…it is among larger families that the household or kin effects on the success of children are the weakest. Not only do children from larger families do worse on average than those from smaller families, but they also experience more randomness. In other words, parents’ class status affects their offspring more weakly in large families than in small ones. Likewise, siblings resemble one another less in larger families. For example, sibling similarity in income levels is approximately 24 percent stronger in families with three siblings or fewer than in families with four or more children. For net worth levels…the degree of sibling similarity is 59 percent greater in smaller families.
Those are significant effects, what will they mean?
What all this means for the class structure of American society is that as families get smaller, there may be less socioeconomic mobility, not more. Parents will be able to more carefully control the environments of their offspring and thereby reproduce their own class status.
There is a caveat:
Of course, this depends not just on the overall average number of kids that families are having, but also on the distribution. If the most dramatic declines in family size occur among the economically disadvantages segments of American society then we could have more equality of opportunity.
In other words, having larger families, across the board, will mean that subsequent income is less correlated with family history. But on average it will not raise the prospects of people from poor families.