It occurs to me that an unintended collateral effect of having smaller families in the US is that fewer boys grow up with sisters, who could help to give them a female perspective; and fewer girls grow up with brothers, who could help to give them a male perspective.
Obviously, I am not advocating for any pressure to choose a family of any size (or to choose either to be or not to be childless).
But at my university, we know that an increasing fraction of our arriving first-year students have never shared a bedroom with anyone, and we think about the consequences of that. I am wondering whether not having a sibling of the opposite gender might affect people’s thinking.
In a single-child family, obviously a boy has no sisters and a girl has no brothers.
In a two-child family, if a boy is the first born, he has a 50% chance of having a sister (raw odds). 50% of two-child families have two boys or two girls.
In a three-child family, if a boy is the first born, he has a 75% chance of having at least one sister. 25% of three-child families have all boys or all girls.
In a four-child family, if a boy is first born, he has an 87.5% chance of having at least one sister. Only 12.5% of all four-child families are all boys or all girls.
The odds that a boy has a sister keep rising as the family size grows, and the odds of single-gender families keep dropping. Given that a family contains one boy, the odds that he has at least one sister are independent of his birth order.
I would not be surprised to learn that 2 generations back, most American families were larger.
The statistics on assault are hard to gather, because of the rate of non-reporting. But I wonder whether having a sister might make a young man more likely to treat women respectfully. (Obviously, I am excluding fraternities with “sister” sororities, who are “sisters” in name only.)