Some time in the 60s and 70s something let the illegitimacy genie out of the bottle in the US. In the early 1960s we went from out of wedlock birth rates consistently under 5% to a dramatic upslope leading to our present rate in excess of 40%. You can see this in Figure 1 from NCHS Data Brief No. 18 May 2009, Changing Patterns of Nonmarital Childbearing in the United States:
It is difficult to separate legal verses social changes because each tends to greatly impact the other. However, Betsey Stevenson and Justin Wolfers offer some insight into the legal part of this dynamic in their paper Marriage and Divorce: Changes and their Driving Forces:
Supreme Court rulings in the 1960s and 1970s also changed the nature of family relationships by eliminating many of the legal distinctions stemming from the marital status of a child’s parents. In 1968, the Supreme Court ruling in Levy v. Louisiana (391 U.S. 68) granted equal protection under the Fourteenth Amendment to “illegitimate” children. Five years later, the 1973 ruling in Gomez v. Perez (409 U.S. 535) overturned state laws exempting men from financial responsibility for “illegitimate” children. These rulings reduced both the social and economic cost to women of bearing a child out-of-wedlock, and thus may also help to explain the decline in shotgun marriages. This remains an under-researched topic in need of further scrutiny.
As they say, this is “under-researched”. More accurately this bit of history is outright denied. Try finding any reference to a legal distinction between legitimate and illegitimate children here, for just one example.
Similarly, neither Levy v. Louisiana nor Gomez v. Perez are referenced in the fairly lengthy history of child support included in the paper by Daniel L. Hatcher in Wake Forest Law Review, Child Support Harming Children: Subordinating the Best Interests of Children to the Fiscal Interests of the State (2007). This omission leaves the impression that the legal basis for compelling child support for illegitimate children was a settled issue in the US going back to at least the 1800s:
Along with the poor laws and criminal nonsupport laws, states enacted bastardy statutes aimed at forcing putative fathers to support their illegitimate children. The statutes were both criminal and civil in their focus and, like the nonsupport laws, their purposes were mixed. For example, a Maryland court explained that “[w]hile the prime object of the Maryland Bastardy Act is to protect the public from the burden of maintaining illegitimate children, it is so distinctly in the interest of the mother that she becomes the beneficiary of it.”
Yet clearly prior to the 1960s there were competing legal views on the degree to which mothers of illegitimate children had a “right” to compel the father to pay support.
Another massive change is the social acceptance of mothers who bear children out of wedlock. Even the name of the bastardy statutes indicates a profoundly different social view of illegitimacy than our current embrace. This vastly different social order comes through in one of the court cases Hatcher quotes in the same paper, the 1832 decision of the Court of Appeals of Kentucky in Burgen v. Straughan (emphasis mine):
[N]or can we perceive how it can be unlawful or immoral, or inconsistent with the policy of the law, for the mother of a bastard to agree with the father that, if he will co-operate in the maintainance [sic] of their child, she will not proceed under the bastardy act . . . . It should not be deemed injurious to the community or county. It is not the public duty of the mother of an illegitimate child to assert her statutory right. Her voluntary forbearance is no breach of any moral or civil obligation. Her child may become a burthen to her county; but this might happen, and would, perhaps, be more likely to occur, if such contracts as that we are now considering should be declared illegal and void. Many, in her condition, might prefer all the wretchedness of destitution and poverty, to a voluntary promulgation, in a county court, of all the circumstances necessary to coerce contributions under the bastardy act.
See also: From Hawthorne to Povich