When you think how often we tell our eldest that they are the 'grown-up' ones who must look after baby, set an example, be good and help mummy, etc etc, it is little wonder that they grow up to be (usually) responsible and reliable, and high achievers but perhaps less happy-go-lucky than their younger siblings.
We also stress over every little development or (lack of) with firstborns whereas with subsequent babies we are more experienced, less paranoid, less demanding, and frankly we just don't have the time or inclination to give that same level of (slightly obsessive!!) attention to them. They learn go with the flow, whereas firstborns set our routine for us, and rule us with a rod of iron, younger babies arrive in a routine already set. They learn that they get by in life by generally being smiley and happy and a team player, whereas our firstborns get praise and attention by achieving, and reaching milestones.
I have three boys. DS1 was 2 and a 1/2 when DS2 was born. I am ashamed to say now that I expected more from him in terms of mature behaviour then (because at barely three he became a 'big boy' ) than I expected of DS3 by the time he was 7, because he was my baby and I indulged him! So naturally, he's charming and delightful, knows how to play to his audience, but is a spoilt and demanding brat at times as well. Entirely my fault.
DS1 is so reliable, well behaved, decent, sensible, utterly dependable and I'm really proud of him. He never let's me down. But he is also cantankerous curmudgeonly, risk-averse, and a little lacking in confidence - takes life a bit too seriously sometimes.
DS2, 2/3 years younger than DS1 and a 4/5 years older than DS3 is an extremely popular, optimistic self starter, very sociable, everyone's friend, class clown. However, he is the one who gives me the biggest headaches. He goes out of the way to be the opposite to DS1 (not always in a good way!) He's very bright but refuses to be a 'geek' or a 'nerd'. My gut feeling is that he will be extremely successful in life, but he's turning my hair grey and drving me to the gin in the meantime!