Measuring Impatience in Intertemporal Choice

In general terms, decreasing impatience means decreasing discount rates. This property has been usually referred to as hyperbolic discounting, although there are other discount functions which also exhibit decreasing discount rates. This paper focuses on the measurement of the impatience associated with a discount function with the aim of establishing a methodology to compare this characteristic for two different discount functions. In this way, first we define the patience associated with a discount function in an interval as its corresponding discount factor and consequently we deduce that the impatience at a given moment is the corresponding instantaneous discount rate. Second we compare the degree of impatience of discount functions belonging to the same or different families, by considering the cases in which the functions do or do not intersect.


Introduction
Impatience was already defined in 1960 by [1] as the decrease in the aggregate utility with respect to time. In his work, he stated: "this study started out as an attempt to formulate postulates permitting a sharp definition of impatience, the short term Irving Fisher has introduced for preference for advanced timing of satisfaction" ( [1] referred to the 1930 work of Fisher [2]: "The Theory of Interest" (Chapter IV)). This idea of a preference for advancing the timing of future satisfaction has been used in economics since the appearance of Böhm-Bawerk's work: Positive Theorie des Kapitals [3].
Some authors use the term impulsivity as a synonym of impatience, e.g. [4]. In effect, [5] define impulsivity in intertemporal choice as a "strong preference for small immediate rewards over large delayed ones". We can find a similar and earlier definition in [6] who defined the impulsiveness (in choices among outcomes of behavior) as "the choice of the less rewarding over more rewarding alternatives". Observe that impatience has usually been presented in relative terms by comparing the values shown by two intertemporal choices. [7] states that the term impulsivity is often utilized in psychiatric studies on intertemporal choice and cites some examples of impulsive subjects such as smokers, addicts and attention-deficient hyperactivitydisorder patients. The opposite behavior to impulsivity is self-control.
2. Inevitably, most researches on this issue show the discount functions which, in each case, better fit the data. In effect, [34] estimate the parameters of the main intertemporal choice models: exponential, simple hyperbolic, quasi-hyperbolic, and q-exponential. Subsequently, they compare the impatience shown by two groups by simply comparing the discount rates of the corresponding discount functions. Obviously, this is not an accurate procedure because some discount functions are biparametric and so it should require a comparison of both parameters defining the function. Moreover, this is a simplification because it would be interesting to compare the impatience in a certain time interval where, among other circumstances, the relative position of the impatience levels can change. Even the use of the qexponential discount function assumes working with an exponential, a hyperbolic or a generalized hyperbolic discount function, depending on the concrete values of q and k q . Therefore, the most important thing is to obtain the discount function which better fits the collected data, and then it is likely that the subsequent comparison can involve discount functions belonging to different families. Even the comparison between two discount functions belonging to the same family (for instance, two hyperbolic discount functions) is also noteworthy because they usually exhibit different parameters.
3. Several researches have considered the impatience shown by individuals of different nationalities, genders or socio-economic levels. The comparison of the discount functions involved in these studies is important in order to design, for example, a market segmentation strategy according to the former criteria. [35] state that the intertemporal impatience can be applied to the acquisition of material objects instead of money. This makes the issue of impatience very interesting in marketing and consumer behavior. They point out some culture-related differences between western and eastern participants in the empirical study conducted by them: the former valued immediate consumption more than the latter. In the same way, [14] experimentally compared intertemporal choices for monetary gains and losses by American and Japanese subjects, demonstrating that Westerners are more impulsive and time-inconsistent than Easterners. [36] also recognize the accuracy of discounting to explain impatience in marketing. Finally, [37] have found that gender and autobiographical memory can have an effect on delay discounting: there is a significant difference between men and women because, in the case of higher memory scores, the former showed less impatience when discounting future rewards. In the experimental analysis, they used the standard hyperbolic and the quasihyperbolic models. It is therefore apparent that the comparison of discount functions will be of interest to segment a market depending on the impatience exhibited by individuals who are classified by different criteria (geographical, gender, culture, etc.).
