GCOE Discussion Paper Series
Global COE Program

Human Behavior and Socioeconomic Dynamics
Discussion Paper No.123

Risk and Uncertainty in Health Investment

Takao Asano and Akihisa Shibata

March 2010

GCOE Secretariat
Graduate School of Economics

OSAKA UNIVERSITY

1-7 Machikaneyama, Toyonaka, Osaka, 560-0043, JapanRisk and Uncertainty in Health Investment ∗Takao Asano †Akihisa Shibata ‡Faculty of Economics
Okayama University

Institute of Economic Research
Kyoto University

March 8, 2010

Abstract
Extending the Grossman [12] model of health capital into a stochastic one, we analyze how the presence of Knightian uncertainty about
the eﬃcacy of health care aﬀects the optimal health investment behavior of individuals. Using Gilboa and Schmeidler’s [11] model ofmaxmin expected utility (MMEU) with multiple priors, we show thatan agent retains the initial level of health capital if the price of healthcare lies within a certain range. We also show that the no-investment
range expands as the degree of Knightian uncertainty rises.
Journal of Economic Literature Classification Numbers: D81, I11
Key Words: Health Investment, Risk, Uncertainty∗
We would like to appreciate two anonymous referees for their very helpful comments
and suggestions. We acknowledge financial support from the Grant-in-Aid for Scientific
Research, JSPS, and the Global Center of Excellence (GCOE) program, the Ministry of
Education, Culture, Sports, Science, and Technology. Needless to say, we are responsible
for any remaining errors.
†
Faculty of Economics, Okayama University, 3-1-1 Tsushimanaka, Kita-ku, Okayama
700-8530, Japan. e-mail: asano@e.okayama-u.ac.jp. Tel: +81-86-251-7668‡
Institute of Economic Research, Kyoto University, Yoshida, Sakyo-ku, Kyoto 6068501, Japan. e-mail: shibata@kier.kyoto-u.ac.jp. Tel: +81-75-753-7126

1Introduction

Uncertainty on the quality of products in medical care is more intense thanthat in standard commodities, as Arrow [1] pointed out in a seminal article.
This study is an analysis of the eﬀect of uncertainty on health care behavior. In the course of the study, a distinction is made, along the lines thatof Knight [15], between “uncertainty” and “risk.” Knight argued that risk
is characterized by randomness that can be measured precisely while uncertainty relates to randomness that cannot be expressed by specific probability
distributions. According to this terminology, risk takes place when a uniqueprobability measure is assigned to each event, and uncertainty takes placewhen only an imprecise information is available to assign a unique probability to each event. Based on Knight’s argument, Ellsberg [10] suggested theempirical importance of the distinction between risk and uncertainty.1
Extending Grossman’s [12] model of health capital, in this study, we
analyze how this kind of “uncertainty” about the eﬃcacy of health care affects health investment behavior, and show that an agent retains the initiallevel of health capital if the price of health care lies within a certain range.
We also show that the no-investment range expands as the degree of Knightian uncertainty rises. It is natural to assume that if people remain lessconfident about their health condition and the eﬃcacy of health investment,
then they will hesitate to invest in their health capital because of the lack ofprecise information on their health condition. Let us consider the problem
of being overweight. According to Puska, Nishida, and Porter [21], there
exist 1 billion overweight adults, at least 300 million of them are clinically1
Ellsberg conjectured that individuals would prefer bets with precise odds to thosewith imprecise odds. For example, consider a bet on drawing a yellow ball from two urns.
An urn includes 50 yellow and 50 white balls, while, the other urn includes 100 yellow
and white balls, but the exact numbers of the two kinds of balls are unknown. Ellsberg
asserted that, in this situation, most decision-makers would prefer the first urn to thesecond. His conjecture was confirmed by several experimental studies, such as those by
Becker and Brownson [5] and Slovic and Tversky [24]. See Oliver [19] and Wakker [25]for a comprehensive account of the Ellsberg paradox in the context of health economics.

