They try to get at that causation problem with the random assignments: they randomly assigned the couples into three groups: no intervention, must do joint fina

They try to get at that causation problem with the random assignments: they randomly assigned the couples into three groups: no intervention, must do joint finances, and continue their separate finances (all 230 couples in the study started with separate finances). After two years the joint finance groups were happier in their relationship and less likely to have broken up. So, if you had RTFM'd, you'd have seen that they are trying to get at causation by doing a randomized controlled trial, the gold-standard for proving causation.