I'm an only child too. The reason is nowhere near as complicated as you think it is.
It's a mathematical probability that you would have siblings. Here's why:
I'm not going to use real statistics for this because I don't believe anyone has ever done a world wide study. Most studies are done regionally, so what the stats are where you are may differ from any real study I could find anyway. Also, the numbers change over time, plus many who are only children one year, are not the next. I am though, basing the numbers I'm choosing on a few real studies I looked at in both the UK and the US.
Let's say that this year where you are, forty percent of families have only one child. That's a pretty realistic number and actually a little higher than the numbers I looked at. Well, forty percent seems like a pretty high number. If forty percent of families have one child it shouldn't be that unusual for people to encounter people with no siblings, right? It would seem so, but you'd be wrong. Here's why:
(I'm NOT counting families with no children because in the end what matter is the probability of whether a child has siblings, and you're never going to encounter a child from a childless family.)
If forty percent of families have only one child, that means the other sixty percent have two or more. Let's assume for the sake of argument that that all of the sixty percent have only two. That would mean that out of every one hundred families in the area, there are 120 kids with siblings (60 X 2), and only forty without. This means even with the max number of children per family being two, the chances of meeting a child with siblings is three times higher than meeting one without siblings.
All you have to do now is mathematically factor in families with three, four, five or more kids, and what you'll find is that only children are far more unlikely to be met than children with siblings.
Statistically, we're quite unusual. It's pretty much that simple.
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I'm not likely to be around much longer. As before when I first signed up here years ago, I'm finding that after a long hiatus, and after only a few days back on here, I'm spending way too much time here again already. So I'm requesting my account be locked, banned or whatever. It's just time. Until then, well, I dunno...