1

obese. Puska, Nishida, and Porter [21] state that “obesity and overweightpose a major risk for serious diet-related chronic diseases, including type 2
diabetes, cardiovascular disease, hypertension and stroke, and certain formsof cancer.” Even though we recognize that losing our weight is necessary
for our health, in general, quite a few of us do not diet in order to lose
their weight. This paper shows that we tend to behave in this way when weare cautious about the eﬀects of eﬀorts such as cutting the amount of fatty
and sugary foods and engaging in daily physical exercise. The reason thatpeople have diﬃculty in controlling their diet can be also explained fromthe point of view of time-inconsistency caused by hyperbolic discounting as
mentioned in Laibson [14]. Or it may be because people cannot resist thetemptation of good food and have a self control problem as analyzed in Gul
and Pesendorfer [13]. This paper provides another angle to understand theoverweight problem from the Knightian uncertainty viewpoint.
While this paper studies the optimal health investment under Knightian uncertainty and shows that the degree of Knightian uncertainty can
reduce the amount of health investment, in the previous literature on healthinvestment, there are several related studies which also analyze the optimal
health investment under stochastic environments. Using a stochastic versionof the Grossman model, Dardanoni and Wagstaﬀ [8] showed that, under mild
conditions, a greater risk stimulates health care (health investment). Selden[23] showed, however, that the result by Dardanoni and Wagstaﬀ can be reversed according to how the risk is introduced. Applying Arrow’s [2] modelof portfolio selection to health care management, Chang [7] constructed aunified model including Dardanoni and Wagstaﬀ’s and Selden’s models as
its special cases. Moreover, Picone, Uribe and Wilson [20] constructed a
dynamic version of the Grossman model and examined the eﬀects of uncertainty on health investment. Simulating the model, they showed that either
a first-order stochastic dominant shift or a second-order stochastic dominant
shift in risk will stimulate health capital investment, especially in the early2

stage of life.
All of the aforementioned studies, however, included only risk factors
and, therefore, were limited to the eﬀects of risk on health investment behavior, while the role of uncertainty was entirely ignored. Because mostpeople have only imprecise information about the eﬃcacy of health carebehavior, it is quite natural to formulate decision-making problems by introducing Knightian uncertainty. Thus, in this paper, using a version of the
maxmin expected utility (MMEU) model of Gilboa and Schmeidler [11], weconstruct a simplified version of the Grossman model with Knightian uncertainty and analyze how the presence of Knightian uncertainty aﬀects thehealth investment behavior of individuals.
In the MMEU theory, an agent’s beliefs are captured by a set of probability measures and his preferences are represented by the minimum of
expected utilities over the set of probability measures. In a closely related
paper, Schmeidler [22] axiomatized Choquet expected utility (CEU) theory
and showed that an agent’s beliefs are captured by a non-additive measure and his preferences are represented by the Choquet integral. Moreover,
Schmeidler [22] proved that, when an agent’s beliefs are captured by a convex non-additive measure, CEU is equivalent to a special case of MMEU,
which endows CEU with the meaning of uncertainty aversion and makes it
possible to analyze his behavior under Knightian uncertainty through theconvexity of non-additive measures.2 Because we would like to consider a2
Let μ be a convex non-additive measure,be the set of probability measures on(S, 2S ), and let B(S, ) denote the space of bounded functions from S into . Define|(∀E ∈ 2S ) μ0 (E) ≥ Q(E) ≥ μ(E) , where μ0 is thethe core of μ by core(μ) = Q ∈conjugate of μ. Then(∀X ∈ B(S, ))X(s)μ(ds) = min

X(s)Q(ds) Q ∈ core(μ) ,where the left-hand side of the equality is in the sense of the Choquet integral. This relation
implies that it is the convexity of non-additive measures that serves the bridge between the
MMEU theory and the CEU theory, and makes it possible to analyze the decision-makes’
behavior under Knightian uncertainty through the Choquet integral. For definitions of
non-additive measures, the convexity, and the Choquet integral, see Appendix.

