As an example, about 80 percent of 20-year-old Jewish Russians marry non-Jewish partners, according to 1994 figures. Most often, the children of mixed marriages do not choose to be Jews, said Sergio DellaPergola, a professor who compiled a survey on Jewish demographics as part of the study.

Only by 2050 can the Jewish people hope to approach the number of Jews lost in the Holocaust. There were 18 million Jews in 1939, before the war.

Despite a growth rate among Jews in Israel of about 2.6 percent, assimilation has brought the growth rate abroad so low that it almost cancels out the increase in Israel, DellaPergola said.

Although the Sept. 11 terrorist attacks in the United States have not yet led to an increase in the number of American Jews immigrating to Israel, they did raise their level of Jewish identification with Israel, Sharansky said.