Abstract McNab, C. & Murray, A. 1985. Family Composition and Mathematics Achievement. Scandinavian Journal of Educational Research 29, 89‐102. The question examined in this paper is whether or not there is a significant difference in the mathematics test scores of Swedish junior high school students growing up in one‐parent as compared to two‐parent families. Controlling for parental educational attainment, the difference in mathematics test scores is significant only for children with both parents educated beyond elementary school level. However, in all but one case (father only with more than elementary school education) the same trend can be seen, i.e. higher test scores for children in two‐parent families. A similar pattern emerges in an analysis of 18‐year‐old science and technology students at senior high school. A parallel analysis of a much more limited word knowledge test also reveals differences between children of one‐parent and two‐parent families. Reference is made to Zajonc and Markus's theo...