Twenty years ago today, Magic Johnson told the world that he was H.I.V.-positive. What had been largely a “gay disease” in the public’s mind now had a very different face. Did this make a difference?

Philip H. Pollock III, a political science professor at the University of Central Florida, had an answer, thanks to a bit of good luck: he came upon a November 1991 poll in Florida that included questions about AIDS and, most crucially, he was in the field when Mr. Johnson made his announcement. By comparing the responses of people interviewed before and after the announcement, he could estimate its effect on public opinion about AIDS.

In a 1994 article, he found that Mr. Johnson’s announcement changed the underpinnings of opinion. Although views of homosexuals were associated with opinion about AIDS — those with less favorable views of homosexuals were in turn less likely to support spending for AIDS treatment and research — Mr. Johnson’s announcement made attitudes toward heterosexual sex a more important underpinning of opinion about AIDS. In particular, those with more conservative values (in this case, those who believe premarital sex is always or almost always wrong) became slightly less supportive of spending to fight AIDS. But those who believed that premarital sex was only sometimes or never wrong became more supportive of AIDS spending — by 15 points, in fact. Mr. Johnson’s announcement had shifted the types of values that people drew on when forming opinions about AIDS.

The same finding showed up in a separate 1992 poll in which respondents were asked to name a celebrity with AIDS. Approximately 50 percent named Mr. Johnson. Among these respondents, attitudes toward premarital sex were again associated with attitudes toward AIDS; this association was substantially weaker among those who did not recall Mr. Johnson.

Professor Pollock argues that having a famous heterosexual acknowledge his H.I.V.-positive status changed how the problem of AIDS was “constructed” in the public sphere:

…It took dramatic symbolism to communicate this construction of the problem to the public at large, to disrupt the way AIDS was discussed and argued in families, among peers and co-workers, and in other social settings. As we have seen, that was the role — and may define the legacy — of “Magic” Johnson.