- I can only assume that if a kid doesn't have an older sibling "to keep an eye on them" the parents take this role, and parents tend to be more risk-adverse and responsible caretakers than siblings.
- I've never lived in the American suburbs, but they don't seem very dense. I know that, growing up in Mexico, the kid density was great. Some of it probably has to do with the fact everyone had siblings. Matching ages with nearby neighbors is more likely when there's more kids too. We were outside all the time, older siblings introduced younger ones to the crowd. Friendships were made, broken and fixed. Adventures were had. More than once we got into situations that I knew my mom wouldn't be happy about too.