The book "Unequal Childhoods" by Annete Lareau is about parenting differences in (upper) middle class, working class, and poor families. The author's argument is primarily about how middle class parents engage their children in structured, organized activities ("concerted cultivation") whereas working class and poor parents tend to let their children have free time ("the achievement of natural growth").

Although the book is mostly about "concerted cultivation," there is a discussion of sibling relationships. The author says that in her research the upper-middle class children fought more than the working class and poorer children did. She says that the upper middle class children would sometimes insult each other, say they hated each other, and fantasize about doing physical damage to each other, whereas the working class and poorer children tended to get along lovingly.

Do you think that this observation has any validity as an overall generalization?