Fights were more common for couples in three- to seven-year-long relationships than for those 21 years or more. That makes sense, of course: one would hope that if a couple kept arguing about the same topic for years and years, they'd eventually give up the fight -- or the relationship. This particular study doesn't say anything about the reported happiness levels of the couples, though, which I would like to see before drawing any conclusions. I've always wanted to know if my high school health class teacher was right about how it's not whether a couple fights or not, it's how a couple fights.
Couples in longer relationships argue less.