Walk into McDonald's and the self-service kiosks look like a simple upgrade. Faster ordering, fewer queues, more convenience. But from a behavioural psychology and business perspective, they serve a much more strategic purpose. They are designed to influence how customers make decisions and ultimately increase how much they spend.

The average spend on fast food in the UK is £110 a month and even higher in the US. Here is how fast-food giants get you to spend more.