This paper is organized as follows. After this introduction, in Section 2 we will formally define the impatience (impulsivity) ranging from the discount corresponding to $1 in an interval [t 1 , t 2 ] (a two-parameter function, referred to as impatience-arc) to the instantaneous discount rate at an instant t (a one-parameter function, referred to as instantaneous impatience). The value of the instantaneous rate at t = 0 (a constant) can also be taken into account. Obviously, any simplification in the measurement of impatience will result in a reduction in the amount of information thus obtained. Therefore, in Section 3, we will compare the impatience associated with two discount functions, considering two cases: when the functions do not intersect and the functions do intersect. In Section 4, all the obtained results will be applied to wellknown families of discount functions. Finally, Section 5 summarizes and concludes.

Defining impatience (impulsivity) in intertemporal choice
In economics and other social sciences it is common practice to try to simplify the complexity of the models describing the behavior corresponding to a group of people. This is the case of discount functions in the framework of intertemporal choice within the field of finance. In effect, a (dynamic) intertemporal choice can be described by a two-variable discount function ( [38]), that is, a continuous function where F(d, t) represents the value at d (delay) of a $1 reward available at instant d + t. In order to make financial sense, this function must satisfy the following conditions: Fðd; tÞ > 0, for every d 2 R.
Regular discount functions are the most usual valuation financial tools. Nevertheless, and as indicated at the beginning of this Section, this discounting model can be simplified by using a function F(t) independent of delay d. More specifically, a one-variable discount function F(t) ( [38] and [39]) is a continuous real function defined within an interval [0, t 0 ) (t 0 can even be +1), where F(t) represents the value at 0 of a $1 reward available at instant t, satisfying the following conditions: The following theorem of representation provides the relationship between the preferences existing in a scenario of intertemporal choice and its associated discount function. Theorem 1. A discount function F(t) gives rise to the total preorder ≽ defined by satisfying the following conditions: Reciprocally, every total preorder ≽ satisfying conditions (i) and (ii) defines a discount function.
Theorem 1 shows that, in intertemporal choice, an agent can indistinctly use a discount function or a total preorder. In this way, the concept of impatience has been mainly treated with a total preorder. For example, [40] propose the following choice: "$10 in a year or $15 in a year and a week". In this way, they state that: "If an individual A prefers the first option ($10 in a year) while B prefers the second option ($15 in a year and a week), it is said that A is more impulsive than B because A prefers a smaller, but more immediate reward, whereas B prefers to wait a longer time interval to receive a greater reward". Nevertheless, our aim here is to define the concept of impatience by using discount functions. In effect, given a one-variable discount function F(t), the patience associated with F(t) in an interval [t 1 , t 2 ] (t 1 < t 2 ) is defined as the value of the discount factor f(t 1 , t 2 ) corresponding to this interval, viz: where dðxÞ ¼ À d ln FðzÞ dz j z¼x is the instantaneous discount rate of F(t) at instant x. Obviously, the inequality 0 < f(t 1 , t 2 ) < 1 holds. Observe that the greater the discount factor, the less sloped is the discount function in the interval [t 1 , t 2 ]. In this case, people are willing to wait for a long time to receive a future amount because they have to renounce a small part of their money. On the other hand, the impatience associated with F(t) in the interval [t 1 , t 2 ] (t 1 < t 2 ) is defined as the value of the discount D(t 1 , t 2 ) corresponding to this interval, viz: which lies in the interval [0, 1]. Some comments: 1. It is logical that the impatience can be measured by the amount of money that the agent is willing to lose in exchange for anticipating the availability of a $1 reward.
2. Any function with the same monotonicity as f(t 1 , t 2 ) (resp. D(t 1 , t 2 )) can be used as a measure of patience (resp. impatience). For example, R t 2 t 1 dðxÞdx is a measure of the impatience. Consequently, for an infinitesimal interval (t, t + dt), the measure of the impatience is given by δ(t).
3. The term impulsivity is used on most occasions as a synonym of impatience, but we prefer its use for intervals of the type [0, t] or, from an infinitesimal point of view, δ(0).
4. [40] use the term "self-control" as the opposite of impulsivity and therefore as a synonym of patience.

