I History and Definition of the Concept.- The Evidence for the Concept of Intelligence.- On Defining Intelligence.- II Measurement and the Problem of Units.- The Absolute Zero in Intelligence Testing.- Is Intelligence Distributed Normally?.- III Development and Constancy of the IQ.- The Effect of the Interval between Test and Retest on the Constancy of the IQ.- The Limitations of Infant and Preschool Tests in the Measurement of Intelligence.- Intellectual Status and Intellectual Growth.- IV Types of Intelligence.- Primary Mental Abilities.- Organization of Abilities and the Development of Intelligence.- A Culture-Free Intelligence Test.- Ability Factors and Environmental Influences.- Personality and Measurement of Intelligence.- V Analysis of IQ Performance.- Intelligence Assessment: a Theoretical and Experimental Approach.- Intellectual Abilities and Problem-Solving Behaviour.- The Speed and Accuracy Characteristics of Neurotics.- Individual Differences in Speed, Accuracy, and Persistence: a Mathematical Model for Problem Solving.- VI Heredity and Environment: I. Twin and Familial Studies.- Genetics and Intelligence: a Review.- Twins: Early Mental Development.- IQs of Identical Twins Reared Apart.- VII Heredity and Environment: II. Foster and Orphanage Children.- A Critical Examination of the University of Iowa Studies of Environmental Influences upon the IQ.- The Relative Influence of Nature and Nurture upon Mental Development: a Comparative Study of Foster Parent-Foster Child Resemblance and True Parent-True Child Resemblance.- VIII Intelligence and Social Class.- Intelligence and Social Mobility.- Achievement and Social Mobility: Relationships among IQ Score, Education, and Occupation in Two Generations.- Differential Fertility and Intelligence: Current Status of the Problem.- Does Intelligence cause Achievement? A Cross-Legged Panel Analysis.- Ability and Income.- IX The Biological Basis of Intelligence.- Evoked Cortical Potentials and Measurement of Human Abilities.- Effects of Glutamic Acid on the Learning Ability of Bright and Dull Rats.- Effects of Heredity and Environment on Brain Chemistry, Brain Anatomy and Learning Ability in the Rat.- X The Paradigm and Its Critics.