The argument that a person's birth order impacts not only their personality development but that their birth also impacts their sibblings. For example, an only child isn't and only child anymore when a baby brother or sister is born. And if another child is born after that, the "baby sibbling" is not longer the "baby" anymore. Imagine your own families and your birth order, including if you are an only child (like me!) How do you feel your birth order impacted your personality?