Evolution and the Trolley Problem: People Save Five Over One Unless the One is Young, Genetically Related, or a Romantic Partner

Abstract

We investigated men’s and women’s responses to variations of an ethical thought experiment known as the Trolley Problem. In the original Trolley Problem, readers must decide whether they will save the lives of five people tied to a track by pulling a lever to sacrifice the life of one person tied to an alternate track. According to W. D. Hamilton's (1964) formulation of inclusive fitness, people's moral decisions should favor the well-being of those who are reproductively viable, share genes, and provide reproductive opportunity. In two studies (Ns = 652 and 956), we manipulated the sex, age (2, 20, 45, and 70 years old), genetic relatedness (0, .125, .25, and .50), and potential reproductive opportunity of the one person tied to the alternate track. As expected, men and women were less likely to sacrifice one life for five lives if the one hypothetical life was young, a genetic relative, or a current mate.

Full text

Journal of Social, Evolutionary, and Cultural Psychology
www.jsecjournal.com - 2010, 4 (3): 115-127.
2010 Journal of Social, Evolutionary, and Cultural Psychology
115
Original Article
EVOLUTION AND THE TROLLEY PROBLEM: PEOPLE
SAVE FIVE OVER ONE UNLESS THE ONE IS YOUNG,
GENETICALLY RELATED, OR A ROMANTIC PARTNER
April Bleske-Rechek
University of Wisconsin-Eau Claire
Lyndsay A. Nelson
University of Wisconsin-Eau Claire
Jonathan P. Baker
University of Wisconsin-Eau Claire
Mark W. Remiker
University of Wisconsin-Eau Claire
Sarah J. Brandt
University of Wisconsin-Eau Claire
Abstract: We investigated men’s and women’s responses to variations of an ethical
thought experiment known as the Trolley Problem. In the original Trolley Problem,
readers must decide whether they will save the lives of five people tied to a track by
pulling a lever to sacrifice the life of one person tied to an alternate track. According to
W. D. Hamilton's (1964) formulation of inclusive fitness, people's moral decisions should
favor the well-being of those who are reproductively viable, share genes, and provide
reproductive opportunity. In two studies (Ns = 652 and 956), we manipulated the sex, age
(2, 20, 45, and 70 years old), genetic relatedness (0, .125, .25, and .50), and potential
reproductive opportunity of the one person tied to the alternate track. As expected, men
and women were less likely to sacrifice one life for five lives if the one hypothetical life
was young, a genetic relative, or a current mate.
Key Words: Trolley Problem, Moral Decisions, Morality, Hypothetical Scenarios
org
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Evolution and the Trolley Problem
Journal of Social, Evolutionary, and Cultural Psychology – ISSN 1933-5377 – volume
4(3). 2010.
116
Evolution and the Trolley Problem: People Save Five over One Unless The One Is
Young, Genetically Related, or A Romantic Partner
In the study of moral reasoning, philosophers have used an ethical thought
experiment known as the Trolley Problem to understand implicit rules that govern
humans’ moral decisions. A classic formulation of the Trolley Problem reads as follows:
A trolley is running out of control down a track. In its path are five people who
have been tied to the track by a madman. Fortunately, you can flip a switch that
will lead the trolley down a different track to safety. Unfortunately, there is a
single person tied to that track.
Would you flip the switch?
The reader must decide whether he or she would save the lives of five people tied
to the main track by flipping a switch to sacrifice the life of one person tied to the
alternate track. One key decision in the Trolley Problem involves a judgment of whether
the utilitarian benefit of saving five exceeds the perceived immorality of killing one. In
response to the scenario above, the majority of people do opt to flip the switch (sacrifice
the life of one). However, like other artificial dilemmas such as the burning building
dilemma (Burnstein, Crandall, & Kitayama, 1994), the trolley problem is useful because
researchers can manipulate its parameters, one by one, to shed light on which aspects of
the dilemma influence people’s decisions (Hauser, 2006). For example, if the wording is
slightly modified so that one must push a lone person onto the track to save the five
people, thus harming the one person as a means to save five (rather than as an unintended
