also a first-born (well, to my dad . . . my half-brother is a lot older than me so we have two in the family). And I'm a real contrarian. Even when I agree with someone, if they tell me I "should" do something, I'm so tempted to do the opposite!
For instance I almost never eat red meat, but I crave steak after a conversation with a vegan.
I am aware that this tendency could make me really easy to manipulate - just tell me I "should" do the opposite of what you want me to do.