Heraclitus or Zimbardo? That's the question raised by pictures of John Kerry and Tony Blair cosying up to Bashar al-Assad.

What I mean is that if Heraclitus was right and character is destiny then Kerry and Blair are, at best, lousy judges of character for befriending a "thug and murderer" and at worst mere hypocrites.

But there's another view - Philip Zimbardo's, that destiny is character, that what we do is shaped by circumstance and social structures:

Good people can be induced, seduced, and initiated into behaving in evil ways...Most of us can undergo significant character transformations when we are caught up in the crucible of social forces...Any deed that any human being has ever committed, however horrible, is possible for any of us - under the right or wrong situational circumstances (The Lucifer Effect, p211*)

This is consistent with Assad's tutor's memory of a "quiet, polite" man, and with Roger Boyes' account in the Times (£):

The idea was that Bashar with Asma at his side would not only de-brutalize the regime but modernize it...It did not work out like that, partly because of the US and British invasion of Iraq in 2003, partly because of the huge security machine that crushed reform and partly because so many of Bashar Assad's own clan profited from the system.

If you were to become a dictator overnight, faced with powerful lobbyists demanding violent repression and opponents determined to kill you, would you really remain a decent person? I fear an affirmative answer owes more to self-love than self-awareness.

On this view, Kerry and Blair are guilty not of misjduging Assad's character, but of underestimating the extent to which dicatorship degenerates into brutality under the wrong conditions.

Though Zimbardo's claim seems radical, it's quite widely shared across the political spectrum. When Warren Buffett said "'When a management with a reputation for brilliance tackles a business with a reputation for poor fundamental economics, it is the reputation of the business that remains intact" he was recognizing the importance of structure over individual character. The Marxian claim that people are bearers of social relations and the neoclassical claim that people respond to incentives similarly say that circumstances shape what we are.

You could read Adam Smith as saying the same thing. The belief that the invisible hand causes ordinarily self-interested people to promote general well-being amounts to a claim that the right social structures will overcome the impulses of bad or indifferent character. Isn't the obverse also possible - that the wrong structures can cause ordinary men to behave abominably?

The best-known experimental evidence here is, of course, the Milgram and Stanford Prison experiments. But there are other clues. There's evidence that measurable personality is as variable (pdf) as income. And whilst researchers have found some correlation between personality (pdf) features and economic outcomes, their explanatory power is quite low.

This issue matters for a simple reason. It's tempting to think that governments will be well-run if only good people were in charge - or failing that "our bastard". That's the Heraclitus view. But if Zimbardo is right, what matters isn't so much people as structures. If these are wrong, they can horribly warp the moral impulses of those in charge. As the man said, absolute power corrupts absolutely.

* I fear Zimbardo is exaggerating for effect here. There are some saints who'd do the right thing regardless of circumstance. The correlation between situation and "character" might be high, but it's not unity.

** Note for the hard of thinking: none of this is to excuse Assad's actions.