3

situation in which an agent is uncertainty-averse, we assume that his beliefs
are described by a convex non-additive measure and analyze our MMEU
maximizer’s problem using the CEU theory.3
The organization of this paper is as follows. Section 2 is an illustrative
example based on the main result of this study. Section 3 is the main resultof this study presented in a formal framework of the CEU theory. Section 4contains further analyses of the main result. Section 5 is the conclusion. The
definitions and some mathematical results regarding the Choquet integrals
and proofs of theorems in this study are in the Appendix.

2

An Illustrative Example

Incorporating Knightian uncertainty into a simple two-period version ofGrossman’s [12] model of health capital, we provide an illustrative example
that shows that an agent keeps the initial level of health capital if the priceof health care lies within a certain range.4 Suppose that there is an agent
who invests in his health capital, either in a positive or a negative amount.5
Furthermore, we assume that he is an MMEU maximizer and that he determines his health investment on the basis of the worst-case scenario, thatis, his beliefs are captured by a set of probability measures, and his

preferences are represented by the minimum of expected utilities over.

Let p be the price of health care at t = 1, and let X(s) be the healthcondition at t = 2 in state s. We assume here that there are two states, good3

The models by Gilboa and Schmeidler [11] and Schmeidler [22] can explain the Ellsberg
paradox fairly well and have been widely employed by applied studies. See, for example,
Dow and Werlang [9] and Berliant and Konishi [6].
4This example is based on Nishimura and Ozaki [18].
5
Typical examples of negative investment are to smoke heavily and to drink heavily.
It seems natural to consider only non-negative health investment. Even if we restrict ouranalysis to the case of non-negative health investment, we can easily obtain essentially thesame results as those derived in this paper, that is, we can show that there exists a lower
bound beyond which he never invests in his health capital and the lower bound becomes
smaller as the degree of Knightian uncertainty increases.

4

(g) or bad (b), and that the good and bad health conditions are X(g) = G
with probability qg and X(b) = B with probability 1 − qg , respectively.6
Moreover, we assume that he is risk-neutral and an MMEU maximizer andthatis the set of probability measures qg .7
The expected return from a positive health investment is representedas−p + min {qg G + (1 − qg )B | qg ∈

}.

(1)

Similarly, the expected return from a negative health investment is given by
p+min {− (qg G + (1 − qg )B) | qg ∈} = p−max {qg G + (1 − qg )B | qg ∈},(2)
where ”negative health investment” stands for the agent’s behavior thatcauses damage on individuals’ health. As easily understood, the agent makes
a positive health investment if (1) is positive, and he makes a negative healthinvestment if (2) is positive. If both (1) and (2) are negative, he never makes
this health investment, that is, he does nothing for his health capital andkeeps his initial health capital intact if
min {qg G + (1 − qg )B | qg ∈} < p < max {qg G + (1 − qg )B | qg ∈}.
(3)

For simplicity, we assume that the true probability of his good healthcondition, qg , is 1/2, but because of the presence of Knightian uncertainty,
he is not confident about qg . In order to capture this situation, we assume
here that his beliefs are represented by a set of probability measures=[1/2−ε, 1/2+ε] for any ε ∈ (0, 1/2),8 where we can interpret ² as the degree
6

We assume that G > B.
Since there exists Knightian uncertainty about his health condition at t = 2, his beliefs
cannot be captured by a unique probabilty measure.
8This class of multiple priors is referred to as ε-contaminations. See Nishimura andOzaki [17] for an axiomatic foundation of the ε-contamination. In Section 4, we provide thedefinition of ε-contaminations, and analyze the eﬀect of increases in Knightian uncertainty
on an interval in which the agent never invests in her health capital.
75

