And now Illinois …

This is starting to get serious. Illinois this week became the 16th state of the United States to authorise same-sex marriage, when governor Pat Quinn signed legislation to that effect (it comes into operation on 1 June next year). Just a week earlier, Hawaii had become the 15th.

That means that about three in every eight Americans now live in states with marriage equality. But that includes three of the five most populous states, and the nation’s three biggest urban areas (New York, Los Angeles and Chicago). This is no longer something that can be dismissed as a fringe movement; it has become thoroughly mainstream.

Further progress may not be quite so rapid as the last couple of years, since the majority of the remaining states have constitutional bans on same-sex marriage – enacted at a time when public opinion was rather different. But the way things are going it’s only a matter of time before some of those are repealed. Wisconsin, Colorado and Oregon are obvious prospects; even more so Pennsylvania, whose ban is only by statute and which is now the only north-eastern state without same-sex marriage.

Earlier this year, Nate Silver modelled the movement of public opinion on the issue and projected that by 2016, there would be majority support for same-sex marriage in 31 of the 50 states.

That would push over a lot more dominoes, but it would also increase the likelihood that some sort of nationwide solution would be found. (Otherwise some deep southern states might hold out for a long time.) The most likely outcome would be a Supreme Court ruling that section 1 of article IV of the US Constitution – the “full faith and credit” clause – requires states to recognise marriages validly performed in another state, same-sex or not.

That’s how more conservative states were during the twentieth century obliged to accept divorce, and how Nevada’s liberal divorce laws in practice became available nationwide.

There’s been some recent publicity for the shift in American opinion on marijuana legalisation, which on some polling now enjoys majority support. As a graph at New York Magazine showed last month, both issues have been on a similar trajectory for about the last twenty years, with the drug issue playing catch up.

But looking at a longer time frame, same-sex marriage seems much more distinctive. Its growth in acceptance has been remarkably steady since it first appeared on the radar; indeed, the same has been true of gay rights in general. Other social issues show less consistency.

As I said back in March, when discussing capital punishment:

This trend mirrors a range of other social issues where reform seemed to be on the brink of full acceptance in the 1960s and ’70s but then struck unexpected resistance. Legalisation of marijuana is another obvious example: the drive halted and then went into reverse, recovering only in the last decade or so. The defeat of the Equal Rights Amendment in the 1970s appears to show the same dynamic.

After that long detour, drugs, gay rights and abolition of capital punishment now seem to be back on more or less the same path. But the detour is a valuable reminder that even social change that seems irreversible might not be. Things can go backwards as well as forwards; the fact that hasn’t happened to date with gay rights doesn’t necessarily mean that it can’t.

There’s also the question of why marijuana legalisation seems to have had less legislative success in the last few years. I suspect there are at least two reasons. One is that although the contours of ideological support and opposition on the two issues are similar, there are powerful vested interests opposed to legal marijuana that have no analogue in the case of same-sex marriage.

Lots of people are passionately opposed to gay people marrying, but hardly anyone has a strong economic interest in that position – unlike, say, those in the alcohol industry who might want to limit competition.

The other reason is that same-sex marriage is a fundamentally simple, yes-no issue. Everyone knows what it means to issue marriage licences to same-sex couples, and there’s really only one way to go about it. But there are many different models of drug legalisation, so it’s easy to muddy the waters and difficult for proponents of reform to agree on a single strategy.