Taking glucose seriously

People sometimes talk about lacking the ‘mental energy’ to complete a certain task. But relatively few professionals – or their employers – probably give it further consideration.
Yet when it comes to our ability to make decisions, mental ‘depletion’ is a more serious issue than we might imagine. Our brains are energy-efficient machines, which, when low on glucose, limit our expenditure by avoiding effortful mental computations. Cognitive tasks and logical decision-making, such as complex product purchases, consume a lot of energy, so can cause depletion. But they are also the tasks most affected by it.

A powerful example of this is shared by Daniel Kahneman in his new book ‘Thinking, fast and slow’, which relates to the performance of eight parole judges in Israel. These experienced judges have the task of reviewing parole cases and deciding whether to grant freedom from imprisonment. Each case takes just a matter of minutes, with around 35% of requests being approved on average. The researchers found compelling evidence of ego depletion when tracking the correlation between the judges’ decisions and their food breaks:

‘The proportion [of approved parole requests] spikes after each meal, when about 65% of requests are granted. During the two hours or so until the judges’ next feeding, the approval rate drops steadily, to about zero just before the meal.’

Besides being a cautionary tale about the impact of depletion, it’s also an interesting perceptive on the role of defaults: the judges defer to the default of denying parole when lacking in energy, but go the other way when fuelled-up. Defaults are one of the most accessible behavioural economics principles – that humans are prone to ‘going with the flow’ of what’s already in place. But this case is a reminder neither to see defaults in a vacuum, nor to view behavioural economics as a silver bullet.


At the same time, studies like this prompt us to question long-held assumptions about the ability of humans to make rational decisions – particularly in a professional context. It's certain that behavioural economics equips us with useful tools with which to better analyse these choices. For disciplines like B2B marketing, where a model of rational decision-making is still often assumed, this should be valuable food for thought.