This study investigated the role of aptitudes and experience in sex-related differences in proportional reasoning. Subjects were 778 7th, 9th, and 11th graders. Fewer than half the subjects consistently displayed proportional reasoning, with females slightly less successful than males. Sex-related differences in proportional reasoning were not accounted for by various measures of spatial ability, field dependence-independence, or Piagetian formal reasoning. Course experience in mathematics and science did not explain the differences. A single-aptitude explanation for the observed sex differences in proportional reasoning seems unlikely, and many additional influences of experience remain to be investigated.