The psychology of purchasing decision-making (part 2) – watch your diet!

(We're pleased to feature part 2 of our guest post from Ginny Smith, a psychologist with a particular interest in cognition and decision making.)

If you have ever been to America, (or indeed live there), you have probably experienced decision fatigue first hand. In the morning, it can seem like a great idea to have 27 different options when you are ordering your coffee- it means you can make sure the cup is perfect for you. After a hard day at work, however, or in your lunch break, when you are starving and in a hurry, the decisions are less appealing. In this situation it can feel like you will just say 'yes' to anything for an easy life (“do you want pickled rabbit's foot with that sir?” “Yes, whatever, just give me my lunch!”).

This is an example of decision fatigue, and it is something that affects us all. During the course of a normal day, we all have to make a multitude of decisions. Some of these are important, others are trivial, but they all use mental energy. We may not feel tired, but after making a series of decisions, our mental reserves become depleted, and our reasoning abilities are compromised. In this condition, we are more likely to make a reckless decision without thinking through the consequences, or avoid making any decision at all by sticking with the status quo.

A recent study (i) looking into bail decisions made by judges over the course of the day revealed some surprising results. Most judges take 2 breaks for food during the day, giving 3 “decision sessions” each day. It was found that the percentage of decisions to grant bail dropped from around 65% to 0% over the course of each of these sessions, before rising to 65% again after the food break.

In this situation the decision not to grant bail is the easier one, as it leaves the situation the same as it was before the hearing. Judges are a group of people chosen to be unemotional and rational, and to concentrate solely on the facts of the case. This study, however, showed that they are hugely affected by their personal circumstances, and decision fatigue, so make easier decisions towards the end of a session when they are more fatigued.

The judge study shows that it is not just the number of decisions we have made that affect our fatigue levels- they are also affected by blood glucose levels. Willpower also seems to be involved in decision making, so if you have made a lot of decisions during the day, you may be less able to stop yourself from indulging in a big slice of chocolate cake, or a glass of wine, that evening. In the cake example there is a slight paradox however, as the glucose contained in the cake would actually give you the boost you need to improve your willpower and decision making abilities!

People who have had to use their willpower to avoid eating tempting chocolates give up more quickly when presented with unsolvable problems (ii), and the same thing is found when it comes to decision making. When one group of students were asked to make repeated decisions about a selection of items, they could then hold their hand under iced water for less than half the time a control group, who just looked at the items, managed. (iii)

This also works the other way around, so someone who is on a strict diet, and  denying themselves treats on a daily basis, may have reduced willpower (iv) and be handicapped in their decision making abilities in two ways. Firstly the low blood sugar in itself may lead to poorer decisions, as decision making relies on willpower, meaning in a procurement situation that they may be less likely to continue negotiating a lower price, instead settling once the going gets tough. The willpower being used to stick to the diet will also be sapping mental energy, leading to a more rapid onset of decision fatigue.

So what does this mean for the procurement executive? Well, it is probably a good idea to schedule any particularly tough negotiations for the morning (after a good breakfast of course), and to avoid doing anything too taxing just before lunch or in the late afternoons. Diet is important, especially with the rise of obesity, but being sensible about it is vital - a low GI diet, which avoids fluctuating blood sugar levels may be helpful, as may having a small, healthy snack before an important meeting.

And if you are going to have the occasional lapse in willpower (after all, we are only human), giving in to that amazing looking cake is probably better than agreeing an inflation busting price increase and a relaxed delivery schedule from your key supplier!


i  Social Sciences - Social Sciences: Shai Danziger, Jonathan Levav, and Liora Avnaim-Pesso Extraneous factors in judicial decisions PNAS 2011 108 (17) 6889-6892; published ahead of print April 11, 2011, doi:10.1073/pnas.1018033108

ii  Ego depletion: Is the active self a limited resource? Baumeister, Roy F.; Bratslavsky, Ellen; Muraven, Mark; Tice, Dianne M. Journal of Personality and Social Psychology, Vol 74(5), May 1998, 1252-1265.

iii  Making choices impairs subsequent self-control: A limited-resource account of decision making, self-regulation, and active initiative. Vohs, Kathleen D.; Baumeister, Roy F.; Schmeichel, Brandon J.; Twenge, Jean M.; Nelson, Noelle M.; Tice, Dianne M. Journal of Personality and Social Psychology, Vol 94(5), May 2008, 883-898

iv  Self-control relies on glucose as a limited energy source: Willpower is more than a metaphor. Gailliot, Matthew T.; Baumeister, Roy F.; DeWall, C. Nathan; Maner, Jon K.; Plant, E. Ashby; Tice, Dianne M.; Brewer, Lauren E.; Schmeichel, Brandon J. Journal of Personality and Social Psychology, Vol 92(2), Feb 2007, 325-336.

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