The authors are emmersed in a comprehensive review of the literature on intellectual sex differences. This paper consists of the first progress report and the tentative hypotheses of the work completed so far. Discussion is initially concerned with verbal and spatial abilities. It is concluded that girls learn language earlier, and may continue to have a very small lead over boys. In spatial abilities, it has been found that by the fourth grade, boys begin to excel and that the sex difference increases in high school students. It is noted that there may be an artificial compression of the sex difference in which girls and an exaggeration of the difference where boys excel because of the differential dropout rate. New physiological hypotheses about sex different are also presented. Factors discussed are hormones, prenatal administration of sex hormones, cognitive style related to hormones, and brain lateralization. It is concluded that alternative explanations are different to separate, since the usual picture is that better infant care and less sex-role differentiation occur together. Suggested for further research are areas of self-esteem and sex differences spatial differences and analytical abilities, and differential sex-related reinforcement patterns. (BW)