of his error about the true probability. Under this setting, we havemin {qg G + (1 − qg )B | qg ∈andmax {qg G + (1 − qg )B | qg ∈1
1
} = G + B − ε(G − B) (4)2211} = G + B + ε(G − B). (5)22Under our assumption that G > B and ε > 0, it is apparent from (4) and (5)that we can have a situation in which (3) holds, that is, there is an interval inwhich he never invests in his health capital. Moreover, an increase in ε leads
to a decrease in the right-hand side of (4), and an increase in ε leads to anincrease in the left-hand side of (5), which implies that the interval in which
he never invests in his health capital expands as ε increases. Since an increasein ε implies an increase in Knightian uncertainty, this result implies that alesser confidence in the probability of his good health condition discourages
his health investment more severely. Finally, we consider a situation in which
ε = 0, that is, there is no Knightian uncertainty in the probability of his
good health condition. In this situation, (4) is equal to (5), which implies
that there is no interval in which he never invests in his health capital.

3

Health Investment under Knightian Uncertainty

In this section, introducing Knightian uncertainty into a simple two-period
version of the Grossman [12] model, we provide a formal model of healthinvestment. In order to analyze how the presence of Knightian uncertainty
about the eﬃcacy of health care aﬀects individual health investment behavior, we assume that an agent is a CEU maximizer, that is, his beliefs arecaptured by a convex non-additive measure, and his preferences are represented by the Choquet expected utility. Under this assumption, we accountfor the existence of an interval in which he never invests in his health capital.
As we explain in Section 2, the existence of the interval is not accounted
for if an agent maximizes the standard expected utility, that is, if his beliefs
are captured by a unique probability measure and his preferences are rep6

resented by the expected utility. Before we present the main result of thisstudy, we will briefly explain the basic setup.
Let S be the space of the states of the world, let 2S be the power set ofS, and let (S, 2S ) be a measurable space. Let B(S, R) denote the space ofbounded functions from S into R. Let X(s) denote the health condition instate s at time t = 2, where X ∈ B(S, R). Let H1 and H2 be health capitals
at t = 1 and = 2, respectively, and let H2 : R × R → R be defined by
H2 (X(s), M ) ≡ (1 − δ)H1 + M X(s),where δ denotes the depreciation rate and M ∈ R denotes the amount ofhealth investment.9 We assume that an agent’s income depends on the levelof his health capital and that the income-generating function Ψ : R → R
is a function of the health capital at t = 2, H2 , where Ψ is concave andcontinuously diﬀerentiable with Ψ0 (·) > 0 and Ψ00 (·) ≤ 0.10
Let A, Y1 and p denote the initial wealth, the income at t = 1, and theprice of health capital, respectively. We assume that u : R → R is a concave

and twice-continuously diﬀerentiable function with u0 (·) > 0 and u00 (·) ≤ 0.
Moreover, we assume that an agent is uncertainty averse and that he has
to decide whether or not to make a health investment at t = 1.

11

The

objective is to choose the amount of the health investment M in order to

9Even if we consider an additive case such as
H2 (X(s), M ) = (1 − δ)H1 + I(M ) + X(s),the result is not essentially aﬀected. That is, we can derive the existence of the noinvestment intaval. Note that in this additive case, the interval obtained is not the sameas the one in Theorem 1.
10Chang [7] defines H2 :→by H2 (M ) = (1 − δ)H1 + I(M ), where I :→denotes some production function satisfying some regularity conditions. Moreover, heis a real-valued function
assumes that the income-generating function Ψ : × →of the amount of the health investment M and some random variable ε, satisfying someregularity conditions, denoted by Ψ(H2 , ε). If we assume that limM →0 I 0 (M ) exists andis finite, then we can also derive our main result within his framework.
11
As we explain in Introduction, we assume that agent’s beliefs are captured by a convex
non-additive measure, and her preferences are represented by the Choquet integral.

