BOSTON — They're smart and charming. They have an aura of success about them and exude respectability. Above all, they instill confidence.

Which is, after all, why they are called con men.

Bernard Madoff, the Wall Street trader accused of running the biggest Ponzi scheme in history — $50 billion — dealt in more astounding numbers than others but shares many of the basic qualities of Ponzi swindlers through history, according to law enforcement authorities and others who have studied such scams.

"They seem trustworthy because of their charm, their command of finance and the unshakable confidence that they portray," said Jacob Frenkel, a former Securities and Exchange Commission enforcement lawyer. "The Bernie Madoffs of the world are the people you want to sit next to on an airplane."

Much like the original Ponzi schemer, Charles Ponzi. He was an Italian immigrant to Boston who worked as a waiter, bank teller and nurse before he talked investors into sinking their money into a complex — and, it turned out, bogus — scheme involving postal currency.

His short-lived swindle in 1919-20 cheated thousands of people out of $10 million but was so wildly lucrative for some early investors that he was hailed as a hero in the Italian community. He was convicted of mail fraud and sent to prison before being deported in 1934.

A Ponzi scheme, or pyramid scheme, is a scam in which people are persuaded to invest in a fraudulent operation that promises unusually high returns. The early investors are paid their returns out of money put in by later investors.

"It used to be called 'robbing Peter to pay Paul,' " said Mitchell Zuckoff, a Boston University journalism professor who wrote a biography of Ponzi in 2005.

Ponzi's scheme was the most famous con game of his time, and his name has been attached to similar frauds ever since.

People who run Ponzis generally fall into two categories: hucksters like Ponzi who plan to cheat investors and get out quickly, often fleeing the country, and people who start a legitimate investment venture but lose money, then try desperately to cover it up and dig themselves into a deeper and deeper hole. Ultimately, it all comes crashing down.

HAVE TO LOOK THE PART

Some have speculated that Madoff — once a highly respected figure on Wall Street and a former Nasdaq chairman — falls into the latter category.

Bookish and bespectacled with a wise smile, Madoff had multiple homes, fancy cars and memberships at exclusive country clubs. He gave millions to charity from his own fortune.

"Looking successful is the key because everyone's first question is going to be, 'If this is such a great deal, then why are you wearing a cheap suit?' " said Eric Sussman, a former federal prosecutor from Chicago who helped on about a dozen Ponzi cases.

Investigators said that only $200 million to $300 million of Madoff's investors' money is left. Where the rest went is a mystery.

NOTABLE SCAM ARTISTS

One of the most famous pyramid schemes occurred just before the turn of the 20th century. William Miller of Brooklyn, N.Y., cheated investors out of $1 million by claiming he had inside information on stocks and promising interest of an astounding 10 percent a week. For that, he was nicknamed "520 Percent."

Modern-day Ponzi operators include Democratic fundraiser Norman Hsu, a former men's sportswear executive who was indicted last year on charges of duping investors out of at least $20 million. He was known for his impeccable attire and warm personality.

Former boy-band promoter Lou Pearlman, who is serving a 25-year prison sentence for cheating investors out of $300 million in a scheme that lasted more than a decade, was a rotund man nicknamed Big Poppa. He was known for his smooth talk and easygoing style.