first born will get parents full attention when new parents were nervous about parenting. they pass their values and expectations to the first born; first born try to conform to the old generation's values and traditions whereas later born/2nd born will be less like that, but perhaps more creative, innovative.
with the later born/2nd born, parents are more experienced and become relaxed, they tend to enjoy kids more than making sure they behave well. 2nd born worry less about parent's rules, so they are more fun to be with, at least to their peers (hence more sociable)
