Here's Bloomberg's (the publication, not the billionaire himself) take on "How to Marry a Millionaire?" the answer is "Be One." Your odds of marrying someone rich rise in proportion to your own income. Even if opposites attract (see opposites-do-attract-but-should-they-marry and some-like-it-quiet) that doesn't hold for income.
But we have to sound a bit more technical than that, so the term used is "assortative mating, " the same terms zoologists apply to "animals with strong preferences about what their mates look like" In the case of humans, social scientists use assortative mating" to describe the predilection for people to marry those who resemble ourselves in appearance, as well as experience. "People, in other words, tend to pair off with those who have a similar educational background and similar earning power. Doctors marry other doctors, lawyers marry other lawyers, Ivy league graduates marry other Ivy league graduates."
Essentially this study says in a rather long-winded way what most of us have known for a long time: money marries money. It's really very simple; people tend to date within their own soci-economic circles. There also happens to be an advantage to coming from similar backgrounds with similar expectations for lifestyle and spending, as so many marital conflicts center around clashing views on money. To find your own position on that, take the Saver or Spender quiz.