A New York Times article reports that crucial factors in the formula include "the spouses' combined age (younger couples divorce sooner)" and "the length of the courtship (quicker to wed, quicker to split)." So William and Kate's lengthy courtship bodes well for their future together, as does the fact that Kate tends to keep her clothes on in public. (And no, I haven't forgotten that she first caught Prince William's attention by walking down a catwalk in that see-through dress.)
This New York Times page shows the equation and a list of past and present celebrity couples with their chances of staying married. There aren't any other royal couples on the list, unfortunately. I wonder what the formula would predict for Prince Albert and Princess Charlene of Monaco.
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