Comparing the impatience represented by two discount functions
Most empirical studies on intertemporal choice present a set of data based on the preferences of outcomes shown by a group of individuals. The analysis of the impatience exhibited by the group is very difficult to realize because individual members of the group will show a wide variety of preferences with regard to amounts and time delays. Therefore, it is preferable to fit the resulting data to a discount function belonging to any of the noteworthy families of discount functions, viz, linear, hyperbolic, generalized hyperbolic, exponentiated hyperbolic, or exponential. The necessary adjustment can be made by using the q-exponential discount function (see [38] and [41]) since it includes the majority of the aforementioned functions as particular cases ( [42]). Once a discount function is obtained which represents all the information coming from the individual questionnaires, it is easier to obtain the instantaneous impatience and the impatience-arc, that is to say, the impatience corresponding to a time interval. To do this, we can make use of all the tools of mathematical analysis. Moreover, the comparison between the impatience shown by two groups of people is more accurate and more easily understood, and the results can be used in designing and implementing future strategies.

Case in which the two functions do not intersect
Let F 1 (t) and F 2 (t) be two discount functions. Assume that the ratio F 2 ðtÞ F 1 ðtÞ is increasing. This implies that, for every t > 0, F 2 ðtÞ F 1 ðtÞ > F 2 ð0Þ F 1 ð0Þ ¼ 1 and so F 1 (t) < F 2 (t). Let us recall that the patience is measured by the discount factor defined by Eq (1). As F 2 ðtÞ F 1 ðtÞ is increasing, for every t 1 and t 2 such that t 1 < t 2 , and so ln f 1 ðt 1 ; t 2 Þ < ln f 2 ðt 1 ; t 2 Þ: In particular, for every t and h > 0, ln f 1 ðt; t þ hÞ < ln f 2 ðt; t þ hÞ; or equivalently ln F 1 ðt þ hÞ À ln F 1 ðtÞ < ln F 2 ðt þ hÞ À ln F 2 ðtÞ: that is to say The converse implication is also true, whereby we can enunciate the following result. Theorem 2. Let F 1 (t) and F 2 (t) be two discount functions. The following three statements are equivalent: 1. The ratio F 2 ðtÞ F 1 ðtÞ is increasing.
2. The impatience represented by F 1 (t) is greater than the impatience represented by F 2 (t), that is to say, f 1 (t 1 , t 2 ) < f 2 (t 1 , t 2 ), for every t 1 and t 2 such that t 1 < t 2 .
Example 1. Let F 1 ðtÞ ¼ 1 1þi 1 t and F 2 ðtÞ ¼ 1 1þi 2 t be two hyperbolic discount functions where According to Theorem 2, d 1 ðtÞ ¼ i 1 1þi 1 t must be greater than d 2 ðtÞ ¼ i 2 1þi 2 t . In effect, > 0 :   Fig 1 shows that it is not easy to graphically observe that the ratio F 2 (t) (shown in red) to F 1 (t) (in blue) is increasing.
For this reason we are going to formulate the following Corollary 1. Let F 1 (t) and F 2 (t) be two discount functions such that F 2 (t) − F 1 (t) is increasing. In this case, any of the three equivalent conditions of Theorem 2 is satisfied. For a proof, see Appendix. Example 2. Let F 1 ðtÞ ¼ 1 1þi 1 t be a regular hyperbolic discount function of parameter i 1 and F 2 ðtÞ ¼ 1þi 2 t 1þi 1 t be a singular hyperbolic discount function of parameters i 1 and i 2 (so necessarily i 1 > i 2 ). Fig 2 shows that the difference F 2 (t) − F 1 (t) is increasing.
