BERLIN - The most talked about man in Germany is a 65-year-old economist whose hot new book and sudden groundswell of popular support have the media dubbing him a folk hero. But that is not the only thing they are calling Thilo Sarrazin these days.

Some are also calling him dangerous. Sarrazin, a board member of the German Central Bank until he resigned under pressure Thursday, has divided the nation by postulating the theory that Germany is being "dumbed down" by Muslim immigrants and their children. Wielding statistics and scientific arguments both in his book and in public comments, he delves into territory largely taboo here since the Holocaust, suggesting that "hereditary factors" are at least partly to blame. Turks and Kurdish immigrants, he asserts, are genetically predisposed to lower intelligence than Germans and other ethnic groups, including Jews.

His statements have shocked many in Germany not only because of a national sensitivity to anything remotely smacking of genetic superiority claims in the post-World War II era. What has also shocked many is that so many Germans have rallied to his side as the central bank and his political party have sought to oust him for his pronouncements.

Though most of his backers are publicly distancing themselves from his genetic arguments, they are lauding him as a straight-talker willing to address the problem of Muslim immigrants, who often eschew German language and culture. By throwing political correctness to the wind, they say, he has dared to speak the truth about higher immigrant unemployment, birthrates and welfare rates.

Among Germany's population of 82 million, about 5 percent are Muslims, most of Turkish descent. A poll published in the national magazine Focus this week showed 31 percent of respondents agreeing that Germany is "becoming dumber" because of immigrants, with 62 percent calling Sarrazin's comments "justified" and 52 percent saying he shouldn't be thrown out of his Social Democratic Party because of them. Since party chiefs began a process to evict him last week, their headquarters in Berlin has been inundated with thousands of e-mails supporting Sarrazin. High-profile politicians, including Chancellor Angela Merkel, have strongly condemned him. But others are praising him for bringing Muslim immigration concerns to the forefront of the national dialogue.

Sarrazin now has more than 21,000 friends on Facebook and an online fan club. In less than two weeks, his book, "Germany Does Away With Itself," is in its seventh printing, topping bestsellers lists with more than 300,000 copies shipped so far and many bookstores in Germany still sold out.

Muslim critics are calling his surging popularity here part of a wave of Islamophobia in the West, citing the move to ban burqa coverings in France and minarets in Switzerland and opposition to the construction of an Islamic center near Ground Zero in the United States. But others say his emergence in Germany, and growing popularity, is fundamentally more disturbing.

German-Jewish groups, for instance, are among Sarrazin's staunchest critics, calling him a dangerous racist. Though Sarrazin has spoken positively of Jews, saying they have "high IQs," he courted controversy after declaring in an Aug. 29 interview that "all Jews share a certain gene." In fact, observers here say that the official outcry against Sarrazin - including the move to expel him from the board of the central bank - would have been far more muted had he simply stuck to his generalizations about Muslims.

But by generalizing about Jewish genetics, albeit positively, Sarrazin also "crossed a red line," said Stephan Kramer, secretary general of the Central Council of Jews in Germany.

"It's absolutely unacceptable with the history here that such a large amount of people follow what he says," Kramer said. "The lesson of the Holocaust is not just about Jews, but that human dignity is indivisible. Yet now, they react if there is a genetic comment about Jews, but not if it's about the Roma or the Turks. We obviously still have some homework to do."

Yet Sarrazin's critique of Muslim immigrants has without question touched a national nerve. In the bars, taxis and offices of Berlin these days, it is the hottest topic of conservation, with his supporters feeling almost liberated by Sarrazin's willingness to throw caution to the wind and speak openly about their concerns. More than anything, he has tapped into German frustrations about the tendency of Muslim Turks - who began large-scale immigration in the 1960s to help run German factories - to live clannish lives, jealously guarding their language and religious traditions.

The bookish, mustached Sarrazin - who declined a request for comment - is no stranger to controversy. While a member of the Berlin state senate, he famously said civil servants had a physical odor and wrote a diet for welfare recipients who claimed they couldn't live on their meager state assistance checks. But in the past year, he has homed in on the issue of Turkish and other Muslim immigrants.

In his book, Sarrazin asserts that Germany is getting dumber in part because of so-called "congenital disabilities" from inbreeding by Turks and Kurds. "But the subject is usually hushed up," Sarrazin writes. "Perish the thought that genetic factors could be partially responsible for the failure of parts of the Turkish populations in the German school system." He goes on to blame crime and bloated welfare roles on Turks, using statistics that, according to German media articles, are often outdated and sometimes just plain wrong.

Many, like Carl Moser, a 26-year-old business school student in Bavaria who launched a Facebook fan site for Sarrazin, say he went too far with some of his comments. But "I support him because he dares to speak out on facts that have come from real data and won't bow down to political correctness," Moser said. "I am not supportive of everything he is saying, but Germany does have problems with immigration and integration and politicians are not willing to talk about it."

In the past, Turkish intellectuals have themselves sharply criticized the community's lack of assimilation, calling for the children of immigrants to integrate more fully into Germany society. But if anything, the book appears to be polarizing Germany, with the Turkish community feeling defensive and ostracized.

In Kreuzberg, a partly Turkish neighborhood of Berlin dotted with kebab houses and women in veils, community leader Kenan Kolat said Sarrazin and his popularity had "damaged" the faith of Germany's Turks in their adopted nation.

"Sarrazin doesn't see a social problem, but an ethnic problem, that we are genetically inferior," said Kolat, chairman of the Turkish Federation in Germany. "And yet, German people are supporting him. How do you think the Turkish community feels?"

faiolaa@washpost.com Special correspondent Jabeen Bhatti contributed to this report.