Most days at work, I grab lunch at one of the many restaurants and food trucks around the University of Texas campus. Occasionally, though, I bring lunch—particularly if I have a number of meetings that will make it hard to take the time to get something. Usually, when I bring lunch, I prepare it the evening before and put it in the refrigerator.

Because I do it infrequently, I am prone to forget to bring my lunch on days when I prepared it—which means that I have an uneaten lunch at home and I’m likely to have trouble getting something to eat at work.

There is a high cost to being wrong. To help ensure I won’t forget, I will put my car keys in the refrigerator along with my lunch. That way, I can’t leave the house without my keys (and my lunch). That is, I am using an external aid to help me remember rather than relying on my own memory.

This is a typical strategy for people. We write things down, keep calendars, make lists, all to make sure that important information is available when we need it rather than relying on our ability to recall something in the moment.

Using these external memory aids has a cost, though. It takes time to write out a note (or to put my keys in the refrigerator). It takes organization to make sure that notes you keep are available when you need them.

An interesting question is whether people are good at determining when they should use an external memory aid. That is, do they weigh the costs of using an aid against the benefits in a way that takes into account their actual likelihood of being wrong? This question was explored in a paper by Sam Gilbert, Arabella Bird, Jason Carpenter, Stephen Fleming, Chhavi Sachdeva, and Pei-Chun Tsai in the March 2020 issue of the Journal of Experimental Psychology: General.

The studies in this paper used a clever procedure. Using a tablet, participants saw six circles on the screen with the letters A through F on them. These circles were yellow. Participants had to move the circles off the screen in alphabetical order by dragging them.

As a circle was dragged off, another appeared with the next letter of the alphabet on it. When that circle first appeared, it might be yellow, or it might be in one of three other colors. Those colors signaled that the circle with that letter on it needed to be dragged off the left, top, or right edge of the screen. After a couple of seconds, colored squares turned yellow, so that the participant had to remember which items needed to be dragged off which edge. Each trial consisted of one pass through the alphabet, and half the new circles that appeared were in a color other than yellow, so participants had a lot to remember.

After practicing the task, participants did three kinds of trials. On some passes through the alphabet, participants went completely from memory. On others, they were forced to use an external memory aid. The external memory aid was to take squares that appeared in a different color and drag them near the edge where they would eventually have to be taken to be removed. Unsurprisingly, people are quite good at the task when they have an external memory aid. They get a little over half the special circles correct when they have to do it completely from memory.

On a third set of trials, participants were told they had a choice. They could do the task completely from memory, and get 10 points for each correct answer. They could do the task using memory aids on each trial, but then they would get fewer points for each correct answer. These trials varied the payoff for using the external memory aid from 9 points for correct answers all the way down to 1 point.

Because participants got a little over half of the special items correct when they had to do the task from memory, on average, they should have used the memory aids when the payoff was 6 points or more. With that level of payoff, they would earn about 55 points using memory alone, but 60 if they got all of the external memory aid trials correct. When memory aid trials yielded lower payoffs, though, they should actually have chosen to do the task from memory.

In fact, participants showed a to use the memory aids longer than they should have given their accuracy in the task. By the time the payouts were very low, people did the task from memory, but they persisted in using the aids longer than they needed to. In one of the studies, the payment participants received for doing the task was based on the number of points they got, so their persistence using the memory aids actually cost them money.

That said, in another study, some participants received feedback before each trial on whether (based on their prior performance) they should use the memory aid or do the task from memory. Participants took this guidance, which suggests that people had trouble figuring out exactly when they should use a memory aid. In fact, people generally underestimated how well they were doing in the task, which may help to explain part of this over-reliance on memory aids. And another study showed that when people had lower in their memory, they were also more prone to use an external memory aid.

What does all this mean, practically speaking? For most of us, most of the time, external memory aids are pretty easy to maintain. They don’t take much time or effort to put together. The cost of forgetting is higher than the cost of using the aid. As a result, most of us don’t really weigh the costs and benefits frequently, we just default to using a memory aid. We may use memory aids even in cases when we would have remembered, but we’re willing to put in that extra effort.

That said, there may be hidden costs to using external memory aids. There is some evidence, for example, that people who use GPS to navigate are worse at learning the spatial layout of a town than those who do not. So, there may be some situations in which people might be better off foregoing the external aids.