INTELLIGENCE IS COMPOSED OF SIX INDEPENDENT ABILITIES--NUMERICAL, SPATIAL, REASONING, TWO VERBAL ABILITIES (VOCABULARY SIZE AND WORK FLUENCY), AND MEMORY. THE INDEPENDENCE OF THESE ABILITIES ARE EXPLORED BY RESEARCH STUDIES WHICH ARE DISCUSSED IN SIX CRITERIA CATEGORIES--(1) DIFFERENTIAL PREDICTION OF SUCCESS, (2) FACTOR STABILITY ACROSS AGE RANGES, (3) CROSS-CULTURAL GENERALITY OF ABILITY PATTERNS, (4) STABILITY OVER ABILITY LEVELS, (5) DIFFERENTIAL EFFECTS OF MENTAL ILLNESS OR BRAIN DAMAGE, AND (6) DIFFERENT RATES OF DEVELOPMENT. EVIDENCE OF THE HEREDITARY COMPONENT OF INTELLIGENCE IS PRESENTED IN TERMS OF RESULTS OBTAINED FROM TWIN STUDIES. STUDIES OF INBREEDING ALSO PROVIDE DATA. THE ENVIRONMENTAL COMPONENT IS ALSO DISCUSSED. IN VIEW OF THE RESEARCH, CO-TWIN CONTROL STUDIES DESIGNED TO STUDY THE EFFECTS OF STIMULATION OF ENVIRONMENT APPEAR TO BE NEEDED. (SK)