Cancer-free women who are from families with an inherited form of breast and/or ovarian cancer (BRCA1/2) are referred to as ‘unaffected non-carriers’ when the results of genetic testing for the inherited gene are negative. Recent studies have identified overscreening behaviours (undergoing more screening tests for cancer than are medically warranted) among these women, even though they do not require specific cancer monitoring. Overscreening behaviours deserve particular attention due to their potential psychological drivers and implications. The principal objective of this study was to examine the factors, including state anxiety, feelings of self-vulnerability, and the comparative perception of cancer risk that might explain these overscreening behaviours. Unaffected non-carriers women (n = 77) were asked about these different variables. Overscreening was associated with and determined by feelings of self-vulnerability and the comparative perception of cancer risk, but was not associated with anxiety. An increase in feelings of self-vulnerability or elevated comparative pessimism (CP) was related to the participants' decision to be frequently screened. Patients' perceptions of the risk should be considered in measures or information aimed at preventing inappropriate overscreening behaviours.