Amy Goyer, AARP multigenerational issues expert, said the most common multigenerational household is one with a grandparent as head of the household and his adult children having moved in with their children, an arrangement usually spurred by the needs of one or both to combine resources and save money. The second most popular is a grandparent moving in with an adult child's family, usually for care-giving reasons. She noted that 2.5 million grandparents say they are responsible for the basic needs of the grandchild living with them.