On the whole, siblings pass on dangerous habits to one another in a depressingly predictable way. A girl with an older, pregnant teenage sister is four to six times as likely to become a teen mom herself, says Patricia East, a developmental psychologist at the University of California, San Diego.

The same pattern holds for substance abuse. According to a paper published in the Journal of Drug Issues, younger siblings whose older siblings drink are twice as likely to pick up the habit. When it comes to smoking, the risk increases fourfold.