Growing up, I had to share a bedroom with my sibling all throughout my childhood up until the point where I left for college. My first roommate ever was an only child, and sharing space with a peer was much more traumatic for them than it was for me. Our third roommate had a habit of ‘forgetting’ which snacks belonged to them, and would often help themselves to whatever was lying around. While I was sometimes mildly annoyed by this, the behavior was nothing new to me. My only child roommate, on the other hand, was absolutely appalled and reacted to these sorts of actions as if they were crimes against humanity. 

There are many subtle differences between people who grew up with and without siblings, and this fascinating Reddit thread discusses some of the funniest and most interesting dead giveaways that someone is an only child.