There are probably many variables other than income here. Many of these kids may be from immigrant families with poor English skills, others may be from native born families that are only marginally literate, most of them probably suffer from non-existent parenting in mother-headed households, etc. In other words, they suffer from cultural deprivation, particularly in the area of language (which obviously affects one’s ability to reason). Poverty in itself is not the cause.
I’m not defending poverty, btw. Being poor certainly makes everything harder. But it’s not the cause of their wretched cognitive development.
However, wretched cognitive development may be the cause of most poverty.