Hey, at least we can blame someone. If you believe this, the gender of your kid's sibling(s) plays a key part in determining their dispositions.  Which confirms my belief that I should've stopped at one. 

I'm kidding. Jeez...

According to a study of 451 American families, with kids ages 9-18, and their parents, children who have older brothers become more aggressive over time, on average, than those who have older sisters. Older siblings with younger sisters become less aggressive over time.  Children with older sisters who are very aggressive become more aggressive and older siblings with younger brothers showed fairly stable levels of aggression over time.  The study also found that older siblings who were aggressive tended to have younger siblings who were also aggressive, and vice versa. This is true for sibling pairs with two boys, two girls, and one boy and one girl. Aggression in younger siblings also predicted increases in aggression in older siblings over time, and vice versa, though the extent varied according to each sibling's gender.  Researchers took into consideration the age difference between the siblings as well as such factors as parenting styles.

Basically, aggression breeds more aggression.  Which is rad.  Oh, and also?  Family "economic pressure," i.e. money stress, also increases overall family aggression - indirectly - through parental hostility.  Sooooo, that's cool.

Do you find that your family reflects the finding of this study?  I certainly don't.  The level of aggression between my two girls is at Code Orange most days... but then, I don't have any boys, so it's hard to compare.  Which is a good thing, sounds like.