Zenilman's anomaly reconsidered: fallible reports, ceteris paribus, and other hypotheses

In the January-February, 1995 issue of Sexually Transmitted Diseases, Zenilman and colleagues reported a null association between incident sexually transmitted diseases (STDs) and self-reported condom use. That anomalous finding generated a flurry of letters to the editor, some of which were quite heated. This article reconsiders the Zenilman team's results.

New statistical analyses were conducted to test two hypotheses that sought to account for the null association: (1) deviation from study protocol, and (2) differential risks of acquiring an incident STD among segments of the study population that varied by reported level of condom use.

No support was found for hypotheses concerning deviation from study protocol and differential risk of acquiring an incident STD by level of condom use. Indeed, for respondents who reported multiple sexual partners, the analyses found increased rates of infection among those who reported more consistent condom use.

Two of the most promising hypotheses for explaining Zenilman's anomalous findings are unsupported by reanalysis of the available empirical evidence. It is still possible that respondents who reported that they used condoms consistently differed from self-reported nonusers or inconsistent users in some way that altered their risk of acquiring an STD and thus obscured the protective effects of properly used condoms. Nonetheless, as Zenilman and others suggest, fallibility in self-reports of condom use remains the primary suspect as the cause of these anomalous results. Such fallibility may be particularly pronounced when self-reported behavioral data are collected in contexts that include strong educational campaigns or other norm-setting interventions.