Serial marriage and divorce has been a foundation of showbiz life since long before Elizabeth Taylor famously pronounced: "What do you want me to do, sleep alone?" There are pressures on celebrities the rest of us can only dream of.
"Film sets are open season," says one insider. "The old code D.C.O.L.D - doesn't count on location, darling - really does apply."
Film casting director Jeremy Zimmerman believes that the glamour and unreality of being "on set" is just too seductive for some actors to resist. "An actor is away from domesticity and playing a different character and it just becomes easier to commit an infidelity. These people are living in a world where people will do anything for them."
Psychologist Aric Sigman says: "The celebrity marriage is a professional amalgamation of two children. Celebrities need to be applauded and a spouse often doesn't do that."
So what hope for Katie Holmes and Tom Cruise, due to wed soon? A New York mathematician has found a formula to calculate the lifespan of the typical celebrity marriage: the crucial factor is fame. He calculates that the chances of marital harmony are reduced with every Google hit each partner receives.
Based on our own research of 20 famous celebrity couples - some listed on the right - the devoted couple can hope to enjoy 7.8 years of married bliss.
Should Holmes and Cruise divorce, lucky Katie, who has never been married before, has a 50 per cent chance of getting hitched again, while Tom, who has been married twice, is more likely to grow old with only a community of religious nut-jobs for company.
The most common cause of celeb break-up is an extramarital affair. Other common obstacles include "star is born" syndrome, where one partner becomes more famous; long spells in rehab (Whitney and Bobby), and writing libellous rants about stuffing your spouse into a car boot and driving her off a cliff (Eminem and Kim Mathers).
Yet hope made flesh in this world is the 48-year marriage of Paul Newman and Joanne Woodward. Their secret, he says, is the understanding that "marriage is three meals a day, sharing the workload and remem-bering to carry out the trash". A long way from Hollywood.
RENEE ZELLWEGER & KENNY CHESNEY
Wedded bliss was shortlived for these Southern sweethearts, who sought an annulment after only four months of marriage, citing "fraud" as the reason.
4 months
DREW BARRYMORE & TOM GREEN
Tom Green made the decision to divorce Drew Barrymore in 2001 after six months of wedded bliss - twice as long as her first marriage to bar owner Jeremy Thomas.
1 year
ANGELINA JOLIE & BILLY BOB THORNTON
Despite exchanging vials of each other's blood, Bob and Angelina split in 2002. He is now on wife No 5: twice divorced, she is now with Brad Pitt.
2 years
JULIA ROBERTS & LYLE LOVETT
The Pretty Woman and the wire-haired country singer were an odd match to say the least, so it was no great surprise when they went their separate ways in 1993. Roberts then married Daniel Moder.
3 years
MADONNA & SEAN PENN
It was a tabloid editor's dream: the platinum pop star and the Hollywood rebel. But fire and fire don't mix - Madonna left Penn in 1989 in a blaze of screaming rows and media hype.
4 years
JENNIFER ANISTON & BRAD PITT
It was too good to be true. In 2000 Mr and Mrs Cleancut were married on a Malibu cliff in a haze of golden loveliness. They divorced in 2005, and Brad was soon with Angelina. There followed a brisk trade in Team Aniston or Team Jolie T-shirts.
5 years
MARIAH CAREY & TOMMY MOTTOLA
Mariah married her manager in 1993 in a ceremony modelled on that of Charles and Diana. There were other similarities: she was a virgin and they ended up divorced. Carey complained that the relationship was not "physical", and sexed up her image after divorcing him.
5 years
REESE WITHERSPOON & RYAN PHILIPPE
The tragic story of Reese and Ryan conformed to another truism about celebrity engagement: if you are the highest-paid actress in Hollywood and you have just won an Oscar, don't expect your husband to stick around. The pair announced their divorce last month.
7 years
JERRY HALL & MICK JAGGER
Fidelity was never Jagger's strong point. Jerry and Mick met in the early 1980s while he was still married to Bianca and she was engaged to singer Bryan Ferry. But bearing Mick's child in 1984 soon put paid to that, and in 1990 the leggy duo were married. Mick continued to play around and Jerry called time in 1999 after yet another model, Luciana Gimenez, became pregnant by Jagger.
9 years
NICOLE KIDMAN & TOM CRUISE
Tom and Nicole were the ultimate Hollywood love story when romance blossomed on the set of Days of Thunder in 1990. The couple vowed never to spend more than two weeks apart, but the union buckled in 2001 due to "divergent careers" and gossip that their marriage was a sham. After years of hard graft and false starts, Nicole is now married to singer Keith Urban. We await the upcoming union of Tom to Katie Holmes.
10 years
WHITNEY HOUSTON & BOBBY BROWN
Sadly the immortal words "I will always love you" did not apply to this troubled union. Married in 1992, the couple endured Bobby's repeated drug-related arrests, spells in prison for refusing to pay child maintenance, numerous accusations of domestic violence and Osama's bin Laden developing a crush on Whitney. Houston was no stranger to Class As herself, despite her famous TV utterance, "Crack is wack." Houston filed for divorce in September.
14 years
HARRISON FORD & MELISSA MATHISON
Harrison Ford may have played the adventuring sexpot on screen, but for 18 years he only had eyes for Melissa. The couple met on the set of Apocalypse Now in 1976 and were married in 1983, setting a glowing example of marital bliss to the rest of Hollywood's floundering inhabitants. However, in 2001, the couple separated. There was speculation in some sections of the tabloid press that their separation was down to Ford's midlife crisis, after he was photographed on several boozy nights on the tiles knocking back tequila shots and limbo dancing with mystery blondes.
18 years
48 years
The 48-year marriage between Paul Newman and Joanne Woodward is considered something of a miracle in Hollywood, and makes other legendary unions such as Ozzy and Sharon (24 years) and Bill Nighy and Diana Quick (25 years) look like the work of beginners. Paul and Joanne met while working on a Broadway production of Picnic in 1953. Newman divorced his first wife, Jackie Witte, and married Woodward in a Las Vegas ceremony in 1958. They have three children together.
Newman is proud to share the secrets of their success. "I never ask my wife about my flaws," he says. "Instead I get her to ignore them and concentrate on my sense of humour." On the thorny subject of infidelity, he says: "Why fool about with hamburger when you have steak at home?"
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