In a related finding, more than one in five self-identified Jews (22 percent) told Pew researchers that they had no religion, a proportion that mirrors the roughly one in five Americans who claim no religious affiliation.
Yet in spite of their weakening adherence to Jewish observances, the report noted that "American Jews overwhelmingly say they are proud to be Jewish and have a strong sense of belonging to the Jewish people."
As if to prove that point, a strong majority of Jews (69 percent) call themselves very or somewhat emotionally attached to Israel – a proportion that has held fairly steady for at least a decade.
Pew's new statistics on intermarriage, raising of children, synagogue membership and attachment to Israel are likely to come under intense scrutiny by Jews and Jewish groups who are focused on the "continuity" question.
In short, that question concerns the fear brought into stark relief by the Holocaust that the survival of the Jewish people is tenuous. In many Jewish eyes, continuity depends on maintaining strong defenses against external enemies who would kill Jews and destroy Israel, but also against the internal threat of intermarriage and its tendency to diminish the number of children who are raised as Jews.
Given that history, it is perhaps not surprising that when the study's surveyors asked what is essential to being Jewish, more respondents (73 percent) named "remembering the Holocaust" than any other answer.
"It's a way of saying Jews must continue," said Rabbi David Saperstein, a leader in the Reform Jewish movement, the largest and most liberal of the three major streams of Judaism. "And therefore we need to build a Judaism that is strong and robust."
Saperstein quoted the theologian and Holocaust survivor Emil Fackenheim, who said: "The 11th commandment is 'You shall not give Hitler a posthumous victory.'"
By contrast, just 19 percent of respondents in the survey called "observing Jewish law" an essential part of being Jewish. After "remembering the Holocaust," the second most popular response to what it means to be Jewish was leading an ethical life, the option chosen by nearly seven in 10 people.
Cooperman said the growing number of "Jews of no religion" is a major new twist in the continuity question. Two-thirds (67 percent) of "Jews of no religion" who are raising children told pollsters that they are not bringing them up as Jews in any way – not even as cultural or secular Jews.
"They're a growing group, and the question is, will they pass along their identity?" Cooperman said.
Other findings from the report include:
• Jews continue to support the Democratic Party over the Republican Party by a wide margin: 70 percent said they are Democrats or lean Democratic, while 22 percent said they are Republican or lean Republican.
• More than eight in 10 Jews (82 percent) said homosexuality should be accepted by society, compared with 57 percent of Americans in general.
• A vast majority of Jews believe a person can still be Jewish and work on the Sabbath (94 percent), and can be strongly critical of Israel (89 percent) or not believe in God (68 percent).
• More than three in 10 Jews believe a person can believe that Jesus was the messiah the belief that's central to Christianity and still be Jewish.
• Of the three major branches of Judaism, the Reform movement the least traditional claims 35 percent of American Jews, followed by unaffiliated Jews (30 percent), Conservative Jews (18 percent) and the Orthodox (10 percent).
• More than four in 10 American Jews (43 percent) have been to Israel, and about the same proportion (44 percent) believes Israel's continued building of settlements on lands claimed by the Palestinians is harmful to Israel's security.
• More than four in 10 Jews (42 percent) said having a sense of humor is essential to their Jewish identity.
Pew interviewed 3,475 Jews in America to produce its 213-page report, which pins the number of adult American Jews who say Judaism is their religion at 4.2 million. That number rises to 5.3 million if cultural Jews are included.
Add to these figures the approximately 900,000 children being raised exclusively as Jews, or the 1.8 million living in households with at least one Jewish adult.
Another report on American Jews, released by Brandeis University in September, found similar numbers and pegged the overall number of Jews in the U.S. at about 1.8 percent of the population. But the Brandeis study drew on previous studies for its estimates and did not include the original research like the new Pew survey does.
"The main difference in approach, I think, is that the Brandeis team uses one (and only one) definition of who is a Jew," Pew's Cooperman wrote in an email.
"Our approach … is to point out that the estimated number of Jews in the United States depends on your definition of who is a Jew, and we provide tables that allow readers to see how the estimates vary if you choose different definitions."
On the controversial question of Jewish intermarriage, the rate has risen substantially during the past five decades, according to the Pew study. Now, of Jews surveyed, 44 percent have married non-Jews, compared to only 17 percent before 1970. Since 2005, that figure has jumped to nearly six in 10 (58 percent) Jews.
But Cooperman notes that the current intermarriage rate is not so different from the rate between 1995 and 1999: 55 percent. "Is the rate leveling off? That's something to look for in the future," he said.
The survey, which cost more than $2 million and was funded by the Pew Charitable Trusts and the Neubauer Family Foundation, was conducted between Feb. 20 and June 13, and has a margin of error of plus or minus 3 percentage points.
Its authors took pains to define precisely who took part in the survey, since the question "who is a Jew?" is hotly debated.
Traditional Jews, for example, say Jewishness is passed from a Jewish mother to her children, while Reform Jews hold that either a mother or father must be Jewish to produce a Jewish child, as long as that child is raised as a Jew.
The study and this story focus on the 3,475 surveyed who said they are Jewish by religion, or have at least one Jewish parent or were raised Jewish and still consider themselves Jewish in some way.