Maryland’s legislature voted last week to become the 18th state of the United States to abolish capital punishment. The legislation was passed 82-56 on Friday in the House of Delegates, having passed the Senate the previous week 27-20. It now goes to the governor, a strong opponent of capital punishment, who will sign it into law.

The US is the only western democracy to retain capital punishment. Last year, 77 death sentences were handed down and 43 executions were carried out, across nine different states.

(Also, I’m sorry to keep picking on the BBC, but they really have some problems in the maths department: after telling us that Maryland is the 18th state to abolish capital punishment, the story says “nearly a third of the 50 states have now renounced executions.”)

It wasn’t supposed to be like this. The Supreme Court halted executions in 1972, and even when in 1976 it opened the door to their resumption, it was expected they would be a rare event (as they are, for example, in India and Japan). Instead, after a slow start, momentum built and by the turn of the century the rate was nearing a hundred a year. Over 1,300 people have been executed in the US since 1976, 493 of them in Texas alone.

But the tide now appears to be turning. Maryland is the sixth state in as many years to embrace abolition, and last year’s number of death sentences was the second-smallest since 1976. A referendum in California last November only narrowly voted against abolition, 52% to 48%. (California has not executed anyone since 2006, but it has upwards of 700 people on death row.)

The change in public opinion is clear. Gallup polls have shown support for the death penalty rising from a low of 42% in the mid-1960s to a high of 80% thirty years later, before falling off to the low 60s now.

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.

It’s a warning against any belief in the inevitability of progress (or of decline, for that matter). But it also raises the question of what is so special about the US. For two centuries it had generally been a world leader in measures to advance individual freedom; in the late 20th century it was overtaken on a number of fronts by western Europe. Why? Why was there no such discernible backlash in, for example, Australia?

Of course, each issue has its own special factors at work. The recent trend against capital punishment, for example, has been fuelled by technical developments, such as the use of DNA evidence, that have exonerated many innocent people – and therefore pushed to the forefront of public opinion the fact that the innocent are often convicted and may be executed. But there also seem to be broader forces at work.

One issue that did not really follow the trend was gay rights. Progress there since the 1970s has been slow, but relatively steady: there was no period during which reforms were wound back. Was there something the gays did differently that other reformers could learn from?