If you’ve ever wondered why your family isn’t crazy about your BF, science has an answer for you: natural selection.

According to a new study , your mom and sister want you to end up with a guy who’s understanding, responsible, helpful, and sensible—zzz—while you tend to prefer men who are fun, sexy, and have a sense of humor. Researchers dubbed this the “Juliet effect,” after Shakespeare’s Juliet and her mom, Lady Capulet, who wanted Juliet to marry Paris, a dude from a good family, over Romeo.

Moms tend to think that “responsible” and “sensible” guys won’t drain the family’s resources, and will help ensure that good, reliable genes get passed on to the grandkids. But, weirdly, sisters get in on the game, too. Why? Scientists found that they also want us to marry a sensible guy, because it can benefit the whole family—ahem, including her.

To reach this conclusion, scientists asked 279 women with sisters to rate how important 133 traits were for their own long-term partner, and for their sister’s future mate. For the most part, the traits were the same, but women tended to prefer that their sis opt for the more reliable dude, while they went for the fun, sexy guy.