In effect, According to Corollary 1, d 1 ðtÞ ¼ i 1 1þi 1 t must be greater than d 2 ðtÞ ¼ i 1 1þi 1 t À i 2 1þi 2 t , which can easily be verified. Then the impatience represented by F 1 (t) is greater than the impatience represented by F 2 (t). Let us now consider a third situation. Let us suppose that the ratio F 2 ðtÞ F 1 ðtÞ reaches a local maximum at instant t 0 . A possible graphic representation is depicted in Fig 4. By Theorem 2, for intervals [t 1 , t 2 ] included in [0, t 0 ] (t 1 , t 2 < t 0 ), the impatience represented by F 1 (t) is greater than the one represented by F 2 (t). After instant t 0 , the opposite situation occurs, that is, the impatience represented by F 1 (t) is less than that represented by F 2 (t), but    this situation can change because t 0 is a local maximum and so there exists the possibility of another local extreme. For example, if F 2 (t) is singular and F 1 (t) is regular, there will exist a neighborhood of infinity where F 2 ðtÞ F 1 ðtÞ is increasing. Example 3. Let F 1 ðtÞ ¼ 1 1þit be a hyperbolic discount function of parameter i and F 2 ðtÞ ¼ 1 ffiffiffiffiffiffiffiffi ffi Obviously, F 1 (t) < F 2 (t) and F 2 ðtÞ F 1 ðtÞ reaches a maximum at t 0 ¼ 1 i (see Fig 4 where i = 0.10). In accordance with the previous paragraph, d 1 1þi 2 t 2 in the interval ½0; 1 i ½¼ ½0; 10½, and contrarily δ 2 (t) is greater than δ 1 (t) in 1 i ; þ1½¼10; þ1½. We can now formulate the following statement. Theorem 3. Let F 1 (t) and F 2 (t) be two discount functions such that F 1 (t) < F 2 (t). If F 2 (t) − F 1 (t) reaches a local maximum at t 0 0 , then the factor F 2 ðtÞ F 1 ðtÞ reaches a local maximum at a later instant t 0 (eventually, t 0 can be +1).
Example 4. Observe that, for the discount functions of Example 1 with i 1 = 0.05 and i 2 = 0.10, F 2 (t) − F 1 (t) reaches its local maximum at t 0 0 ¼ 12:610 and t 0 = +1, as predicted by Theorem 3. Table 1 schematically represents the result obtained in Theorem 3. For the sake of simplicity, we will suppose that both F 2 (t) − F 1 (t) and F 2 ðtÞ F 1 ðtÞ reach a unique local maximum. Although t 0 is the instant which separates the intervals where δ 1 (t) > δ 2 (t) and δ 1 (t) < δ 2 (t), there are some intervals [t 1 , t 2 ], where t 1 < t 0 < t 2 , such that f 1 (t 1 , t 2 ) < f 2 (t 1 , t 2 ). In effect, given t 1 < t 0 , this instant t 2 must satisfy: The maximum value of t 2 must satisfy the following equation: Example 5. Let F 1 (t) and F 2 (t) be the discount functions of Example 3. Taking t 1 = 7, we have to solve the following equation (see Fig 5): for which the solution is t 2 = 14.367.
Finally, this reasoning can be continued by considering the following local extreme of F 2 ðtÞ F 1 ðtÞ (in this case, a local minimum), and so on. doi:10.1371/journal.pone.0149256.t001

Case in which the two functions intersect
For the sake of simplicity, in this Subsection, we will assume that functions F 1 (t) and F 2 (t) only intersect at an instant t 1 . In this case, we will distinguish between the following two subcases: • F 1 (t) and F 2 (t) are secant. This situation does not affect the results obtained in Theorems 2 and 3.
• F 1 (t) and F 2 (t) are tangent. In this case, F 2 ðtÞ F 1 ðtÞ reaches a local extreme at this point and so we can apply Theorem 3. More specifically, F 2 ðtÞ F 1 ðtÞ reaches a local minimum at t 1 (see Fig 6) and so, by Theorem 3, δ 1 (t) is less than δ 2 (t) on the left of t 1 , and contrarily δ 1 (t) is greater than δ 2 (t) on the right of t 1 . But observe also that F 2 ðtÞ F 1 ðtÞ reaches a local maximum at t 0 . Thus, the global situation can be summarized in Table 2.

An application to well-known discount functions
In experimental analysis, it is usual to fit the available data from several groups of individuals to discount functions belonging to the same family. It is therefore necessary to compare the impatience represented by two discount functions coming from the same general family. Comparison of two generalized hyperbolic discount functions These functions are the well-known q-exponential discount functions introduced by [41]. Let F 1 (t) and F 2 (t) be two generalized hyperbolic discount functions: and doi:10.1371/journal.pone.0149256.t002 where i 1 > i 2 . Let us calculate the first derivative of F 2 ðtÞ F 1 ðtÞ : d dt We are going to assume that s 1 6 ¼ s 2 . Otherwise, the comparison between F 1 (t) and F 2 (t) would be the same as two hyperbolic discount functions. Making this derivative equal to zero, we obtain: 1. If s 1 > s 2 , then t 0 < 0 and F 2 ðtÞ F 1 ðtÞ is increasing in R þ . Thus, by Theorem 2(iii), δ 1 (t) > δ 2 (t) and so the impatience represented by F 1 (t) is greater than the impatience represented by F 2 (t).
b. s 2 > s 1 i 1 i 2 , in which case t 0 < 0 and F 2 ðtÞ F 1 ðtÞ is increasing in R þ . Thus, again by Theorem 2, δ 1 (t) > δ 2 (t) and so the impatience represented by F 1 (t) is greater than the impatience represented by F 2 (t).
