No, it isn't compromise, unconditional love, or an even split of the household chores (although those probably wouldn't hurt). It turns out, the key to a long and happy marriage is sex, according to a recent study in "The Journal of Gerontology: Series B."

MORE: 3 Must-Read Sex Secrets from Women Over 70

Sex is secret to long, happy marriage. Tim Hale Photography, Corbis // Sex is secret to long, happy marriage. Tim Hale Photography, Corbis (Sex is secret to long, happy marriage. Tim Hale Photography, Corbis)

Researchers interviewed 732 married couples between the ages of 57 and 85 and found that those who reported having little to no sex reported lower levels of marriage satisfaction. The more often couples reported getting busy, the happier and more positive they said they felt about their marriages. Interestingly, this association held when their hookups involved any sexual act (not just intercourse).

MORE: 10 Reasons You Should Have More Orgasms

Obviously, these findings can go one of two ways: Either couples are happier because they're having more sex, or they're having more sex because they're happier with the relationship. Regardless, it seems pretty clear that it's crucial to make sex a priority in any relationship. Plus, the more time you dedicate to your sex life, the more likely you are to keep things hot in the long run: One recent study found that women who ranked sex as important in their lives were more likely to stay sexually active over time. So get busy--for your relationship's sake.

MORE: 13 Signs You Need to Spice Up Your Sex Life