Time preferences in long-tailed macaques (Macaca fascicularis) and humans (Homo sapiens)

Abstract

Rosati et al. (Curr Biol 17(19):1663–1668, 2007) found in a self-control test in which choice was between a smaller, immediately delivered food and a larger, delayed food, that chimpanzees preferred the larger reward (self-control); humans, however, preferred the smaller reward (impulsivity). They attributed their results to a species difference in self-control. In Experiment 1, monkeys (long-tailed macaques) were exposed to a self-control task in two conditions: where the food was hidden under differently colored bowls and where it was visible. When these two conditions were compared, choice shifted from greater preference for the impulsive alternative in the hidden condition to greater preference for the self-control alternative in the visible condition. Additionally, in both conditions, preference shifted from self-control to impulsivity over sessions. These results were explained in terms of the reversed-contingency effect (a propensity to reach for more over less when rewards are visible) and not to a capacity for self-control. In Experiment 2, humans that demonstrated preference for more over less in choice preferred the impulsive alternative when choice to either alternative was followed by the same intertrial interval—a preference that accelerates trial rates relative to preference of the self-control alternative. When trial rates were equated so that neither choice accelerated session’s end, humans demonstrated self-control. These results suggest that Rosati et al.’s demonstration of impulsivity in humans was due to participants’ desire to minimize session time.

Appendix

Control condition script

This is an experiment about decision-making, and will take no longer than 45 min. In the experiment you are going to be able to make a series of choices about food, and after each decision you can eat the food you choose. How much you receive will depend on the decisions that you make. This part of the experiment will take no longer than 31 min. At the end of the experiment you will be asked to complete a questionnaire. This questionnaire will take about 10 min to complete. Do you have any questions at this point?

In the test, you get to choose between a small, partially filled teaspoonful of yogurt, and a full teaspoonful of yogurt. I will ask you which of the options you prefer, and you can then answer verbally or by pointing at your choice. Once you have indicated your choice, I will say “you can now have the food”, and you can then eat the option you have chosen. Specifically what will happen is that both options will be in teaspoons. I will ask you which teaspoon you want, and once you answer I will remove the other one. Then I will say right away, “you can now have the food”. Each trial will consist of just one choice like this. The total number of trials was randomly predetermined, so the test could stop after any number of trials. Therefore, just choose according to your preferences when I actually ask you. There is no right or wrong way to do this task. Before we start the experiment there will be a practice period where you get to see how the procedure works and experience both options. This practice period can then guide your decisions when you choose between the two options. Do you have any questions at this point? Throughout the test, you must eat all of the yogurt before you can proceed to the next trial—that is, you have to eat it here during the test, and cannot take the it out of the test or save it for later. You can take as long as you’d like to eat the food and eat it in any way you’d like. That’s a glass of water that you should feel free to drink throughout the test if you get thirsty. Do you have any questions at this point? Before the actual test begins, if you are wearing a watch could you please remove it. Thank you. Now for the actual test, you will be eating the same food throughout the test. However you can choose beforehand what type of food you would like to eat. We have three different flavors of yogurt. OK, great. Now there’s going to be the practice period where you get to experience each option two times each. In this practice period I am going to set just one teaspoon in front of you, and then ask you how much is in the teaspoon (is it full or only partially full). Once you answer I will immediately say “you can now have the food”, and then you can consume your choice. Once you have experienced each option twice, I will tell you that the practice period is over and the actual test will begin. Do you have any questions at this point? Then we will begin with the practice period.

Unequal-trial-rate condition script

This is an experiment about decision-making, and it will take no longer than 45 min. In the experiment you are going to be able to make a series of choices about food, and after each decision you can eat the food you choose. How much you receive will depend on the decisions that you make. This part of the experiment will take no longer than 31 min. At the end of the experiment you will be asked to complete a questionnaire. This questionnaire will take about 10 min to complete. Do you have any questions at this point?

In the test, you get to choose between a small amount of yogurt, and a larger amount of yogurt. I will ask you which of the options you prefer, and you can then answer verbally or by pointing at your choice. Once I say “you can now have the food”, you can then eat the option you have chosen. Specifically what will happen is that both options will be in spoons. I will ask you which spoon you want. If you pick the smaller amount, I am allowed to give it to you immediately. However, if you choose the larger amount I am not allowed to give it to you until after a set period of time has passed. That is, if you chose the spoon with the smaller amount of yogurt, I will say right away, “you can now have the food”. But if you choose the yogurt with the larger amount, you will have to wait before I say “you can now have the food”. Each trial will consist of just one choice like this. The total number of trials was randomly predetermined, so the test could stop after any number of trials. Therefore just choose according to your preferences when I actually ask you. There is no right or wrong way to do this task. Before we start the experiment there will be a practice period where you get to see how the procedure works and experience both options receiving the smaller amount right away, and waiting for the larger amount. This practice period can then guide your decisions when you chose between the two options. Do you have any questions at this point?

