Studies on family planning methods traditionally have relied on self-reports of unknown validity and reproducibility. Social desirability bias, a type of information bias, occurs when study participants respond inaccurately - but in ways that will be viewed favorably by others. Several lines of evidence reveal that this bias can be powerful in sexual matters, including reports of coitus, use of contraceptives and induced abortion. For example, studies using vaginal prostate-specific antigen testing reveal underreporting of unprotected coitus and overreporting of barrier contraceptive use. Medication Event Monitoring System studies, which electronically record the time of pill dispensing from a bottle or pack, indicate widespread exaggeration of adherence to pill-taking regimens, including oral contraceptives. Comparisons of provider data and self-reports of induced abortions reveal extensive underreporting of induced abortion. Reliance on self-reported data underestimates contraceptive efficacy. Although techniques to minimize this bias exist, they are infrequently used in family planning studies. Greater skepticism about self-reports and more objective means of documenting coitus and contraceptive use are needed if contraceptive efficacy is to be accurately measured.