Free will, food and obesity

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Studies show watching junk food ads on television ‘primes’ children to eat more.
Credit: Philippe TURPIN / Getty Images

Is free will an illusion? A number of neuroscientists think so. And many of their doubts converge on a 1983 experiment by American physiologist Benjamin Libet.

At first glance, Libet’s experiment looks childishly simple. He asked people to just flick their wrist whenever they felt like it, then note the time they first felt the urge to do it on a clock with a fast-sweeping hand. Scalp electrodes recorded their brain activity.

As expected, people noticed the urge to move their wrist slightly before they did move it. But something odd was showing up. The electrodes were picking up brain activity even before people felt the conscious urge to move – nearly half a second before, which is no snippet in brain time.

Libet used this brain signal – the “readiness potential” – to suggest people were triggering their wrist movements unconsciously. The conscious urge was just an “epiphenomenon”, an after-the-fact rationalisation that sprung up automatically to explain something we’re loath to accept; we might not be the authors of our own actions.

Libet’s findings are still debated but they were just common sense to many neuroscientists, among them Patrick Haggard at University College London.

To get Haggard’s gist, set out in a 2008 article in Nature Reviews Neuroscience, think about how your arm goes up automatically if someone taps your biceps with a tendon hammer. Then consider you can also move your arm up as “an act of will”. Many actions are somewhere in between on the spectrum from reflex to voluntary; an example is when you change gears driving even though you’re in deep conversation with a passenger.

Haggard explains that decision-making isn’t necessarily linear, from agent to intention to action. It may be more like a loop from the environment to an unconsciously processing brain and then to action. This fits with the idea, celebrated by Nobel laureate Daniel Kahneman in his book Thinking Fast and Slow, that we have a “fast” unconscious system that continually extracts data about the world. On this view the “will” is like an autopilot adjusting course as new information comes quietly to it.

In 2009, US psychologist Jennifer Harris showed how food choices might be silently relegated to the unconscious. Her team split school children into two groups to watch a TV program with ads. One group saw ads for games while another group saw ads for waffles, fruit roll ups and potato chips that depicted people having a good time.

Students had a bowl of cheese crackers within reach and were told they could snack. Kids who watched the food ads ate a tummy-turning 45% more crackers than their fellows. This is an example of “priming”; the ads triggered the idea and the reward of food which primed – made more psychologically accessible – the related behaviour “eating”. A similar finding is that adults eat more food when it is visible on a nearby desk rather than placed higher up on a distant shelf.

So food cues are subtle prompts that start you eating. But could they keep you eating too?

In a classic study published in Psychological Science nearly two decades ago, psychologist Paul Rozin fed a standard lunch to two men with such severe amnesia they couldn’t remember anything past the last minute. The men duly polished their plates.

Recalling nothing of that meal, they were offered a second meal of the same size 20 minutes later and promptly ate it too. This went on until a fourth sitting when one of the men complained his “stomach was a little tight”.

The lesson is clear. For the multitudes trying to eat less, feeling full is soundly trumped by the simple reminder: “It’s meal time.” Even an intact memory is no defence against food cues.

Psychologists call the desire to eat what you think is the right amount “unit bias”. And that unit is often the size of a bowl. Behavioural scientist Brian Wansink showed this in a striking 2005 study that served people soup in a bowl with a difference; it was designed to sneakily self-fill. Those people ate 73% more than a control group with normal bowls, yet felt no fuller. Wansink concluded people “use their eyes to count calories and not their stomachs”.

To make matters worse, food cues actually drive changes in the endocrine system. Simply thinking about mouth-watering dishes can have similar effects to eating them, hiking up insulin, dropping blood sugar and perpetuating hunger. So if your belly aches next time you pass that Ben and Jerry’s billboard, maybe cut yourself some slack.

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