Take this with a large grain of salt but Zulkey presents to us some witty analyses and generalizations of what personalities people might develop depending on the order in which they were born.
Of course, people's personalities are built from a variety of factors though it may be influenced by one's environment. Still, if you've been around for quite some time, you start to observe and realize certain similarities in such situations.
I'm an only child and I've heard a lot of stereotypes about it as well. I haven't heard the one given on the list so it's an interesting perspective.
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