Australasian Science Magazine

Credit: tawanlubfah/Adobe

By Bowen Fung

A new study finds that our recent experience with rewards such as food can change our capacity for patience.

In everyday life, we often make commitments to long-term goals, but in many cases we break these commitments (sometimes rather quickly). When we diet, make financial investments or try to quit smoking, it can feel like there is a constant temptation to give up and appease our more immediate desires. This kind of impulsivity – the tendency to act impatiently, without regard for the future consequences – is often considered to be a negative trait, and society praises individuals who display persistence in the face of temptation. After all, we all know that patience is a virtue.

Impulsivity is also mostly considered to be a stable personality trait that doesn’t change significantly over the course of our lives. The classic example of this is the Stanford marshmallow experiment, where Walter Mischel tested whether children were able to wait patiently for many marshmallows or instead would be tempted into taking a single marshmallow immediately. Follow-up studies of these children showed that those who were more patient tended to have better lifestyle outcomes in terms of educational achievement and health. Does this mean that your inability to quit smoking is set in stone?

Despite the “virtue” of patience, in many real-world situations it is unreasonable to be infinitely patient. Imagine waiting at a bus stop and not knowing when the bus will arrive. You are probably happy to wait for a little while, but what if there is a possibility that the bus has broken down? Surely you wouldn’t wait forever? In this situation you might rely on your previous experience with the frequency of bus arrivals, and only give up and take a taxi after the expected time had passed.

This scenario is quite similar to the marshmallow test, but it clearly reveals two important things about impulsivity. First, infinite patience isn’t always viable, and acting impatiently can sometimes save you time and money. Second, it shows that, contrary to being set in stone, our level of patience could be dependent on our recent past experiences and expectations. If I’m used to a efficient public transport system I’m going to be less tolerant of delays, but if I move to a city with poor public transport I might (unhappily) wait longer for a bus.

My laboratory and I set out to test exactly how people behaved in situations like this, and what kinds of factors might make them more or less likely to persist in waiting.

In our study, published in Proceedings of the Royal Society B (http://tinyurl.com/yd8soa84), we asked participants to avoid eating or drinking for 4 hours, and then asked them to complete a computer task that was similar to the bus stop scenario. Instead of buses, participants waited for a monetary reward (that they were actually paid), but they could give up waiting at any time. If they gave up, they received a much smaller reward. Whenever they won money (either the large or small amount), the task was reset, and the time they would have to wait for the large money reward to “arrive” would be randomly shuffled. We gave participants a fixed amount of time to earn money in this task, using whatever strategy they wanted.

Imagine one extreme strategy that people might have used: continuously taking the immediate, smaller reward. While this would ensure a constant cash flow, it meant always missing out on a larger amount of money (which was substantially greater).

The opposite extreme strategy would be to wait for the larger money reward every single round. However, for some rounds this could take a very long time, during which participants wouldn’t be getting any money at all. Thus the key to earning the most money was to give up when they thought they had been waiting too long, because the large reward might come much quicker in the next round.

We experimentally manipulated two factors to see whether they would affect how long participants were likely to wait in this task. First, for some participants we delivered the larger monetary reward slightly sooner on average. In terms of the bus stop scenario, this was equivalent to having an arrival timetable with a higher frequency, so these participants should expect the large reward to “arrive” quicker. This also meant that these participants could potentially earn more money in total.

For the second manipulation, half the participants drank a sweet beverage (very similar to a normal soft drink), while the other half just drank water. Although this was irrelevant to the computer task, it meant that half of our participants had the experience of being physiologically rewarded while the other half remained hungry.

We found that for both of these experimental manipulations the participants gave up waiting earlier. But why should that be the case?

One likely explanation relates to the concept of opportunity cost. This generally refers to the value of forgone opportunities – in our task, this was the potential money that participants might have been able to earn if they hadn’t chosen to keep waiting. In the case of the first manipulation (when we delivered the rewards sooner), we not only changed our participants’ expectation of when the large reward would “arrive” but we also increased the potential amount of money they could be earning per unit time. This meant that their time was inherently more valuable, and every second spent waiting was more costly than the participants who had their large rewards “arrive” later. It is likely that this sense of increased opportunity cost made participants give up sooner.

But what about the participants given the soft drink? The drink was unrelated to the task of earning money, yet despite this, drinking it still affected participants willingness to wait for rewards.

One suggestion is that our perception of opportunity cost is sensitive to all types of rewards, even those that are irrelevant to the task at hand. When our hungry participants drank the energy-providing soft drink, a general psychological mechanism may have signalled that rewards were now readily available in the environment, and that waiting now carried a higher opportunity cost (the cost of not finding more to drink). If humans are indeed sensitive to irrelevant rewards in the environment, then this implies that reward-rich environments like casinos may make us more impulsive simply by increasing our general sense of opportunity cost.

Many people would agree that, at least in their experience, they get more impatient when they’re hungry, yet the results of our study seem to imply the opposite: that we are less impatient when hungry.

Naturally, the task used in our experiment does not capture the complexities of real-world environments like a supermarket. A straightforward explanation is that supermarkets are full of cues and images that represent food, and that these overwhelmingly indicate that the environment is full of rewards that could be exploited.

A good test of this idea would be to repeat our experiment, and instead of giving participants the soft drink, we could simply expose them to the kinds of food cues and brands that we regularly experience in supermarkets. Another possible explanation for this counterintuitive finding is that when we are reasonably confident of receiving food (e.g. if we’ve just ordered at a restaurant), we would prefer to wait rather than waste valuable energy trying to find food elsewhere.

There is also a plausible explanation for these effects from a neurobiological perspective. A recent theory from computational neuroscientist Yael Niv of Princeton University proposes that opportunity costs are encoded in the brain by the neurotransmitter dopamine – the “reward” neurotransmitter. Dopamine increases when individuals receive rewards like food or money. Thus, when people have had more rewards, their levels of dopamine increase, and this is a signal that the opportunity cost of time is higher.

Previous studies have shown that when individuals are rewarded more frequently, their dopamine levels rise and they also act more quickly and vigorously. This makes sense because wasted time is more costly in these situations, and acting faster offsets this cost. While we didn’t measure dopamine levels in our participants, it’s possible that the same neurobiological mechanism can explain our results.

Interestingly, increases in dopamine also affect our perception of time. When dopamine levels increase, people overestimate time intervals (i.e. it feels like time is dragging on). Thus, changes in the perception of time may also partly explain the results found in our experiment: if time was overestimated, participants would give up waiting sooner even if they thought they were giving up at the normal expected time.

It’s important to note that some other studies have reported that calorie consumption can increase self-control and patience. This doesn’t necessarily conflict with our results, as these studies used different tasks to measure impulsivity. However, it does highlight that impulsivity is a rather complicated phenomenon, and the way our behaviour is affected by calorie consumption may not be so clear-cut.

Yet, at least in the case of the bus stop scenario, the results of our study suggest that calorie consumption can increase impatience. This reinforces the idea that our perception of opportunity cost can influence behaviour, even when these costs are irrelevant to our current goals. Our findings also strike a blow to the idea that impulsivity is a stable personality trait, and instead suggest that impulsivity can differ depending on our environmental context and our implicit expectations.

Perhaps bear this in mind next time you are waiting for a bus, and put the snacks aside until later.