In 2001, researchers at Purdue University found that 75 percent of 566 surveyed mothers between the ages of 65 and 75 named a particular child they would want to take care of them as they aged. Seven years later, the researchers checked back in. About half of the mothers, 234 of them, had fallen sick or become injured: Some were being tended to by their golden child, others by a different child, and others not at all. The Purdue team... saw increased anxiety and sadness in mothers who didn’t get the caretaker they wanted. These moms were more unhappy than the ones receiving no help at all. (On the other hand, mothers who secured the ministrations of their favorite kids broke even, psychologically: Apparently, having the “preferred” child care for you doesn’t make you happy; it just keeps depression at bay.)
A pretty good argument for having more than one kid, or just one kid, or no kids, depending on how you read it.