Masa's leaders are mystified after the raucous responses to an initiative they considered to be utterly wholesome and unassailable. American Jewish youth are "assimilating," American Jewish leaders keep complaining. Somewhere, Masa executives heard the oft-quoted, outdated and vague notion that about half of American Jewry are "leaving the fold." This conventional wisdom is the product not only of the 1990 National Jewish Population Survey, which seemed to suggest a 50 percent intermarriage rate among American Jews, but also of research by the likes of Prof. Steven Cohen, who speaks of "half" of American Jewry "distancing" itself from the Jewish community, and another "half" growing closer and stronger in their Jewish affiliation, though he refers to broad identity trends rather than numerical facts. In any case, that sense that half is "lost" and half is "safe" amounts to the only thing most Israelis, even those dealing with the Diaspora, actually know about American Jewry. Faced with such a clear and agreed-upon crisis, they reacted in a quintessentially Israeli way. They mobilized. And, spectacularly, demonstrated a reality I have come to know well from the past few years of following the research conducted by my father, Rabbi Ed Rettig of the American Jewish Committee: The Jewish world does not appreciate and fully understand the vastness of the cultural gulf that divides the two largest communities, Americans and Israelis, which together constitute 80% of all Jews. Perhaps it is time to begin to question what, as communities, we really know about each other. One of the best demonstrations of this breach comes in the form of Israel experience programs like Masa. You can't argue with the data of surveys by the likes of Cohen, Prof. Leonard Saxe of Brandeis University and others showing the profound changes wrought on young American Jews by a mere week-long bus tour of Israel. But no Israeli policymaker can offer a good explanation for what, exactly, is happening to those young people. What is it about Israel that makes young Americans, who are utterly and proudly American and sometimes only conditionally Jewish, react so positively? Americans, too, are befuddled by this gap. Americans fund and encourage their children to go to Israel by the hundreds of thousands, but rarely consider clearly and rationally why a mere ten days in a foreign country can so affect the identity and lifelong affiliation of an ordinary 19-year-old. Here's a theory: Israeli society has a profoundly different and deeply moving way of defining the very notion of Jewishness. Israelis are a product of their heritage and experience. The vast majority of Israelis hail from countries untouched by the Protestant Reformation and the identity-shifting aspects of modernity. In both Eastern Europe and the Muslim world, religious identities are fundamentally collective and couched in familial terms. Meanwhile, for 60 years incessant wars and hostile borders have added an element of collective fate to that Middle Eastern and East European structure of identifying. It is that organic, rooted nationhood, a radically different notion of what it means to be a Jew from anything Americans have ever experienced, that so impresses young American Jews, and makes programs such as Masa and birthright Israel transformative experiences for Americans. The vast majority do not become Israeli or adopt Israeli identity structures, but do seem to come away with a more complex Jewishness; an understanding that there are aspects and layers to Jewish affiliation which they had not experienced before. American Jews, too, are products of their broader environment. Like their surrounding culture, they are radically individualistic, believing that the source of authentic identity, of religious authority and of life decisions, lies within the individual. Where Israelis are profoundly Eastern in the overarching structure of their Jewishness, Americans understand identity in radically individualistic and essentially American ways. Thus, faced with what they believe to be a genuine crisis, the Israelis went about solving it the way Israelis solve everything from Arab invasions to water shortages to poverty - by national mobilization. Assimilation, Israelis believe, is fundamentally a problem to be solved through collective action, not identity education and personal affiliation. The total worldwide Jewish population "is on the verge of negative growth," warned Masa CEO Ayelet Shiloh-Tamir at the launch of the controversial ad campaign last week. "We want Israelis to view assimilation [in the Diaspora] as a national strategic problem," she added bluntly. For Americans, it is hard to hear the campaign as anything more than a denial of individual autonomy and personal authenticity. The core assumptions behind the campaign seem, in an American cultural context, appalling. It is particularly interesting to note how knowledgeable American Jewish observers characterized their discomfort. For the Forward's JJ Goldberg, the imagery of subway and railway stations - where the "Lost" posters are seen hanging - were reminiscent of the Holocaust. Esther Kustanowitz, a well-known blogger in the Jewish blogosphere, had this to say. "But I have to tell you, that as an American citizen and - up until almost a year ago - a longtime New Yorker, I can't look at those 'missing' flyers without thinking of 9/11 - those flyers, hanging in many locations all over Manhattan and beyond, were at once symbols of hope that someone who was missing was 'only missing,' and a denial of the reality that most of the missing were actually dead. Invoking that image to refer to people who are not dead, but presumed 'lost to Judaism' because they 'married out,' seems somewhat inappropriate." Perhaps it was their unique sensitivity as veteran writers, or perhaps their long familiarity with Israeli society, that allowed both Goldberg and Kustanowitz to sense the underlying assumption behind the ad campaign that accounts for another fundamental dividing line between Israeli and American Jewish culture - the importance and sanctity of sacrifice. In a country with so much experience of loss, where every generation has had its own war, where every Israeli knows personally a casualty of war, where there truly are enemies at the gates, the ideal of sacrifice strikes a deep chord in the collective consciousness. Just compare the somber and sacred Yom Hazikaron to the "Happy Memorial Day Weekend" banners festively festooning the entrances to American malls. Sixty years of relative peace have stripped away personal grief from American national commemoration. Loss and grief do not form, as they do in Israel, part of the basic identity of most Americans. One Israeli friend said he saw in the posters not "missing persons," 9/11 victims or Holocaust victims, but war dead. The music of the television spots evoked Israeli war documentaries; the age was right; and the ethnic mix - Americans, Russians, French-speakers of North African origin - suggested the kind of casualty list you would get from an army as ethnically diverse as the IDF. Had I been asked, I would have advised against this campaign. But now that it's been aired, perhaps it's time for the two communities to do something other than shout across the breach. Any conversation about assimilation and continuity inevitably invokes some of the fundamental pillars of communal identity. For Israelis, that almost automatically means collective action, sacrifice and loss. But for Americans, those noble values translate into an imposition on the autonomous, sacred self. Speak to the heads of Masa and you'll discover real bewilderment and resentment at being forbidden by unfathomably sensitive Americans to express some of their most intrinsic values as Jews. They are right. Speak to the Americans, whose existential crisis is indeed assimilation, but who understand this as a call to fashion new worlds of personal meaning and individualistic affiliation, and you'll find real anger at the callous Israeli attempt to define who is "lost" and who is "found." They're right, too. Friends, the experiences of birthright and Masa show that these two communities, blended, strengthen each other. But first we must come to terms with how incredibly different they really are. Let's talk about it.