A question I never thought to ask, but Prof. Terry Turnipseed asks and answers it, in a Slate article about a recent case in which a man did adopt his girlfriend — apparently to shelter money from creditors, given the terms of a trust for his children that he had earlier set up — and also in a full-on law review article from 2009.
The article’s abstract reports that, “For some time now adults — both heterosexual and homosexual — have been adopting their lovers and spouses all over the country for various reasons: to better guarantee the adoptee’s right to inherit directly from the adoptor; to keep collateral relatives from having standing to contest the adoptor’s estate plan; or to add a loved one to a class of trust beneficiaries (allowing the adoptee to inherit “through” the adoptor).” And the article reports that about half the states “are subject to statutory or common laws that include the adopted parent/adult child relationship within the definition of incest.”