An illegitimate child is one whose parents were not lawfully married to each other at the time of his birth. Many states have statutes which disadvantage illegitimate children. The discrimination may take the form of ineligibility to take by intestate succession, inability to sue for the parent's wrongful death, disentitlement to claim a presumption of dependency(relevant for receipt of certain government benefits, such as social security survivor's benefits), etc.
The court's course in reviewing state classifications based on illegitimacy - classifications disadvantaging nonmarital children - has been a wavering one. What is clear is that, although the court has never labeled illegitimacy a "suspect" classification, it has in fact exercised a degree of heightened scrutiny in most of the cases and has struck down illegitimacy classifications with some frequency. Only in the late 1980s, in Clark v. Jeter, did the court explicitly endorse an intermediate scrutiny standard.
The Supreme Court consistently has invalidated laws that deny a benefit to all nonmarital children that is accorded to all marital children. In the cases of "Levy", "Glona", "Cahill", and "Trimble", the laws in question allowed all marital children to receive a benefit that was denied to all nonmarital children. In each instance, the Supreme Court found that the discrimination violated equal protection.
No similar bright-line rule exists when the law provides a benefit to some nonmarital children that it denies to other nonmarital children. In other words, rather than discriminating between marital and nonmarital children, these laws distinguish among nonmarital children. Such statutes are subjected to intermediate scrutiny and evaluated on a case-by-case basis with the courts determining whether there is an important interest served and whether the law is substantially related to that goal.
In response to Supreme Court decisions invalidating laws denying benefits to all nonmarital children, some states adopted relatively short statutes of limitations for establishing paternity. The Supreme Court has been consistently hostile to these limitation periods. So long as the father and child remain alive, paternity can be established at any point in time.