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Human responses to unfairness with
primary rewards and their biological
limits
Nicholas D. Wright, Karen Hodgson, Stephen M. Fleming, Mkael Symmonds, Marc Guitart-Masip
& Raymond J. Dolan
Wellcome Trust Centre for Neuroimaging, Institute of Neurology, University College, London, 12 Queen Square, London, WC1N
3BG, UK.
Humans bargaining over money tend to reject unfair offers, whilst chimpanzees bargaining over primary
rewards of food do not show this same motivation to reject. Whether such reciprocal fairness represents a
predominantly human motivation has generated considerable recent interest. We induced either moderate
or severe thirst in humans using intravenous saline, and examined responses to unfairness in an Ultimatum
Game with water. We ask if humans also reject unfair offers for primary rewards. Despite the induction of
even severe thirst, our subjects rejected unfair offers. Further, our data provide tentative evidence that this
fairness motivation was traded-off against the value of the primary reward to the individual, a trade-off
determined by the subjective value of water rather than by an objective physiological metric of value. Our
data demonstrate humans care about fairness during bargaining with primary rewards, but that subjective
self-interest may limit this fairness motivation.
In humans fairness has been studied extensively using games played for money
1
. The paradigmatic example is
the Ultimatum Game (UG)
2
where one player (the Proposer) is given an endowment (e.g. £10) and proposes a
division (e.g. keep £6/offer £4) to a second player (the Responder), who then accepts (both get the proposed
split) or rejects (both get nothing) the offer. In the UG with money humans typically reject low, ‘‘unfair’’, offers
even at cost to themselves
1
Here, we asked if thirsty humans in an UG instead make self-interest maximising
responses to unfair offers with a primary reward of water, and attempted to maximise our power to induce such
self-interested behaviour.
We sought to enhance our power to induce self-interested behaviour in two ways. Firstly, we physically
presented the water, which has been recently shown to increase food’s propensity to trigger appetitive responses
3
.
Secondly, we amplified this effect by inducing thirst. Such induction of thirst in humans is a non-trivial pro-
blem experimentally, for example requiring prolonged water deprivation under supervision (e.g. over at least
24 hours
4
); or extended heating and exercise as typically employed in military research
5–7
; or intravenous infusion
of hypertonic saline, which given the careful setup needed to prevent potential health risks has previously been
used to study the physiology of thirst in small numbers of subjects under clinical supervision
8,9
. Here we used an
intravenous saline infusion, as this method enabled a double-blind design allowing us to test UG choices in
individuals who were not in a deprived state (infusing isotonic saline similar to normal human osmolarity, so with
little impact on thirst), and also in individuals who received hypertonic saline that markedly increases blood
osmolarity (and thus thirst) and that might increase our power to induce self-interested behaviours.
This manipulation with primary rewards also enabled the possibility of providing insight into the nature of the
self-interested motivation in the UG, although we note that our experiment was not specifically designed to
address this subsidiary issue. Specifically, we probed whether objective need, subjective need, or both are traded-
off against a putative fairness motivation. Objective physiological need, for example the degree of energy deple-
tion, powerfully influences non-social value-based choice. This is seen even in organisms as simple as the
grasshopper, in which the value of stimuli is higher when learning about those stimuli originally occurred in a
lower than higher state of nutritional reserve
10
. However, when humans choose in an UG, even an increase in the
stakes equivalent to many months’ salary does not necessarily increase acceptances
1,11
. In contrast, subjective
motivations in the UG have been highlighted more recently in relation to a fairness motivation, with disruption to
SUBJECT AREAS:
BEHAVIOUR
ANIMAL BEHAVIOUR
NEUROENDOCRINOLOGY
SYSTEMS BIOLOGY
Received
2 May 2012
Accepted
26 July 2012
Published
23 August 2012
Correspondence and
requests for materials
should be addressed to
N.D.W. (n.wright@fil.
ion.ucl.ac.uk)
SCIENTIFIC REPORTS | 2 : 593 | DOI: 10.1038/srep00593 1
right dorsolateral Prefrontal Cortex (dlPFC) using Transcranial
Magnetic Stimulation (rTMS) leading to reduced rejections but leav-
ing subjective assessments of fairness unaffected
12
.
