John Allen Paulos was nursing a dangerous infatuation, and keeping it secret from his wife. Worse, he had a check in his pocket that day in Philadelphia.

Leaving his wife to browse in a bookstore, the Temple University mathematics professor dashed over to his brokerage office to meet a margin call – a demand for more money to cover a losing stock bet.

“My illicit love affair with WCOM continued,” Paulos writes in his new book, “A Mathematician Plays the Stock Market.”

WCOM is the former ticker symbol for WorldCom Inc., which was then tracing its impressive arc into bankruptcy court. Buying stock on margin is the risky practice of borrowing the purchase price from your broker.

And Paulos’ book is an amusing – and sobering – study of the limits of intelligence in investing.

Paulos doubled and redoubled his ill-starred bet on WorldCom, which costarred with Enron Corp. in the late, great stock-market bubble.

He slipped to his computer in the middle of the night to check on the stock’s progress on European markets. He not only bought the stock on margin, he purchased options locking in the price for more shares if it turned around.

“I did every dumb thing it was possible to do,” Paulos said in an interview. “Normally, I am a hardheaded fellow.”

Investing turns most people daffy to some degree, said Meir Statman, a Santa Clara University finance professor. As a specialist in behavioral finance, he explores the human reaction to risk and reward.

Investors are intoxicated by their prospects for wealth and disdainful of evidence to the contrary, he said.

“All people, mathematicians included, make the same dumb mistakes,” he said. “We are all alcoholics when it comes to the stock market. The best way to deal with it is not to buy individual stocks.”

Paulos agrees in his book, to a point. While describing the mathematics that can be used to counter market psychology, he remains skeptical that stock pickers can outstrip the market’s overall gains.

“Do you make more than you would investing in a low-fee index mutual fund?” Paulos asked. “The evidence just isn’t there” to say so affirmatively.

But Paulos is not ready to give up the adrenaline rush. “The entertainment value of the market is not to be dismissed,” he said.

The market rivaled baseball as the national pastime in 2000, and Paulos was holding a small, unexpected check. Among the sources of investment funds, “mad money” certainly counts as an endangered species.

Paulos writes that he was seduced by the promise of the Internet, and captivated by WorldCom’s central role as the operator of its telecommunications “backbone.”

He was not the first to pay homage to cyberspace. WorldCom shares soared 111 percent between September 1998, and the following summer, closing at $64.50 on June 21, 1999.

He bought his initial shares at $47, expecting it to bounce back to its former glory. Instead it kept falling, dropping back to its 1998 price by September.

Paulos bought more shares, “averaging down” in investing parlance. Equal purchases at $47 and $30 lowered the average price to $38.50. In hindsight, Paulos writes that he succumbed “to an unexpected gambling instinct and a need to be right.”

In any event, Paulos plunged ahead, buying more shares of WorldCom, with some of these purchases completed on margin, and then, with WorldCom’s price under $10, buying “many thousands” of options reserving the right to buy WorldCom at $15 until January 2003.

Paulos declines to say how much money he lost on WorldCom, except to describe the sum as substantial.

In his book, Paulos reviews the many ways that bright people latch onto bad ideas and refuse to let go of them.

Statman takes a homelier approach. “Buying stocks is like getting married,” he said. “Selling them is like getting divorced.”

Few people are able to segue effortlessly from visions of bright futures to assignment of blame.

On the latter score, Paulos accepts responsibility for his decisions despite a surplus of villains in the WorldCom soap opera. Investors “are little too quick to see themselves as victims,” he said.

So when did he break the news to his wife, and how did she take it?

Paulos said he told her after he finally dumped the stock. Her reaction was “meteorologically normal: an initial storm of anger and depression, punctuated by intermittent upset and uncertainty, gradually clearing into resignation and acceptance.

“Overall, she reacted more generously than I would have.”