How long does it take to change someone’s mind?

A lot longer than it takes to read a newspaper article, for a start. A lifetime isn’t enough, in some cases. And that’s why there has been some scepticism about Starbucks’ decision to close thousands of stores to give staff so-called “unconscious bias” training, or sessions in recognising and overcoming the ingrained prejudices most of us don’t even know we have.

The move follows an infamous incident in one of its Philadelphia stores where staff called police after noticing two black men hanging out without buying anything; both were taken away in handcuffs even as fellow customers protested that they hadn’t done anything wrong. When it turned out the two men had just been innocently waiting for a friend, Starbucks found itself branded racist overnight. Would a store manager really call the cops on two white businessmen waiting for a colleague, or a bunch of giggly teenage girls sharing one gingerbread latte? So why were black men deemed uniquely threatening? The whole thing was particularly excruciating in a country where older people of colour still vividly remember segregation in restaurants, or being chased out of drugstores as children by white owners, but the backlash spread well beyond the US. And so Starbucks joined an increasing number of household names (including Guardian News and Media) and public sector organisations known to use unconscious bias training.

It’s not hard to see why it’s a growth industry; from the BBC’s gender pay scandal to the current furore over whether Oxbridge admits enough black students, the hidden assumptions most of us can’t even admit we make are increasingly likely to land organisations in trouble. What is less clear is whether a morning of sitting through diversity videos can really achieve anything beyond some free publicity.

Police officers arresting Rashon Nelson (pictured) and Donte Robinson

At best, the jury is out on whether unconscious bias training works. A recent report from the Equality and Human Rights Commission found “mixed results” for sessions aimed at reducing bias and “limited” evidence that they change behaviour. One-off sessions conducted as a management exercise in box-ticking almost certainly don’t work; even if they briefly prick consciences, people soon forget and slide back into bad habits. Badly delivered training may even reinforce stereotypes by making people feel that bias is universal and therefore pretty much inevitable, or provoke a resentful backlash from staff who take umbrage at being deemed racist or sexist.

But if deep-seated attitudes don’t change overnight, employers are in an unusually good position to change behaviour. If nothing else, by instigating training Starbucks is signalling to its staff that racism is taken seriously and might cost you your job. But arguably the crucial thing it’s doing is to change the rules, giving staff fewer opportunities to express any prejudice they might have. It’s now company policy that customers can sit in coffee stores or use their loos (one of the flashpoints in the Philadelphia case) without shelling out for a latte, so staff no longer have to make potentially dodgy judgments about who looks as if they “shouldn’t” be there.

If humans can be persuaded to do the right thing for long enough then it eventually becomes second nature

There’s a fine line between weeding out opportunities for bias, and making your workforce feel like robots by removing all freedom to exercise their discretion. But so-called “bias mitigation strategies” – practical techniques to nudge people into more objective decisions, such as introducing more structured recruitment interviews that test what applicants can actually do rather than how much the interviewer instinctively warms to them – do at least recognise how social change actually happens. It doesn’t necessarily start with attitudes, but with behaviour. Humans are creatures of habit, and if they can be persuaded to do the right thing for long enough then it eventually becomes second nature; the feeling follows the behaviour, not the other way round. Change what people do, and not only do fewer people get unnecessarily dragged out of coffee shops in handcuffs, but eventually, the very idea starts to seem bizarre.

So no, you can’t change hearts and minds in an afternoon. But you can change the rules, and sometimes that’s what really matters.

• Gaby Hinsliff is a Guardian columnist