Finally, whilst in the UG with money humans typically reject low
offers
1
, chimpanzees with a food primary reward behave solely as
self-interested maximisers in an UG and accept unfair offers
13
. Our
data help address one potential source of this discrepancy, although
we note a more complex picture emerges across different primate
species and tasks
14,15
, while other potential sources for this discre-
pancy may relate to inter-species differences in time horizon
16
.
In summary, we tested the behaviour of humans in an UG with
primary rewards, against the hypothesis that they would be rational
self-interest maximisers who would accept any offer. We sought to
maximise our chances of eliciting such self-interested behaviour by
physically presenting the water, and also by examining choice not
only in a non-deprived state (i.e. following isotonic infusion) but also
following a manipulation that might increase self-interested choices
(i.e. following hypertonic infusion). We also hoped to provide tent-
ative evidence concerning whether objective and subjective measures
of need might impact on a possible fairness motivation.
Results
Thirst manipulation.As predicted, administering hypertonic saline
markedly altered objective and subjective measures relating to thirst.
Subjective thirst did not differ between treatments at t
baseline
(hyper-
tonic 2.561.9 and isotonic 2.561.7; t(19)50.057, P50.96, d50.02),
but differed at t
UG
(7.361.6; 3.562.0; t(19)54.68, P,0.0005,
d52.06). Similarly, osmolarity at t
baseline
did not differ between
treatments (hypertonic 293 mOsmL
21
6s.d. 4; isotonic 29567;
t(19)51.27, P50.22, d50.57), but at t
UG
was higher for the
hypertonic (31065) than isotonic group (29565; t(19)57.58,
P53.7310
27
,d53.30).
Fairness influences choice.Our data show fairness influenced
responses in the UG despite the use of a primary reward. In an UG
with a primary reward of food, chimpanzees in a non-deprived state
accepted almost all offers, shown by only around 1 out of the 11
individuals tested rejecting a low offer similar to that presented
here
13
. However, our human subjects, who were also in a non-
deprived state (i.e. following isotonic infusion), showed the opposite
pattern with 8 of 11 individuals rejecting the unfair offer (binomial test
versus no influence of fairness, P,0.001). Furthermore, even
following the induction of severe thirst in the hypertonic group, we
still observed an effect of fairness with 5 of 10 individuals rejecting the
unfair offer (binomial test versus no influence of fairness, P,0.001).
There was no difference between groups (hypertonic versus isotonic)
in the proportion of rejections (likelihood ratio test between groups,
X251.16, P.0.25), as we discuss further below. When we compare
the frequency of rejections here with that previously reported for
similarly low monetary offer proportions (approximately 50–60%
rejections, see Methods for details), we find this was not significant
for either the isotonic or hypertonic group (likelihood ratio test P.0.3
in both groups).
Subjective and objective measures of self-interest.In addition to
our central question of whether humans were rational self-interested
maximisers with primary rewards, we next sought evidence that this
fairness motivation was traded-off against self-interest. When self-
interest was defined as subjective thirst, measured by a rating scale,
this was the case. Subjectively thirstier individuals at t
UG
were more
likely to accept the unfair offered water, indicated by a main effect of
choice in a 2 choice (accept, reject) by 2 treatment (isotonic, hyper-
tonic) ANOVA with subjective thirst as the dependent variable
(main effects of choice F
(1,17)
59.37, P50.007, g
p
2
50.36; and treat-
ment F
(1,17)
519.53, P,0.0005, g
p
2
50.54; with no interaction,
F
(1,17)
50.15, P50.7, g
p
2
50.01). Further, our data revealed that the
degree to which hypertonic infusion increased subjective thirst was
related to choice. This was evident in a significant interaction of
choice (accept, reject) and treatment (isotonic, hypertonic) in an
ANOVA with change in subjective thirst as the dependent variable
(interaction F
(1,17)
57.19, P50.016, g
p
2
50.30; main effect of treat-
ment F
(1,17)
527.40, P,0.0001, g
p
2
50.62; no main effect of choice,
F
(1,17)
53.52, P50.078, g
p
2
50.17). This interaction was driven by the
degree to which hypertonic saline increased subjective thirst
(Fig. 1b).
