That's pretty polarizing. I think I said something to the effect of not developing the same, not "failing to develop any empathy at all". I and many of my friends have kids. They don't come into contact with each other more than occasionally, but the ones who are only children are much less willing to create something out of nothing and, say, play with the one toy that was available. Social situations, like having very close friends or cousins help, but something like preschool isn't 24/7. From observing the little guys, the ones who pitch screaming fits when their mum leaves them and I'm dropping off my kids are not the ones with siblings. And they're losing resources they haven't learned to share yet. I wouldn't say it's impossible for only children to learn empathy, just that it happens later, differently, and maybe doesn't achieve the same end goal of having a human being who's always had to share things 24/7.