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 Democratic presidential nominee Sen. Barack Obama, Olympic swimmer Michael Phelps and others who were reared by single mothers?

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.