Examinations of participant characteristics suggest that particular people volunteer to take part in social science research, and women have been found to be more likely to volunteer than men. However, within sex research it seems that this cooperation diminishes, and women are more likely to refuse to participate (particularly if a study seems personal or invasive). Evidently, this affects the reliability and validity of research and could lead to women's sexual health care decisions based on unrepresentative sampling. Given the need to include women in sex research, this article examines reasons for refusal and suggests improvements in research methods.