Last week, when Barack Obama declared his support for gay marriage, the president was lionized by many, who thought it was an act of courageous leadership, and mocked by others, who saw it as nothing more than a politician scrambling to keep his feet dry after the tide had turned.

Both takes were true, though not in equal proportions. Obama led, to a modest extent. But mostly he followed.

That's not a criticism. In fact, what Obama did - getting out in front a little, but mostly following - is what leadership usually amounts to.

Of course we tend to think of leadership in grander terms. Leaders have unique visions. They inspire others to share their visions. Collective action is taken. The vision is made real. In short, leaders decide what change will happen and make it so.

This is the Great Man theory of history. In formal terms, it was popular in the 19th century but soon fell out of favour among historians who looked seriously at how and why change happens. Less formally, it lived on. And it thrives today.

Prime ministers and presidents have little ability to control free-market economies, especially over the short term, and yet they are routinely credited or blamed for the performance of the economy. That's Great Man thinking. Nor do leaders have much control over the intellectual climate, or social mores, or cultural trends, or developments in science and technology. But when change happens we routinely personalize these impersonal forces and imagine it happened because a Great Man - a Ronald Reagan, maybe, or a Steve Jobs - grabbed History by its shirt collar and dragged it along, kicking and screaming.

And that is simply wrong.

Consider the British election of 1945, when Winston Churchill and the Conservatives warned that a victory for Clement Attlee's Labour would mean the implementation of a socialist welfare state and Britain's ruin. Attlee won. And he implemented the ambitious program he had promised. It's easy to see that as a moment when Great Men decided the fate of a nation's future: Surely, if Churchill had won, history would have taken a very different course.

But in the 1930s, Conservatives had taken tentative steps in the same direction. And the blueprint for Labour's post-war welfare state was the Beveridge Report, published in 1942 by a prominent Liberal. The reality is that the post-1945 changes were the culmination of long-developing political, economic, and cultural trends that transcended parties and had little or nothing to do with the individual politicians. "If the Tories had been returned to office in 1945," observed historian David Kynaston, "they almost certainly would have created a welfare state not unrecognizably different from the one that Labour actually did create."

Or consider the 1968 decriminalization of homosexual sex in Canada by then-justice minister Pierre Trudeau. Trudeau's leadership has been hailed ever since, but he was no more acting in advance of the times than Barack Obama did last week. Poland decriminalized homosexual sex in 1932, Denmark in 1933, Sweden in 1944. Countries all over the Western world followed. The first American state to decriminalize was Illinois, in 1961. England started seriously considering decriminalization in 1957 and actually did it a year before Trudeau introduced his bill. (Trudeau himself was modest about the change. In the same statement in which he famously declared "there's no place for the state in the bedrooms of the nation," Trudeau said the bill "is bringing the laws of the land up to contemporary society.") It's not simply a matter of the times making the man, to use the old saw. It's that the times convince the man to do what he might never have imagined doing. The times may even grab the man by his shirt collar and drag him along, kicking and screaming.

Look at Richard Nixon's domestic policy legacy. One liberal landmark after another. This, from a lifelong conservative who worried that the Beatles were spreading socialism. Why? The times insisted.

Or consider Winston Churchill again. "I have not become the King's first minister in order to preside over the liquidation of the British Empire," declared the proud old imperialist in 1942, not many years before he presided over the liquidation of the British Empire. Not even the last lion could defy the tectonic forces at work.

Or think back to 1999, when the House of Commons passed a motion declaring that marriage "is and should remain the union of one man and one woman to the exclusion of all others." Jean Chrétien voted in favour. So did Paul Martin. Liberals today sometimes claim their party led on same-sex marriage but in reality the courts led, and they followed, until it became clear that the tide had turned - and they scrambled to keep their feet dry.

But as I said about Barack Obama's support for same-sex marriage, that alone does not amount to criticism. Political leadership routinely involves sensing when change is coming, and then, when the time is judged to be right, getting out in front of it, assisting it, and guiding it, to a happy conclusion. Politicians are like midwives: They are useful, sometimes even essential, and they can justifiably take pride when the job is done well. But they cannot take credit for the existence of the baby.