So what really led Adolf Hitler to evil?

A billy goat who bit off the tip of his penis? A Jewish prostitute who gave him syphilis? Parents who abused him? Rumors of a Jewish grandfather? An inborn inclination to kill? The German society surrounding him?

None of these theories has ever been proven. But for more than five decades, historians, theologians, psychoanalysts and others have been positing such ideas about this infamous man.

And today, to author Ron Rosenbaum's amazement, there is still a complete lack of agreement regarding Hitler's motivations and beliefs.

Rosenbaum's new book, "Explaining Hitler: The Search for the Origins of His Evil," details the ongoing debates. Published last month, it hit the New York Times bestseller list this week.

"You scratch the surface of the Hitler literature on any controversy — biographical, his ancestry, his motives, the origins of his anti-Semitism — and there is a bitter battle. Schisms, divisions. We don't have the answers," the 51-year-old author said last week during a stop in San Francisco.

"If we had the answers, if we understood what made Hitler Hitler, then I wouldn't have wasted my time. But I think what's almost shocking, stunning is how little consensus there is on so many important issues."

The book includes interviews with "Shoah" documentary director Claude Lanzmann, Nazi hunter Simon Wiesenthal and historians Yehuda Bauer, Lucy Dawidowicz and Daniel Goldhagen. Rosenbaum even interviews Holocaust revisionist-denier David Irving in a chapter titled "The Big Oops."

After more than 10 years of research, Rosenbaum said he has concluded that too many theories somehow diminish Hitler's responsibility for the genocide.

"It is important to say Hitler is evil," Rosenbaum said bluntly.

Because so little about Hitler's motivation is certain, Rosenbaum feels particular disgust for theorists who form conclusions without solid evidence.

Those who most upset him are a handful of psychoanalysts who with "an amazing overconfidence" and "deplorable egotism" have diagnosed Hitler without ever seeing him on the couch.

With "dubious evidence," for example, a few psychoanalysts have determined that "poor little Adolf" was beaten by his father, or that a dysfunctional family and a lack of self-esteem led Hitler to choose evil.

Other theorists have sought out a single Jew to blame for the Holocaust: the family doctor who bungled the cancer treatment of Hitler's mother, the unknown man who impregnated Hitler's paternal grandmother or a possible Jewish suitor pursuing Hitler's niece, who was in fact Hitler's own love-interest.

Again, none of those theories about a sole Jewish catalyst can be proven as the root of Hitler's anti-Semitism.

Instead, Rosenbaum suggests, "Why not look for other anti-Semites?"

Through numerous interviews and paper trails, the author believes he has disproved the story that Hitler was missing a testicle. And Rosenbaum dismisses as unprovable another story that a young Hitler had his penis nipped when he tried to urinate in a goat's mouth.

After all the years of research, Rosenbaum hasn't come up with his own theory on Hitler's evil. But the author's beliefs now embody a few theories, particularly those of Dawidowicz and philosopher Berel Lang.

Dawidowicz asserts Hitler had a mission to destroy the Jews as far back as 1918, at the end of World War I. To Rosenbaum, Hitler's actions over the years seem to fit that concept better than theories that view him as indecisive about the fate of the Jews.

Likewise, Lang sees Hitler as a connoisseur of the "art of evil," who relished his criminality.

Despite his current work, Rosenbaum's interest in Hitler has not been a lifelong obsession. The Manhattanite has spent his career as an investigative journalist, diving into other enigmas such as the Kennedy assassination and the Dead Sea Scrolls.

Wearing sneakers, an oversized suit, a rumpled shirt and an uncentered tie, Rosenbaum better fits his image as a New York Observer columnist than his job as a Columbia University journalism instructor.

Rosenbaum became interested in writing about Hitler in 1982 after his father revealed that a French cousin died in the Holocaust.

A short while after learning about his cousin, he also met a group of Jewish militants who planned to kill Nazi war criminals living in the United States.

When they asked Rosenbaum if it would have been right to assassinate Hitler in pre-Nazi Germany, Rosenbaum answered that the forces in Germany were so intense that the Nazis would have come to power anyway.

Today, he feels differently, believing more strongly in the "No Hitler, No Holocaust" theory.

The author now says he would even strangle baby Adolf in the crib if he could travel back in time.

With such attitudes, it's not surprising that Rosenbaum eventually found his project extremely depressing.

"It was a toll," he said.

And after all the work, Rosenbaum acknowledges that he cannot explain Hitler — though he does better understand this figure of history.

"I think I may understand the texture of his lies better," he said. "I sort of recognize the tone of his deceit."

Basically, it's important never to trust what came out of Hitler's mouth — unless there is corroborating evidence. He was a liar extraordinaire.

"The more you tune in," Rosenbaum said, "the more you distrust just about everything that comes from him alone."