Communication privacy management theory informed this study of nine mothers and their 18 or 19‐ year‐old daughters who were interviewed to understand privacy rule foundations that influence their decisions to reveal or conceal sexual information. This study shows the salience of motivation and the risk‐benefit ratio when making decisions about revealing or concealing private information. Namely, mothers may have many motivations to talk to their daughters, whereas daughters are motivated to discuss sex with a trusted source. Mothers’ perceived risks of talking about sex included judgement from other parents and daughters were concerned about disappointing their parents. Additionally, a privacy rule emerged during joint mother‐ daughter interviews that stipulated ‘we talk about everything but the details’, and mothers volunteered their privacy rule acquisition of talking about sex with daughters differently from the ways their mothers talked to them. The findings augment sex education research by showing how mothers and daughters who talk about sex assess their decisions to do so.