However, our objective measure of thirst (blood osmolarity) was
not related to choice, either when used as the dependent variable or as
a covariate in the previous ANOVAs. Furthermore, there was no
difference between groups (hypertonic versus isotonic) in the pro-
portion of rejections (likelihood ratio test between groups, X
2
51.16,
P.0.25). Thus, although our experiment was primarily designed to
ask whether humans behaved as rational self-interest maximisers in
an UG with primary reward, together these results present tentative
evidence that the primary driver of the self-interested motivation
here was subjective, rather than objective, thirst.
Discussion
Humans’ closest relatives, chimpanzees, appear to be rational self-
interest maximisers who do not reject unfair offers in the canonical
fairness task, the UG, with primary rewards
13
. In contrast, here we
Figure 1
|
Change in subjective thirst and responses to unfairness. (a) All participants were assigned the Responder role in an UG and faced this
proposed division of water. They had received an intravenous infusion of either isotonic or hypertonic saline. (b) The change in subjective thirst induced
by the saline infusion is calculated by the difference in subjective thirst at baseline and at testing. In the hypertonic group the degree to which the infusion
increased subjective thirst was related to choice, such that a greater increase was seen in those who accepted (6.461.7) relative to those who rejected
(3.161.6; independent samples ttest, t
(8)
53.19, P50.013, d52.02). Error bars indicate s.e.m.
www.nature.com/scientificreports
SCIENTIFIC REPORTS | 2 : 593 | DOI: 10.1038/srep00593 2
show that humans remain motivated by fairness even with primary
rewards and in a deprived state. Outside such a bargaining context,
capuchin monkeys have been reported to be more likely to reject a
cucumber slice after seeing that another capuchin has received a
more attractive grape
14
, although we note debate concerning these
findings
17
; for a more general discussion of experiments on social
preferences in primates see the review by
15
. Further, in contrast to
that behaviour reported in capuchins
14
, humans tend not to reject
inequitable distributions of money when such rejection would have
no effect on the proposer’s payoffs, for example in a modification
of the UG where rejection does not alter the Proposer’s payoff
(an ‘‘Impunity game’’) the rate of rejections is markedly reduced
18
.
Taken together, such work across species and tasks has begun to
delineate the particular contexts in which particular species may
exhibit particular fairness-related behaviours. We do not suggest that
fairness-related behaviours are uniquely human, but that reciprocal
fairness (i.e. punishment of others’ unfair behaviours) even when
bargaining with primary rewards, may be particularly prominent
in humans.
In addition to our data examining responses to unfair offers with
primary rewards, recent work examining proposals has also shown
an influence of fairness in children making offers with sweets as
rewards
19,20
. Children given the opportunity to divide sweets between
themselves and others develop more egalitarian tendencies with age,
by the age of 7–8 they prefer resource allocations that remove advant-
ageous or disadvantageous inequality
19
. It would be interesting to
examine the response to unfair offers of sweets in children, although
this may require large subject numbers, deception or other such
methods. For example, in a recent study of 34 pairs of preschool
children only two responders faced a non-zero offer of ,4 out of
10 sweets, although none of 23 fair offers (even split or greater) were
rejected
20
.
Whilst our participants were not solely self-interested, neither
were they solely motivated by fairness, and instead they exhibited a
trade-off between these motivations. In terms of behavioural eco-
nomic theory, such a trade-off maps conceptually onto models where
choice is determined by utility functions containing both self and
other regarding components
1,21
. Previous work has suggested that a
subjective aspect of the fairness motivation can be dissociated from
choice, for example where TMS to right dlPFC reduces rejections but
does not alter subjective assessments of fairness in the UG
12
.
