I'm a psychologist who studies couples: The happiest couples never fight about this 1 thing (it's not sex or money)

When people ask me what causes the most damage in a relationship, they expect me to list out the usual suspects, like financial trouble or intimacy problems. And they're not wrong to guess that, since both money and sex come up constantly in research on conflict. But as a psychologist who's spent years studying couples, I've noticed something else: Couples who feel quietly resentful of each other are, in my experience, almost always running a mental ledger. They tally who cooked last night, who's more exhausted, who canceled plans last time, who "owes" whom. Whether they admit it or not, there's an invisible math humming beneath the surface of every other disagreement, and it wears down their relationships.The happiest couples I've seen, however, have stopped fighting about who's doing more. They're not keeping score in the first place.Two types of relationshipsPsychologists have names for the two different operating systems a relationship can run on:In an exchange relationship, partners informally track what they give and what they get, and they expect things to even out fairly soon. For example: I did the school pickup Monday, so you're doing it Tuesday. In a communal relationshi...