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 presidential candidate Barack Obama, Olympic swimmer Michael Phelps and others who were reared by single mothers?
Obama, a U.S. senator from Illinois, tonight will accept the Democratic presidential nomination. Phelps just 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, says fatherhood expert Michael Lamb, a psychology professor at the University of Cambridge in England.
"The key point is yes, there is a risk," he says. "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 says 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 says.
Another expert on fatherhood, sociologist Tim Biblarz of the University of Southern California-Los Angeles, says the evidence shows economics plays a significant role in the risk for negative outcomes, such as 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 says. "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 USA.
Lamb says children do better if they have a good relationship with the in-home parent, as well as if the parents have low conflict; if the parent has economic resources; and if children have individual resilience to adverse circumstances.
"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.
"In the case of Obama, his mother was not particularly well off, though she was well-resourced intellectually and had been to college and had supportive parents," Lamb says.
Michael Kimmel, a sociologist and gender studies expert at Stony Brook University in New York, says the resident parent has a huge effect.
"We see constantly children of single-parent families who thrive because the parents are so devoted because they're compensating for the absence of the other parent," he says.
But Biblarz says 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 says. "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, says Peggy Drexler, author of the 2005 book Raising Boys Without Men.
"Phelps was born with a gift that his mother nurtured," says Drexler, an assistant professor of psychology at Cornell University's Weill Medical College in New York City.
Such mothers "don't hold them back," she says. "They encourage their talents, and drive and encourage independence and a sense of adventure."