Sex differences in human performance have been documented across a wide array of human endeavours. Males tend to exhibit higher performance in intellectually demanding and competitive domains, and this difference tends to be more pronounced the higher the level of performance. Here, we analyse publishing performance for the whole population of associate and full professors in relatively sex-balanced disciplines, namely Education, Nursing and Caring Science, Psychology, Public Health, Sociology, and Social Work, comprising 426 women and 562 men. We find that sex differences in the number of publications, citations, and citations per publication were small across low and medium levels of productivity, but become more pronounced the higher the level of performance. In the top performing 10% the female proportion decreases from the average 43.2% to 26% (25 F, 71 M), which further decreases to 15% in the top 5%. The results are discussed with respect to the greater male variability hypothesis, sex differences in psychological traits, and environmental factors such as sex discrimination.