"The whole definition of what constitutes America's Sweetheart is a really complex one and different from what constitutes a Tiger Beat teeny-bopper sex symbol for males," said Robert Thompson, a professor of popular culture at Syracuse University.
James Robert Parish, a Hollywood historian and author of "Fiascos: A History of Hollywood's Iconic Flops," says that the actresses who have historically been labeled sweethearts "project not only sweetness but vulnerability, which is very important because then people can relate and don't feel threatened.
"Not only do you need to be a beautiful, wholesome American, but you need one or two defining roles that put you into that category where people say 'Oh, whenever I go to see a Julia Stiles picture I know it's going to be this romantic comedy and it's not going to be threatening that my husband or boyfriend is going to want to run off with her and forget about me,'" Parish said.
The current queen of sweetness is Witherspoon, 29, whose Southern charm and comedic timing won her praise in movies such as "Election," "Legally Blonde" and "Sweet Home Alabama."
Witherspoon took such fare to the next level with her Oscar-winning performance as June Carter Cash in "Walk the Line," and in March, she reportedly surpassed Roberts to become the highest-paid actress in Hollywood, earning $29 million for the 2007 horror film "Our Family Trouble."
Delaina Dixon, a staff writer at OK! Magazine, says Witherspoon is "stepping into Julia's role with the Oscar."
But behind Witherspoon is a pack of beautiful actresses clamoring for the attention. Dixon thinks 29-year-old McAdams (who was actually raised in Ontario) and Bynes, the WB's 20-year-old "What I Like About You" star, are definitely women to watch.
"McAdams is interesting in the sense that she doesn't follow the Hollywood rule of being in the spotlight all the time," Dixon said. "It's really all about the work."
McAdams starred in the highly successful comedies "Wedding Crashers" and "Mean Girls" and the sleeper hit "The Notebook."
As for Bynes, "she's fun and down to earth," Dixon said. "I think a lot of people enjoy watching her." Bynes' current flick, "She's the Man," garnered mixed reviews.
Smokey Fontaine, the editor in chief of Giant magazine, points to women like Bryce Dallas Howard, who will star in "Spider-Man 3," Michelle Monaghan, who will star in "Mission: Impossible 3," Kate Bosworth, who will star in "Superman Returns" as the American actresses to watch this year.
"It's interesting for a lot of these young actresses to be in these big Hollywood blockbuster summer movies," Fontaine said. "If you do well and you kind of steal a couple scenes, your name recognition will go through the roof and it can really set you up in a good way."
And like Bynes, who got her start on Nickelodeon, the new guard doesn't necessarily have to begin their careers on the big screen. They do need, however, need to have a sense of comedic timing.
"When they can do a comedy, you think of them being nice if they can make fun of themselves," Dixon said.
There comes a time, though, when America's Sweetheart outgrows the label.
Roberts seemed to give away her fears of the future when she played a movie star in the 1999 film "Notting Hill" and delivered the line: "One day, not long from now, my looks will go. They will discover I can't act, and I will become some sad middle-aged woman who looks a bit like someone who was famous for a while."
For Roberts, who won her Academy Award for "Erin Brokovich," starring in a Broadway production is another attempt to broaden her artistic resume. "Even Julia did fluffy movies at the beginning," Dixon said. "Now she can afford to do Broadway."
Some, however, say Roberts is starring on Broadway to step into the next phase of her career, as the dearth of gritty Hollywood roles for women in their 40s ultimately removes them from the Sweetheart spotlight.
"By that point both their physical look and newer people coming up demand that they sort of mature into other roles," Parish said, adding that Hollywood doesn't "write roles for mature actresses."
Parish said being America's Sweetheart might hurt a "serious" actress' artistic integrity, even if it's only for a few years.
But when you're paid $29 million a picture, even the most devoted thespian is usually willing to suffer for her art.