Conventional wisdom is that boys who grow up without fathers are at greater risk of problems, from doing poorly in school to substance abuse. So how does that account for the high-profile successes of standouts such as Sen. Barack Obama, Olympic swimmer Michael Phelps and others reared by single mothers?

Obama is the Democratic presidential nominee. Phelps won eight gold medals at the Beijing Olympics. They — as well as Tour de France-winning cyclist Lance Armstrong, Massachusetts Gov. Deval Patrick and actor Benjamin Bratt — are just some of the accomplished men who grew up in single-parent households for most or all of their youth.

For decades, researchers have said children from two-parent families do better than those raised by a single parent. That's still true, they say. But newer research pokes holes into that all-or-nothing approach, said fatherhood expert Michael Lamb, a psychology professor at the University of Cambridge in England.

"The key point is yes, there is a risk," he said. "But it's not really a risk inherent in the single-parent family, per se. You can't assume that every child raised by a single parent is going to have difficulties. The majority don't."

Lamb said that decades ago, researchers were concerned about risks to children, and "their concerns were driven by a lot of cultural assumptions, which led them to propose kids are better off in the traditional family.

"The evidence, on the whole, hasn't supported that, but the beliefs have persisted in society," he said.

Another expert on fatherhood, sociologist Tim Biblarz of the University of Southern California, said the evidence shows economic factors play a significant role in increasing the risk of poorer grades and lower educational attainment, substance abuse or poor social adjustment.

"Those who grow up with single mothers with adequate socioeconomic resources tend to do well. The children of poor single mothers are more at risk," Biblarz said. "Many of the results that say that kids are at increased risk for negative outcomes have to do with economics."

According to the most recent data for 2007 from the U.S. Census, 8.4 million boys under 18 were living with a single mother. That's 22 percent of all boys in that age group in the United States.

"What's important is not whether they are raised by one or two parents. It's how good is the relationship with the parent, how much support they're getting from that parent and how harmonious is the environment," Lamb said.

But Biblarz said the idea "that boys in particular need fathers in the way girls need mothers" doesn't hold true.

"I can tell you there's almost no evidence supporting that," he said. "For a variety of reasons, children who grow up with single fathers, for example, are at higher risk than those who grow up with single mothers for either sex."

In the case of swimmer Phelps, mothers such as Debbie Phelps have the right approach, said Peggy Drexler, author of the 2005 book "Raising Boys Without Men" (Rodale Books, $23.95).

"Phelps was born with a gift that his mother nurtured," said Drexler, an assistant professor of psychology at Cornell University's Weill Medical College in New York City.

Such mothers, she said, "encourage their talents, and drive and encourage independence and a sense of adventure."