Sibling fights

It's pretty obvious that siblings fight a whole hell of a lot more than they would with their friends, but here is some data. Po Bronson and Ashley Merryman have a new book out called NurtureShock: New Thinking About Children. They discovered that young siblings on average come to conflict 3.5 times per hour, which adds up to 10 minutes of every hour just arguing. They make 700% more negative and controlling statements to each other than to their best friends.

Why? Basically, familiarity breeds contempt. Siblings know they'll be with each other for a very long time; that's not necessarily true for their friends. So there's no incentive to act as nicely to your sibling as you would to your friend. You're stuck with your sibling.