These studies are rubbish. If it were true, for example, that moms would earn between $60,000 to $70,000 annually for "doing things such as cooking, cleaning and nursing wounds" then why do nannies earn half that (or less)? There may be a point for maid services, who probably can pull in much closer to that number, but their work is not comparable as they work at multiple homes/offices and are thus cleaning every day. Moms have but one house to manage, 2.5 offspring to manage.
"Personal density is directly proportional to temporal bandwidth."