Throughout the test, you must eat all of the food before you can proceed to the next trial—that is, you have to eat it here during the test, and cannot take the food out of the test or save it for later.

You can take as long as you’d like to eat the food and eat it in any way you’d like. Throughout the test, the smaller amount of yogurt will always be available immediately, and the larger amount will always be associated with the same delay. That’s a glass of water that you should feel free to drink throughout the test if you get thirsty. Do you have any questions at this point? (If you choose the larger amount, during the delay period I will sit at that table over there, and we are not allowed to talk to each other. Then when the delay finishes you can eat the food.) Before the actual test begins, if you are wearing a watch could you please remove it. Thank you. Now for the actual test, you will be eating the same food throughout the test. However, you can choose beforehand what type of food you would like to eat. We have different flavors of yogurt. Feel free to try these options before you decide. OK, great. Now there’s going to be the practice period where you get to experience each option two times each. In this practice period I am going to set just one spoon, and then ask you if it is a small amount or large amount. If the spoon is only partially filled, once you answer I will immediately say “you can now have the food”. But if the spoon has the larger amount, there will be a delay before I say you can eat it. Once you have experienced each option twice, I will tell you that the practice period is over and the actual test will begin. Do you have any questions at this point? Then we will begin with the practice period.

Equal-trial-rate condition script

This is an experiment about decision-making, and it will take no longer than 45 min. In the experiment you are going to be able to make a series of choices about food, and after each decision you can eat the food you choose. How much you receive will depend on the decisions that you make. This part of the experiment will take no longer than 31 min. At the end of the experiment you will be asked to complete a questionnaire. This questionnaire will take about 10 min to complete. Do you have any questions at this point?

In the test, you get to choose between a small amount of yogurt, and a larger amount of yogurt. I will ask you which of the options you prefer, and you can then answer verbally or by pointing at your choice. Once I say “you can now have the food”, you can then eat the option you have chosen. Specifically what will happen is that both options will be in spoons. I will ask you which spoon you want. If you pick the smaller amount, I am allowed to give it to you immediately. However, if you choose the larger amount I am not allowed to give it to you until after a set period of time has passed. That is, if you choose the spoon with the smaller amount of yogurt, I will say right away, “you can now have the food”. But if you choose the yogurt with the larger amount, you will have to wait before I say “you can now have the food”. Each trial will consist of just one choice like this. There will also be delay before the next trial begins. The duration of this delay may not be the same for both choices. That is, the delay following choice of the delayed food may differ in length from the delay to the next trial if you choice the food that is immediately available.

The total number of trials was randomly predetermined, so the test could stop after any number of trials. Therefore, just choose according to your preferences when I actually ask you. There is no right or wrong way to do this task. Before we start the experiment there will be a practice period where you get to see how the procedure works and experience both options (receiving the smaller amount right away, and waiting for the larger amount). This practice period can then guide your decisions when you chose between the two options. Do you have any questions at this point?

Throughout the test, you must eat all of the food before you can proceed to the next trial—that is, you have to eat it here during the test, and cannot take the food out of the test or save it for later.

You can take as long as you’d like to eat the food and eat it in any way you’d like. Throughout the test, the smaller amount of yogurt will always be available immediately, and the larger amount will always be associated with the same delay. There may be different delays until the next trial depending on how you have chosen. That’s a glass of water that you should feel free to drink throughout the test if you get thirsty. Do you have any questions at this point? If you choose the larger amount, during the delay period I will sit at that table over there, and we are not allowed to talk to each other. Then when the delay finishes you can eat the food. Before the actual test begins, if you are wearing a watch could you please remove it. Thank you. Now for the actual test, you will be eating the same food throughout the test. However, you can choose beforehand what type of food you would like to eat. We have different flavors of yogurt. Feel free to try these options before you decide. OK, great. Now there’s going to be the practice period where you get to experience each option two times each. In this practice period I am going to set just one spoon, and then ask you if it is a small amount or large amount. If the spoon is only partially filled, once you answer I will immediately say “you can now have the food”. But if the spoon has the larger amount, there will be a delay before I say you can eat it. Once you have experienced each option twice, I will tell you that the practice period is over and the actual test will begin. Do you have any questions at this point? Then we will begin with the practice period.