Illegitimacy. — After wrestling in a number of cases with the question of the permissibility of governmental classifications disadvantaging illegitimates and the standard for determining which classifications are sustainable, the Court arrived at a standard difficult to state and even more difficult to apply.1747 Although "illegitimacy is analogous in many respects to the personal characteristics that have been held to be suspect when used as the basis of statutory differentiations," the analogy is "not sufficient to require 'our most exacting scrutiny'." The scrutiny to which it is entitled is intermediate, "not a toothless [scrutiny]," but somewhere between that accorded race and that accorded ordinary economic classifications. Basically, the standard requires a determination of a legitimate legislative aim and a careful review of how well the classification serves, or "fits," the aim.1748 The common rationale of all the illegitimacy cases is not clear, is in many respects not wholly consistent,1749 but the theme that seems to be imposed on them by the more recent cases is that so long as the challenged statute does not so structure its conferral of rights, benefits, or detriments that some illegitimates who would otherwise qualify in terms of the statute's legitimate purposes are disabled from participation, the imposition of greater burdens upon illegitimates or some classes of illegitimates than upon legitimates is permissible.1750The issues involve technical legal issues, such what to do with a parent's assets if he dies without a will. The cases do have a common thread -- the supremacist rejection of the idea that the law should use marriage as an incentive for the proper upbringing of children.
One surviving law on illegitimacy is that the foreign-born child of an American man has a harder time getting US citizenship if the dad is not married to the mom. See Nguyen v. INS, 533 U.S. 53 (2001).
The Wikipedia article on Legitimacy law reports:
Latin America has the highest rates of non-marital childbearing in the world (55–74% of all children in this region are born to un-married parents). In most countries in this region, children born outside of marriage are now the norm. Even during the early 1990s the phenomenon was very common: in 1993 the rate of children born out of wedlock was: in Mexico was 41.5%, in Chile - 43.6%, in Puerto Rico - 45.8%, in Costa Rica - 48.2%, in Argentina - 52.7%, in Belize - 58.1%, in El Salvador - 73%, in Panama - 80%.Sometimes Latin America is praised for its high Catholic Church membership and its presumed conservative values. But as you can see from the illegitimacy numbers, they do not have pro-family values.
A few decades ago, an incentive for marriage was to remove the social stigma of illegitimacy, and to insure that the kids will have the support of a legal dad and mom. Today, child custody and support laws ignore the marital status of the parents, and the social stigma has disappeared from many cultures.