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How same-sex marriage’s spread across states mirrors past social reforms

When social change bubbles up from the states — as same-sex marriage has — when does it go national? In an interesting article for the Pacific Standard last year, writer Joel Warner and computer scientist Aaron Clauset tackled that question. Their dive into the relevant political science found that the pace of a policy change's spread across various states often occurs in an S-curve — slowly at first, then very rapidly in the middle, and then more slowly again at the end.

The authors plotted the spread of same-sex marriage across states along an S-curve, and here's how it looked as of last November:

SSM S Curve

(Joel Warner, Aaron Clauset, and Abigail Z. Jacobs. Pacific Standard, November 2014.)

Since this chart was made, though, federal court rulings have made the number of states with same-sex marriage rise far higher — to 37, currently. That speed is rapidly outpacing the S-curve. (Of course, if the Supreme Court rules against same-sex marriage, those federal court rulings will be reversed.)

Warner and Clauset also pulled some key historical examples of how many states enacted major policy changes before they were instituted nationally (either by constitutional amendment, Supreme Court ruling, or federal law). They found:

  • 40 percent: Prohibition
  • 40 percent: abortions permitted
  • 54 percent: prescription drug benefits for seniors
  • 58 percent: a minimum wage
  • 68 percent: interracial marriage
  • 72 percent: no ban on sodomy
  • 81 percent: women's suffrage

While Warner and Clauset argue that on average there is some "tipping point" of around 60 percent, there's quite a bit of variation there, so we shouldn't overgeneralize. Overall, though, there's quite a bit of precedent for the Supreme Court stepping in when a policy becomes as widely adopted as same-sex marriage is now. Head over to the Pacific Standard to check out the full article.