Decreased Family Size & Social Class Concentration

From a purely statistical point of view, smaller family sizes decrease the likelihood of having close relatives who fall into more economically disadvantaged status. For instance, in a middle-class family of 6 siblings, it is not hard to imagine that 1 or 2 might, for various reasons, wind up experiencing substantially lower standards of living. This might tend to create empathy in more well-off layers of society, since they have close experience of those in a less-advantaged economic class.

In a family of 2 siblings, the statistics are greatly reduced. Then on top of that, there is the generational wealth factor. A comfortable family will be able to deploy more resources to help prevent or assist a child who might be in danger of slipping economically.