The results of a study investigating heredity components of mathematical ability as well as sexual differences related to this ability are reported. A test containing 26 arithmetic problems was administered to 300 pairs of twins. Analysis of the data included within and between pair variance, and intraclass correlations for each sex and zygosity group. In addition, the data were also analyzed by a new technique called "dichotomic analysis" to test for evidence of an underlying biomodality in the distribution of scores. The author concludes that there is an underlying heredity component for proficiency in quantitative reasoning, and that this component fits a sex-linked recessive model fairly well. The paper also includes summaries of previous studies of heredity and environmental components of mathematical abilities, and a bibliography of related research. [Not available in hardcopy due to marginal legibility of original document.] (CT)