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Actresses know to hang onto their men when Oscar comes to call, but has the dreaded curse associated with the Academy Award been broken?

A startling number of Best Actress Oscar winners have ended their relationships within a year or two after taking home the award, a trend that has come to be known as the "Oscar curse."

"It's not a theory, it's been scientifically proven," said Bradley Jacobs, a movie editor for Us Weekly. "Sometimes they end right away, other times it takes six months or a year, but they do end."

Natalie Portman, who won the Best Actress Oscar last year for her role as a deeply disturbed ballerina in "Black Swan," is one of the few whose relationship hasn't been doomed by the award, yet.

Portman met dancer Benjamin Millepied while they were filming "Black Swan" in 2009. Soon after the film wrapped, the couple announced their engagement and pregnancy. Portman gave birth to their son Aleph Portman-Millepied last year. But can their seemingly whimsical romance last? Some are skeptical.

"That relationship doesn't seem totally on solid ground," Jacobs said. "I'm pulling for her, but I'm worried for her relationship because of the curse."

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Researchers at the University of Toronto's Rotman School of Management and Carnegie Mellon University produced a study last year that looked at married women nominated for Best Actress from 1936 to 2010. They concluded that 60 percent of Best Actress winners got divorced after going home with Oscar.

Shannon Fox, a relationship expert and author of "Last One Down the Aisle Wins," said while winning an Oscar can take its toll on a relationship, there are numerous reasons why a Hollywood marriage, which has a "shorter shelf life" than most, would fail.

"The movie was probably made two years before it ever made it to the Oscars so the issues have probably been going on for a while, whether they won the award or not," Fox said. "[But] the person knows when they shoot it and they know after they've seen it, it's an Oscar contender ... so there is definitely a pressure on the spouse to stay with it until the Oscars are over, let that breath out, then figure out what's wrong with the marriage."

One of the biggest reasons celebrity marriages fail, Fox said, is because actors are away for months at a time, so there's a lack of physical contact -- several of the Best Actress nominees who divorced their spouses were once married to other actors or people in the entertainment industry -- but also a lot of intimate time spent with other people while filming.

"Most marriages that I have ever worked with would never be able to withstand another partner kissing or even fake making love with another person," Fox said. "The fact that you're on set with that person and your partner isn't there, and you're acting like you're in love with them and you're physical with them and those feelings happen."

Another reason is the incredible, emotional drain an actress goes through when she is getting into character, Fox said, and sometimes, that actress can come back to her spouse a completely different woman.

"An Oscar-winning role is a very demanding role," she said. "Not just demanding of time ... it's a very emotionally demanding role ... you really have to be completely inhabited by your character and a lot of women will experience different emotions than they have before."

Not to mention that winning an Oscar greatly increases an actress' asking price, and marriages across the board can buckle when the woman is the breadwinner.

Jacobs of Us Weekly predicts that Viola Davis will take home the Oscar for Best Actress Sunday for her role in "The Help," which would be a big deal, he said, because she would only be the second African-American woman to ever win the award, after Halle Berry.

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But if Davis were to win, could she be stricken with the curse? She has been married to actor Julius Tennon since 2003. He has two sons from a previous relationship, and the couple just adopted a baby girl in October.

"It takes a real man to be married to an Oscar-winning actress," Jacobs said. "She's in demand ... everyone wants to meet you, and be the only one on the red carpet with you ... and for the men to hold your purse. Men don't want to hold purses. Men want to also be vibrant."

Here are just some of the Best Actress Oscar winners who have been hit with the curse since 1938.

Sandra Bullock Sandra Bullock won the Best Actress Oscar in 2010 for her critically acclaimed performance as Leigh Anne Tuohy in "The Blind Side," a biopic about Baltimore Ravens offensive tackle Michael Oher. Bullock mentioned her husband, "Monster Garage" host Jesse James, in her Oscar acceptance speech, saying, "I love you so much, and you're really hot. And I want you so much." The Oscar Curse: Mere days after Bullock secured her Oscar win, rumors emerged that James had an 11-month affair with tattoo model Michelle "Bombshell" McGee. The affair led to a nasty, public divorce, all the while the couple had been in the process of adopting a baby. Bullock filed for divorce in April 2010 and began raising their adopted son, Louis, now 2, on her own. In an interview with "Nightline" last year, James revealed that while he and Bullock both have houses in Austin, Texas, they don't speak to each other. "I've never seen Louie since everything happened," he said, adding that his daughter, Sunny, from a previous marriage has only seen Bullock "a couple of times," but that there has been "no contact at all for several months. "I think, you know, a marriage ending, all this crazy stuff ... that was like a lesson to work on myself and fix what's wrong with me," James said. "So I don't transfer it and keep it going, like my parents did."

Kate Winslet Kate Winslet won the Best Actress Oscar for her role as Hanna Schmitz in "The Reader" in 2009. That same year, Winslet also won the Golden Globe award for Best Actress in a Drama for her performance in "Revolutionary Road," directed by then-husband Sam Mendes. In her Oscar acceptance speech, Winslet thanked her "wonderful husband and two beautiful children who let me do what I love and who love me just the way that I am." The Oscar Curse: In March 2010, Winslet announced she was ending her almost seven-year marriage to Mendes. "Kate and Sam are saddened to announce that they separated earlier this year," their lawyers said in a statement. "The split is entirely amicable and is by mutual agreement. Both parties are fully committed to the future joint parenting of their children."

Reese Witherspoon Reese Witherspoon won the Best Actress Oscar for her critically acclaimed performance as June Carter in the Johnny Cash biopic, "Walk The Line," in 2006. In her Oscar acceptance speech, Witherspoon thanked her "wonderful husband," actor Ryan Phillippe. The couple first met at Witherspoon's 21st birthday in 1997, and later worked together on the 1999 film "Cruel Intentions," which included a steamy sex scene between bad-boy Sebastian, played by Phillippe, and the virginal Annette, played by Witherspoon. The couple married when Witherspoon was 24, a "ridiculously young" age, she admitted to the U.K.'s version of Elle magazine in an April 2011 interview. The Oscar Curse: After a seven-year marriage, Witherspoon, 35, and Phillippe, 36, who have two children together, Ava, 11, and Deacon, 7, divorced in October 2007. Three years later, Witherspoon tied the knot with Hollywood agent Jim Toth.

Hilary Swank Hilary Swank won her second Best Actress Oscar in 2005 for her role as the amateur boxing star Maggie Fitzgerald in "Million Dollar Baby.: Her first Oscar was for her performance in the 1998 film "Heartwood." In her Oscar acceptance speech for "Million Dollar Baby," Swank told her husband, actor Chad Lowe, "you are my everything." The Oscar Curse: In January 2006, 11 months after winning the Oscar, Swank separated from Lowe, ending their nearly nine-year marriage. At the time, Swank said his "substance abuse" played an important role in their sudden separation.