[Why researchers excluded women from their trial populations].

Women are still, but to a lesser extent than twenty years ago, excluded as subjects of medical research on diseases that are prevalent among both men and women. To discover the basis on which women were excluded, the research ethics committee requested a written explanation. In all, 26 such project applications were identified during 1997-1999 (2% of the total number of applications during the period). Most researchers had more than one reason for exclusion. Qualitative analysis revealed that these explanations could be grouped into three categories, depending on whether women were excluded for scientific, historical or economic reasons. The scientific reasons correspond mainly to a lack of pertinent knowledge of the physiology and metabolism of women of childbearing age. Consequently, results lacked external validity. Perhaps the lack of knowledge of women's physiology and metabolism could be explained by a lack of female experimental animals in pre-clinical studies. One notes however a general concern not to harm women of childbearing age. The historical reasons underlie the tendency to repeat studies on former study populations that happened to be composed of men. Finally, tight research budgets restricted the participation of women but not of men. The Swedish Medical Research Council issued a policy document in 1998 to the effect that research ethics committees could require additional information concerning choice of study population. This study demonstrates an avoidable occurrence of gender bias in medical research.