Dodi's amorous track record is already starting to look a bit iffy. Last Friday, alleged ex-fiancee Kelly Fisher, a Californian model, tearfully announced that she and Dodi were meant to be tying the knot on the very day he was photographed kissing Di. And his previous marriage to ex-model Suzanne Gregard lasted all of eight months. So, are Dodi and Di likely to sail off into the sunset? How can one tell if the path of true love will run smoothly, whether that new partner is the John to one's Yoko, the Joanne Woodward to one's Paul Newman - or the Larry Fortenski to one's Liz Taylor?
Dr Catherine Surra of the University of Texas is a veteran researcher into the process of commitment. Her latest study was published earlier this year. "I was very interested in how people arrived at the commitment necessary to wed," she explains. "I wanted to know how people get close to marriage - how they decide to go ahead or not." She monitored 54 couples and asked them to record their attitudes, interests and relationship history; then she asked them to provide monthly updates on how likely they were to marry - and why they were more (or less) sure that their partner was "the one".
Her research identified two types of relationship. The first, which Surra calls "relationship-driven commitment", progresses in an orderly fashion, with each partner reporting a regular increase in commitment. The second type, "event- driven commitment", is a roller-coaster ride, with commitment soaring one month and plummeting the next.
In event-driven relationships, explains Dr Surra, the couples react strongly to what has happened recently; attending a friend's wedding, for example, or having a serious argument. Di and Dodi must be one of the most event- driven couples in the world, by events on a scale that most other pairs can barely dream of: front page stories in the world's press, for example, or knowing that the Queen is taking an interest in one's progress.
This does not, however, make for a cosy life. "With event-driven couples, there is often a pattern of big fights, splitting up, talking it over and being back together two weeks later," says Dr Surra. Her research paper notes: "Women in event-driven commitments reported significantly higher amounts of conflict and negativity than those in relationship- driven commitments."
Having a turbulent, event-driven relationship does not necessarily mean that a couple is any less in love than a more stolid pair, but "I would predict lower marital satisfaction in this kind of relationship", warns Dr Surra. "I would think, in the long run, event-driven couples are more likely to run into trouble." Oh dear.
But even those pairs who thrive on controversy and fuss (and Di and Dodi must be pretty much used to ups and downs by now) will find other pitfalls on the way. In the first throes of romance, one must beware of being blinded by the fleeting attraction caused by, say, an enormous bank balance or a yacht in the Med. "There is evidence that it takes a good two or three years to get a reasonable idea of what another person is like and how you fit together," says Dr Maryon Tysoe, author of The Good Relationship Guide (Piatkus). "You can certainly fall in love fast, but it takes longer to be sure. One study showed that a courtship of two years makes a couple more likely to stay together. And an American study showed, even 21 months into a relationship, that around 20 per cent of couples will still be hiding things that they believe spoil their image." Ahem, quite. Like an ex-fiancee who is now suing for breach of contract, for example.
A couple that shares the same basic view of life are more likely to stay together, says Dr Tysoe. "Opposites attract, but they tend not to stay the course. That doesn't mean you need to find a clone of yourself, but you want someone who will reinforce your view of the world." The English rose and the Egyptian playboy do share a famous interest in shopping - a beacon of hope for their future happiness.
Courting under the glare of publicity must make the process extra nerve- racking, but seeking out "the one" is hardly the exclusive preserve of the rich and famous. For some it's easy. "I knew straightaway when I met my wife, and I was only about 20," says Richard Carey, 32. "I always carried a torch for her. We didn't get married till eight years after we met - she took some persuading, but now we're very happy. Even now we've been married for four years, I still sometimes get that fluttery feeling in my stomach over her."
It can all end in tears. "You can decide very quickly, and be quite sure in your own mind, then be horribly disillusioned," warns Jane Miller, 30, divorced after only 12 months of marriage. "It may be old-fashioned, but I'd say marry in haste, repent at leisure. Or even live together in haste, repent at leisure."
Perhaps it's simply not possible to be sure. "I don't think you can ever be certain that you've found 'the one' - that someone better isn't round the next corner," says one thirtysomething woman, married for five years. "You just have to ignore that possibility and work on the partner you've plumped for. You can't spend your whole life as if you were talking to someone at a party, but all the time looking over their shoulder, waiting for someone more interesting to come along."