7

maximizeZu ((A + Y1 − pM ) + Ψ(H2 (X(s), M ))) μ(ds),where μ is a convex non-additive measure and the integral is in the senseof the Choquet integral. This agent is assumed to maximize his Choquet
expected utility of his terminal wealth with a convex non-additive measure.
In this setting, we have the following theorem.
Theorem 1. Suppose that the price of health capital p satisfies the following:
Z
Z
dΨ
dΨ
X(s)μ(ds) < p <X(s)μ0 (ds),
S dH2
S dH2
where μ0 is the conjugate of μ. Then, the agent never invests in this healthcapital.
The integrand dΨ/dH2 X denotes the marginal product of the healthinvestment evaluated at M = 0. If this agent’s beliefs are captured by aprobability measure, then the interval does not exist. This theorem implies that the decreased confidence in his health condition at t = 2 makes
him hesitate about the health investment in either a positive or a negative
direction.
It is worth mentioning the economic mechanism that leads to our main
result. In the two-period model of our paper, the reason that individuals invest in health is because it will bring a better health status tomorrow, which
in turn will increase the income tomorrow. The presence of Knightian uncertainty makes the health condition tomorrow very uncertain. It discounts
the expected return of the health investment and makes individuals more
likely not engaged in the health investment.

4

An Increase in Knightian Uncertainty

In this section, we analyze the eﬀect of increases in the degree of Knightian
uncertainty on the interval in which the agent does not invest in his health8capital and show that an increase in the degree of the Knightian uncertainty
expands the length of the interval. For that purpose, we assume that his
beliefs are captured by the ε-contamination of the true probability measureP0 .12First, a formal definition of ε-contaminations is presented. Let P0 be
the true probability measure about his health condition at t = 2, and letε ∈ (0, 1). The ε-contamination of P0 is defined byP0 ≡ {(1 − ε)P0 + εQ | Q ∈}.

If his beliefs are captured by the ε-contamination of P0 , then, with (1 −ε) × 100%, he is certain about the true probability measure of his future
health condition, but, with ε × 100%, he fears that his beliefs in his future
health condition are completely wrong. If ε = 0, then the set of probability
measures P0 is reduced to the singleton, P0 . An increase in ε implies thatan agent is more uncertain about the true probability measure P0 . Thus, εcan be considered to be a parameter that captures the degree of Knightian
uncertainty.13
Let ε0 and ε be in (0, 1), let ε0 > ε, and define θ1 and θ2 by½(1 − ε0 )P0 (E) if E 6= S(∀E) θ1 (E) =1
if E = S½(1 − ε)P0 (E) if E 6= S(∀E) θ2 (E) =1
if E = S,(6)(7)

where E is an event and P0 is a probability measure on (S, 2S ). It can beshown that θ1 and θ2 are convex non-additive measures and that core(θ1 ) =P1 and core(θ2 ) = P2 , where P1 ≡ {(1 − ε0 )P0 + ε0 Q | Q ∈{(1 − ε)P0 + εQ | Q ∈} and P2 ≡}. Therefore, the intervals of an agent whose beliefs

are represented by the ε0 -contamination of P0 and the ε-contamination of
12
As we explain in this section, the ε-contamination of P0 can be represented by core ofsome convex non-additive measure. Therefore, an agent can be considered to be a CEU
maximizer as in Section 3. For the relation between MMEU and CEU, see footnote 2.
13
For applications of ε-contaminations to economics, see Nishimura and Ozaki [16] andAsano [3].

9

P0 areZZdΨdΨX(s)θ1 (ds) < p <X(s)θ10 (ds), and
S dH2
S dH2
Z
Z
dΨ
dΨ
X(s)θ2 (ds) < p <X(s)θ20 (ds),
S dH2
S dH2

respectively.
Our main theorem on the eﬀect of Knightian uncertainty is as follows:
Theorem 2. If the degree of Knightian uncertainty increases, then the interval in which the agent does not invest in his health capital expands.
Proof. See Appendix.
This theorem states that the more uncertainty averse he is about his
future health condition, the more he hesitates about his health investment
in either a positive or a negative direction. As mentioned in Introduction,even though we recognize that losing our weight is necessary for our health,in general, quite a few of us do not diet in order to lose their weight. Thiskind of behavior can be explained by Theorem 2. Moreover, this theorem
implies that the more cautious we get about eﬀects of diet, the less we diet.

