tl;dr; There is a historic difficulty in polling on this topic, such that one poll from 1993 suggested 20% of young Americans thought it possible the Holocaust did not happen, only to find that a repoll, with a different question, put the number of Holocaust deniers at just over 1 percent, and "Many of those who questioned or denied that the Holocaust occurred cited lack of information as the reason for their uncertainty, or displayed ignorance of basic facts about the Holocaust."

I didn't see any mention of historical trends. Is this higher than it used to be? Is it lower?

In trying to answer this for myself, I found https://slate.com/human-interest/2020/10/millennials-holocau... ;

> In April 1993, in conjunction with the opening of the United States Holocaust Memorial Museum, the Roper Organization conducted a poll to determine the extent of Americans' knowledge of the Holocaust. Neither the Roper Organization nor the American Jewish Committee, which sponsored the poll, expected any startling results. But they were surprised by the response to one of the questions. When asked "Do you think it possible or impossible that the Holocaust did not happen?" 22 percent of American adults and 20 percent of American high school students answered, yes, it was possible. The response shocked many people who had long dismissed Holocaust denial as a wacky phenomenon of no more validity than the claim that the earth is regularly visited by alien beings. The poll's results, coupled with the deniers' recent forays onto college campuses in order to publish ads in campus newspapers denying the Holocaust, convinced many people that Holocaust denial constituted a clear and present danger.

If it's 20% of young Americans in 1993 and only 10% now, then things have gotten better, right?

In looking for more about the Roper survey, I found https://historynewsnetwork.org/article/177607 from 2020:

> Recently, you may have heard that many young Americans are unaware of what happened in the Holocaust, or who was to blame for it. Worse, many believed it was a “myth” or “exaggerated.” “Nearly Two-Thirds of US Young Adults Unaware 6m Jews Killed in the Holocaust,” a headline blared in the Guardian, over a piece that asserted “shocking levels of ignorance about the greatest crime of the 20th century.” NBC News also reported the numbers, writing that the survey “showed that many respondents were unclear about the basic facts of the genocide.” ...

> Whether or not the assumptions in the Claims Conference survey are fair, and how to tell, is at the core of a decades long debate over Holocaust knowledge surveys, which are notoriously difficult to design. In 1994, Roper Starch Worldwide, which conducted a poll for the American Jewish Committee, admitted that its widely publicized Holocaust denial question was “flawed.” Initially, it appeared that 1 in 5, or 22 percent, of Americans thought it was possible the Holocaust never happened. But pollsters later determined that the question—“Does it seem possible or does it seem impossible to you that the Nazi extermination of the Jews never happened?”—was confusing and biased the sample. In a subsequent Gallup poll, when asked to explain their views on the Holocaust in their own words, “only about 4 percent [of Americans] have real doubts about the Holocaust; the others are just insecure about their historical knowledge or won’t believe anything they have not experienced themselves,” according to an Associated Press report at the time.

So, how well was this YouGov/Economist poll designed, given how it is "notoriously difficult to design" for this topic? I saw no discussion.

I also found https://www.jta.org/archive/new-ajcommittee-poll-shows-1-1-p... from 1994 :

> A new survey commissioned by the American Jewish Committee and conducted by Roper Starch Worldwide Inc. found that just over 1 percent of respondents deny that the Holocaust occurred, contradicting the group’s disputed 1992 poll which had put denial at more than 20 percent.

> The new survey, released July 6, also found a strong correlation between denial of the Holocaust and knowledge of basic facts about it. Many of those who questioned or denied that the Holocaust occurred cited lack of information as the reason for their uncertainty, or displayed ignorance of basic facts about the Holocaust.

> “Not only are the levels of denial low, but few are dedicated, committed deniers. Most of the questioning of the Holocaust reflects ignorance rather than anti-Semitic commitment,”

A switch from 20% to 1%, just based on the polling question, highlights how hard it is to poll on this topic.