Whilst we acknowledge that our experiment was not designed to
make strong inferences regarding the nature of the self-interested
motivation, our data do provide tentative evidence that this self-
interested motivation may also be subjective in nature. With thirst,
objective measures of physiological need of water (e.g. blood osmo-
larity or sodium concentration) are closely monitored peripherally
and in the brain
9
to enable homeostasis. The subjective sensation of
thirst (e.g. individuals’ subjective reports of their thirst) is closely
related to such objective monitoring but is dissociable, for example
after drinking when thirsty, and involves a wide network of brain
regions
9,22
.
This observation that this self-interested motivation may be sub-
jective in nature helps interpret previously discrepant results between
UG studies with money that tried to manipulate the self-interested
motivation. In previous work that manipulated individuals’ subject-
ive feeling of entitlement by having subjects ‘‘earn’’ the endowment
before an UG, this affected behaviour and led to lower (i.e. less fair)
offers in the UG
23
. However, other studies that increased the stakes of
the UG relative to individuals’ wealth, often in countries such as
Slovakia or Indonesia where the equivalent of many month’s salary
may be used experimentally, did not lead to a clear cut pattern of
more self-interested choices
1,11,24
. The former manipulation may
affect the subjective value of the self-interested motivation more than
simply raising the absolute amount of the stake. Further, our data
also shows that raising the stakes by increasing osmolarity (analog-
ous to reducing wealth) can matter, but only when these stakes
impact upon the individual’s subjective motivational state. In addi-
tion, it is known that culture can affect UG responses
25
, and unlike
previous high-stakes games with money, our method has the advant-
age of operationalising changes in ‘‘wealth’’ within a single country
and culture as a physiological need. Future work could usefully focus
on these aspects concerning the nature of the self-interested motiva-
tion using more subjects, perhaps also examining whether subjec-
tively thirsty individuals are also more likely to reject low monetary
offers.
Finally, a further interesting consideration here is the concept of
satisficing used in economics
26
and foraging theory
27
, where because
of constraints (e.g. information, time, other costs) a decision-maker
attempts to meet an acceptability threshold rather than choosing
optimally. Here this may have led to not bothering to drink the
62.5 ml of water (about 1/5
th
of a standard drink can). However,
we note there was no cost difference between drinking and not
drinking in the current experiment. Furthermore, individuals valued
even small amounts of water, as shown here by subjectively thirstier
individuals becoming more likely to consume this amount. Finally,
considerable previous work has shown that in thirsty individuals very
significantly smaller amounts of amounts of water than used here
reduce thirst
28
, are rated pleasantly
29
and modulate neural activity in
reward-related brain regions
28,29
.
In summary, our data demonstrate that humans care about being
treated fairly when bargaining with primary rewards, and, together
with a broader literature
13,14
, suggest that such reciprocal altruism
may be particularly prominent in humans. Our data also provide
tentative evidence that subjective self-interest may limit this fairness
motivation.
Methods
Participants.21 healthy participants provided informed, written consent (11 male,
mean age 25 (range 20–32) years; 2 further participants did not complete testing) for a
study approved by University College London Ethics Committee. Note that including
gender as a factor in our analyses with respect to fairness or subjective measures of
thirst did not alter our findings below.
Thirst manipulation.On the testing day, participants were asked to refrain from
drinking after 08:00 am. They arrived at 09:00 am. We manipulated thirst using saline
administered via an intravenous line for 50 minutes, at a rate 0.15 ml/kg/min for
males and 0.12 ml/kg/min for females. The sessions lasted four to five hours
(including time for preparation beforehand and to ensure subject safety afterwards)
and experienced physicians were present throughout to ensure participant safety.
In a double-blind, randomised design. 11 participants received isotonic saline
(0.9% NaCl) similar to normal human osmolarity, with a minimal impact on thirst;
and 10 received hypertonic saline (5% NaCl) that markedly increases blood osmo-
larity and, as a consequence, thirst
9
. After infusion subjects performed one hour of
non-social tasks (not reported here); then the UG; and finally waited a further hour
without water. Participants were fully informed of this timetable at the start of each
testing day (only the UG was unexpected, as described below). The one hour wait
post-testing without water was in order to prevent the value of water being rendered
negligible if participants believed they would have immediate access to water after
testing.
