The incident, which was filmed by another customer, blew up on social media, and Starbucks CEO Howard Schulz responded by admitting he believed his store manager "demonstrated her own level of unconscious bias".

The Starbucks boss put his money where his mouth is and closed 8,000 stores for one day so staff could attend anti-bias training. Other organisations are rolling out similar workshops. They’re keen to make sure all staff and customers are treated fairly. But do these programmes work? Can you train people to be less prejudiced?

Jamie Perry, assistant professor of human resources at Cornell University, thinks you can, but in order for it to be effective, the training must make you aware of both your explicit (or conscious) and implicit biases.

The theory is that a slow drip of stereotypes seeps into each and every one of us. Sometimes inherited by our family, sometimes based on who we do and don’t see in power, and sometimes picked up from friends or the media.

Effective anti-bias training “teaches you strategies to consciously push down these biases”, says Perry. “For example, you might hold in your head that women can only do administrative jobs. So counteracting that is priming yourself every day to think about women in managerial roles…that’s how you might replace a negative stereotype.”

And while Perry believes this training can reduce prejudice at work, she says a one-time workshop is less effective than a series of ones that take place over a larger period of time.

“It’s a journey, not a destination that you reach,” she says.

Under equality laws, firms that fail to tackle discrimination face costly lawsuits, and there’s evidence that happy, diverse workforces are more creative. So, there’s money to be made in anti-bias training from face-to-face workshops to online self-guided courses.

Alexandra Kalev, a sociologist at Tel Aviv University, used to think that diversity training was effective, that it felt like common sense – and then she investigated the impact of training.