As others have said, they did a randomized trial. "Whereas couples assigned to keep their money in separate accounts or to a no-intervention condition exhibite

As others have said, they did a randomized trial. "Whereas couples assigned to keep their money in separate accounts or to a no-intervention condition exhibited the normative decline in relationship quality across the first 2 years of marriage, couples assigned to merge money in a joint account sustained strong relationship quality throughout." Sadly, I don't have access to the paper. May try and find a link to it later. I'm curious to know if they split the "no-intervention" cohort to look at the idea you are mentioning. That is, mayhap the cohort that got to do what they want was dominated by the separate accounts group? More strangely, it would be really weird if the no-intervention group saw an inverse result. That is, if given the choice and you keep joint accounts, you are unhappier than if you had kept separate.