5

Conclusion

Introducing Knightian uncertainty into the Grossman [12] model of healthcapital, we analyzed the eﬀect of Knightian uncertainty on the health investment behavior of individuals. We showed that, in the presence of Knightian
uncertainty, when the price of health care lies within a certain interval, an
agent does nothing, that is, he keeps the initial level of his health capital.
We also showed that the length of the no-investment interval expands as thedegree of uncertainty increases.

10Appendix
Let (S, 2S ) be a measurable space. A set function μ : 2S → [0, 1] is a
non-additive measure (or normalized capacity) if (a) μ(∅) = 0, (b) μ(S) = 1,
and (c) E, F ∈ 2S and E ⊂ F imply μ(E) ≤ μ(F ), where ∅ denotes theempty set. A non-additive measure μ is convex if μ(E ∪ F ) + μ(E ∩ F ) ≥μ(E) + μ(F ) for all E, F ∈ 2S . Let μ be a non-additive measure on (S, 2S ).

Then, the conjugate μ0 of μ is defined by μ0 (E) = 1 − μ(E c ) for all E ∈ 2S ,where E c denotes the complement of E.

Let B(S, R) denote the space of bounded functions from S into R and
let X ∈ B(S, R). The integral of X with respect to a non-additive measureμ is called the Choquet integral, and is defined byZ
X(s) μ(ds)
Z ∞
Z 0
=
μ ({s ∈ S| X(s) ≥ α}) dα +[μ ({s ∈ S| X(s) ≥ α}) − 1] dα,0−∞where integrals on the right hand side are in the sense of Riemann integrals.
Fact 1.
(∀X, Y ∈ B(S, R)) X ≥ Y ⇒Z

X(s) μ (ds) ≥Z

Y (s) μ (ds).

Fact 2.
(∀X ∈ B(S, R))(∀a ∈ R)(∀b ∈ R+ )Z

(a+bX(s))μ (ds) = a+bZX(s)μ (ds).

Fact 3.
(∀X ∈ B(S, R))Z

X(s) μ(ds) = −Z(−X(s)) μ0 (ds),where μ0 is the conjugate of μ.
Fact 4 (Schmeidler [22]). Let μ be a convex non-additive measure, and let
X ∈ B(S, R). Then
½Z
¾
Z
¯
¯
X(s)μ(ds) = min
X(s)Q(ds) ¯ Q ∈ core(μ) , and
S

S

11

Z

0

S

X(s)μ (ds) = −ZS−X(s)μ(ds) = max

½Z

S

¾
¯
¯
X(s)Q(ds) ¯ Q ∈ core(μ) .

Fact 5. Let X, Y ∈ B(S, R), and let μ be a convex non-additive measure.
Then,
Z

S

(X(s) + Y (s))μ(ds) ≥Z

X(s)μ(ds) +SZ

Y (s)μ(ds).

S

Let μ be a convex non-additive measure, let X ∈ B(S, R), and define(μ, X) by(μ, X) ≡ argmin
Since μ is convex, the set

½Z

¾
¯
¯
XdQ ¯ Q ∈ core(μ) .

(μ, X) is non-empty.

Let h : S × R → R satisfy the following:

Then,

R

(∀z ∈ R) h(·, z) is measurable(8)(∀z ∈ R) h(s, ·) is diﬀerentiable and concave(9)

S h(s, z)μ(ds) is concave with respect to z if μ is convex.