In order to compare the impatience of several well-known discount functions, in Table 3, we have considered the linear, hyperbolic, generalized hyperbolic, and exponential discounting both in the column on the left and on the upper row. Each cell of this table has been divided into three parts. We have represented the cases in which two discount functions F 1 (t) and F 2 (t) (F 1 (t) < F 2 (t)) satisfy the three equivalent conditions of Theorem 2. In this case, the first part of the cell shows the relationships to be satisfied by the parameters of F 1 (t) and F 2 (t) in order to satisfy Theorem 2. On the other hand, we have represented in bold those cases where F 1 (t) and F 2 (t) do not satisfy the conditions of Theorem 2. In theses cases, the first level of the cell exhibits the relationships between the parameters of F 1 (t) and F 2 (t) so that F 1 (t) < F 2 (t) in a neighborhood of zero; the second level of the cell includes the maximum t 0 of F 2 ðtÞ F 1 ðtÞ ; and, finally, the third level contains the maximum t 0 0 of F 2 (t) − F 1 (t). The relative position of these time instants was discussed in Theorem 3.

Conclusion
The term impatience was introduced by [2] in 1930 to refer to the preference for advanced timing of future satisfaction. More recently the concept of decreasing impatience has been applied to those situations in which discount rates are decreasing. Usually this property has also been labeled as hyperbolic discounting, although there are other discount functions involving decreasing discount rates. In this paper, we have focused on measuring the degree of impatience of discount functions in both intervals and instants.
In effect, in experimental research into impatience in intertemporal choice, the data from questionnaires are usually fitted to discount functions from different families of functions. Leaving aside the problem of whether this fitting is good, once the experimental discount functions corresponding to two groups of people have been obtained, there arises the problem of comparing the impatience exhibited by each of them.
At first glance, the faster the function decreases, the higher is the degree of impatience. That is, if F 2 (t) − F 1 (t) is increasing, the impatience shown by F 1 (t) is higher than the impatience shown by F 2 (t). But this graphic criterion only represents a condition sufficient to compare degrees of impatience. Nevertheless, it is convenient to state a necessary and sufficient condition for the impatience of F 1 (t) to be higher than the impatience of F 2 (t); this condition could be that the ratio F 2 (t)/F 1 (t) is increasing. In Theorem 2, that allows us to compare the impatience associated with two discount functions, we present two conditions equivalent to the former. Unfortunately, it is difficult to observe this property graphically in most cases (unless we consider the difference ln F 2 (t) − ln F 1 (t)). In other cases, however, the monotonicity of F 2 (t)/ F 1 (t) changes, necessitating the calculation of its maximum and minimum extreme values (Theorem 3). In most cases, the comparison of the impatience will be made using two discount functions belonging to the same family. Therefore, the problem of determining the local extremes of F 2 (t)/F 1 (t) can be solved explicitly or, at least, their existence must be demonstrated.
The main contributions of this paper are Theorems 2 and 3. In Table 3 we compare the impatience shown by pairs of discount functions belonging to the most important families of temporal discounting (linear, hyperbolic, generalized hyperbolic and exponential discount functions). Thus, a restriction in red represents a condition sine qua non for a pair of discount functions in order to satisfy Theorem 2. Nevertheless, there are other pairs of discount functions not satisfying the conditions of Theorem 2. In this case, we have deduced (as shown in bold) the expressions of t 0 and (the equation to be satisfied by) t 0 0 which allows us to check the statement in Theorem 3.
These Theorems allow us to compare the impatience shown by two individuals or two groups of people, once their preferences have been fitted to a suitable discount function belonging to a well-known family of functions. Finally, this methodology can be applied to two-variable (amount and time) discount functions when some anomalies in intertemporal choice (for example, delay or magnitude effect) are taken into account.