delinquents compared with 180 elementary school boys 3. Hypothetical development curves (normal distributions) 253 4. The question of equivalence of year units 265 5. Hypothetical development curves (changing form of 277 distribution.) 6. Tests of the development of memory processes. Medians at 285 each age for the central tendencies of the tests 7. Different types of development. Medians at each age for the 286 central tendencies of the tests 8. Forty tests of development. Distribution at each age for the 287 central tendencies of the tests 9. Relative positions at each age of the median and of 299 corresponding bright and retarded children with the Form Board Test DEFICIENCY AND DELINQUENCY PREFACE In undertaking in 1912 to examine the mental development of delinquents for the clinic started and supported by the Juvenile Protective League of Minneapolis, in connection with the Juvenile Court, I soon became convinced that a safer method for evaluating the limit of feeble-mindedness with tests was more needed than masses of new data. The researches that have been published in the past three years do not seem to have changed this situation. Numerous studies with psychological tests are already available, but they generally treat of average rather than borderline conditions. In the field of delinquency the work of testing has been carried on with especial activity. Here, as well as elsewhere, the conclusions seem likely to be misleading unless social workers better appreciate the real place of mental tests, their value and their limitations.