At pre-infusion baseline (t
baseline
) and the time of testing (t
UG
) we measured sub-
jective thirst (visual analogue scale from 0–10) and blood osmolarity (analysis by
freezing point depression osmometer). Participants completed a similar session 5–7
days before but without the UG (and receiving the alternative infusion), and were
unaware of the prospect of the UG until it was conducted.
Behavioural task.Three participants attended each session where they met and
interacted with each other, and were then tested in separate rooms. At time of testing,
t
UG
, participants first received written instructions stating that two of the participants
(one Proposer and one Responder) would be randomly selected to play an UG, in this
case dividing 500 ml of water for immediate consu mption. Next, all participants were
informed they were the Responder. The experimenter then brought a covered tray,
removed the cover and left the room. For all participants the tray contained two
straight-sided 500 ml capacity glasses, one holding 62.5 ml (12.5%) with ‘‘they offer’’
written below, and the other holding 437.5 ml (87.5%) next to ‘‘they keep’’ (Fig. 1b).
Participants had 15 seconds to circle ‘‘accept’’ or ‘‘reject’’ on a piece of paper.
Participants who accepted then drank the 62.5 ml, and all participants waited one
further hour without water. Note that in the current study the number of participants
was limited by substantial difficulties and potential risks of IV saline administration,
and thus given the typically low rate of unfair offers made by Proposers in the UG
1
,
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SCIENTIFIC REPORTS | 2 : 593 | DOI: 10.1038/srep00593 3
here we employed this deception to examine Responder behaviour in response to low
offers. Participants were informed that they would only play the UG once (i.e. it was
one-shot in nature) and would undertake no further tasks subsequently (i.e. during
this last post-testing wait period).
Analysis.Statistical tests were carried out using independent-samples t-tests or
independent analysis of variance (ANOVA) in SPSS 17.0; reported p-values are
two-tailed. Before implementing each parametric test we applied Levene’s test for
inhomogeneity of variance, and in no case found even trend level significance for
rejection of the null hypothesis. The Kolmogorov-Smirnov test of normality also did
not reach even trend level significance for rejection of the null hypothesis in either
group for either objective or subjective measures of thirst at either time point.
For the purpose of comparison with choice data from our task, we estimated an
average frequency of rejections of similarly low offers of money in human studies. We
used data from an extensive review of the UG literature (Table 2.3 in
1
) where we
averaged across all quoted rejection rates (not weighted by number of subjects and
collapsing over various manipulations). This revealed for offer proportions of
11–20% a mean rejection rate of 49% (mean of 39 entries), and for offer proportions of
1–10% a mean rejection rate of 59% (mean of 24 entries). This rejection rate accords
well with the author’s conclusion that offers below 20% are rejected about half the
time
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Acknowledgements
This work was supported by Wellcome Trust Programme Grant 078865/Z/05/Z to R.J.D.
The Wellcome Trust Centre for Neuroimaging is supported by core funding from the
Wellcome Trust 091593/Z/10/Z.
Author contributions
NDW and SMF originated the idea. NDW, KH, MS and MG collected the data. NDW
analysed the data. NDW and RJD wrote the manuscript, on which all autho rs commented.
Additional information
Competing financial interests: The authors declare no competing financial interests.
License: This work is licensed under a Creative Commons
Attribution-NonCommercial-ShareAlike 3.0 Unported License. To view a copy of this
license, visit http://creativecommons.org/licenses/by-nc-sa/3.0/
How to cite this article: Wright, N.D. et al. Human responses to unfairness with primary
rewards and their biological limits. Sci. Rep. 2, 593; DOI:10.1038/srep00593 (2012).
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SCIENTIFIC REPORTS | 2 : 593 | DOI: 10.1038/srep00593 4