Proposition 1. Let μ be a convex non-additive measure, and let h : S ×R →R satisfy the conditions (8) and (9). Then
½Z
Z
¯
∂h
d
¯
h(s, z)μ(ds) = max(s, z)Q(ds) ¯ Q ∈(∀z ∈ R)
dz−
∂z
½Z
Z
¯
∂h
d
¯
(∀z ∈ R)h(s, z)μ(ds) = min(s, z)Q(ds) ¯ Q ∈dz+∂z(μ, h(·, z))¾(μ, h(·, z)) ,where d/dz− and d/dz+ denote the right derivative and the left derivative,
respectively.
Proof. See Aubin [4].
Proof of Theorem 1. Let f : R → R be defined by f (M ) =Ru ((A + Y1 − pM ) + Ψ(H2 (X(s), M ))) μ(ds). From Fact 5 together with12¾

the assumptions that Ψ and u are concave, it follows that f is a concave
function with respect to M . Therefore, M = 0 is optimal if
Z
¯
d
¯
0
u ((A + Y1 − pM ) + Ψ(H2 (X(s), M ))) μ(ds)¯f+ (0) ≡
dM+ S
M =0
Z
¯
d
¯
<0<
u ((A + Y1 − pM ) + Ψ(H2 (X(s), M ))) μ(ds)¯≡ f−0 (0).
dM− S
M =0

From Proposition 1, it follows that
µ
¶
¾
½Z
¯
dΨ
¯
0
u (A + Y1 + Ψ(H2 (X(s), 0))) −p +X(s) Q(ds) ¯ Q ∈ (μ, u(W ))
min
dH2
S
½Z
µ
¶
¾
¯
dΨ
¯
0
< 0 < maxu (A + Y1 + Ψ(H2 (X(s), 0))) −p +X(s) Q(ds) ¯ Q ∈ (μ, u(W ))
dH2
S
½Z
¾
½
¾
Z
¯
¯
dΨ
dΨ
¯
¯
⇔ min
X(s)Q(ds) ¯ Q ∈ core(μ) < p < max
X(s)Q(ds) ¯ Q ∈ core(μ)
S dH2
S dH2
Z
Z
dΨ
dΨ
X(s)μ(ds) < p <X(s)μ0 (ds),
⇔
S dH2
S dH2
where the first equivalence follows from u0 (·) > 0 and(μ, u(W )) = core(μ),

and the second equivalence follows from Fact 4.
Proof of Theorem 2. In order to prove Theorem 2, we provide the conceptof being more uncertainty averse proposed by Dow and Werlang [9] andpresent mathematical results related to the concept. Let μ, μ1 , and μ2
be non-additive measures on (S, 2S ), and let c(μ, A) ≡ 1 − μ(A) − μ(Ac ).
Dow and Werlang [9] state that μ1 is more uncertainty averse than μ2 ifc(μ1 , A) ≥ c(μ2 , A) for all A.
Theorem 3 (Dow and Werlang [9]). Let μ1 and μ2 be non-additive measureson (S, 2S ). Then, the following statements are equivalent.
(i) μ1 is more uncertainty averse than μ2 .
(ii) For any X ∈ B(S, R),Z
ZZZ
X(s)μ01 (ds) − X(s)μ1 (ds) ≥ X(s)μ02 (ds) − X(s)μ2 (ds),where μ01 and μ02 are conjugates of μ1 and μ2 , respectively.

13

Lemma 1. Let ε0 and ε be in (0, 1). Let θ1 and θ2 be defined by (6) and(7), respectively. If ε0 > ε, then c(θ1 , E) ≥ c(θ2 , E) for all E.
Proof. The proof is immediate.
Combining Theorem 3 and Lemma 1 proves Theorem 2.

14

References[1] Arrow, K. J. (1963): “Uncertainty and the Welfare Economics of Medical Care,” American Economic Review 53, 941-973.
[2] Arrow, K. J. (1965): Aspects of the Theory of Risk Bearing, Yrjo Jahnsonin Saatio, Helsinki.
[3] Asano, T. (2006): “Portfolio Inertia and Epsilon-Contaminations,”
ISER Discussion Paper No. 610, forthcoming in Theory and Decision.
[4] Aubin, J. P. (1979): Mathematical Methods of Game and Economic
Theory, North-Holland, Amsterdam.
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