consequence of saving five), individuals are less likely to sacrifice the lone person
(Hauser, 2006). Individuals also respond to framing effects. For example, they agree
more with the option of “saving” five people than with the option of “killing” one person,
even though the two options are consequentially equivalent (Petrinovich & O’Neill,
1996). Individuals also respond to variations in the species involved, with a tendency to
spare human lives over non-human lives (O’Neill & Petrinovich, 1998).
A selfish gene perspective (Dawkins, 1989) and the logic of Inclusive Fitness
Theory (Hamilton, 1964) implicate genetic relatedness, age, and reproductive opportunity
(of the person on the alternate track) as additional parameters that should undermine a
utilitarian response to the Trolley Problem. Over evolutionary history, individuals who
saved the lives of those who (a) were likely to share genes with them, (b) had their
reproductive lives ahead of them, and (c) were likely to provide reproductive opportunity,
would have been more likely, on average, to pass their genes on to subsequent
generations than would individuals who did not save those lives. Research using the
Trolley Problem has offered support for people’s bias toward saving kin (O’Neill &
Petrinovich, 1998; Petrinovich, O’Neill, & Jorgensen, 1993); and research using other
hypothetical helping dilemmas (Burnstein et al., 1994) has illustrated people’s concern
for the welfare of targets depicted as genetically related or as very young (i.e., one year
old), particularly under life-and-death conditions. However, past studies suffer from two
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Evolution and the Trolley Problem
Journal of Social, Evolutionary, and Cultural Psychology – ISSN 1933-5377 – volume
4(3). 2010.
117
primary limitations. First, they have utilized within-subjects designs; between-subjects
designs provide a stricter test of the hypotheses. Second, they have not investigated
systematically the effects of age, varying degrees of genetic relatedness, and reproductive
viability on people’s decisions.
In the current studies, we use a between-subjects design to investigate the effects
of multiple independent variables. We refer to the one life on the alternate track as the
lone target. In the first study, we investigate the effects of target sex, target age, and
target genetic relatedness on participants’ decision to sacrifice the target in order to save
five lives. In the second study, we investigate the effect of target’s reproductive
opportunity on participants’ decision.
STUDY 1
In Study 1, we hypothesized that participants would be less likely to flip the
switch on genetically related targets. Because youth is an indicator of life left to live and
reproductive viability, we also hypothesized that participants would be less likely to flip
the switch on younger targets. Because past research has documented a greater tendency
toward inaction among women than among men in helping situations (Lay, Allen, &
Kassirer, 1974) and in response to life-or-death dilemmas (Petrinovich et al., 1993), we
also expected that female participants would be less likely than male participants, overall,
to flip the switch.
Method
Participants
A total of 239 men and 413 women from a suburban community in the United
States participated. Participants ranged from 15-86 years (M = 31.23, SD = 15.41).
Measures
We created 32 versions of the original Trolley Problem by manipulating the sex,
age, and genetic relatedness of the person tied to the alternate track (see Table 1).
Participants were exposed to one version of the problem. An example is as follows (key
manipulated area in italics):
A trolley is running out of control down a track. In its path are 5 people who have
been tied to the track by a madman. Fortunately, you can flip a switch that will
lead the trolley down a different track to safety. Unfortunately, your 20 year-old
male cousin is tied to that track.
We provided no additional information about the “five people” tied to the main
track. The experiment included one subject variable (participant sex) and three true
independent variables (target sex, target age, and target relatedness), for a 2x2x4x4
between-subjects design.
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4(3). 2010.
118
Table 1. Study 1: Independent Variables Combined to Form 32 Versions of the Lone Target
Depicted in the Trolley Problem, and Number of Participants (N) Exposed to Each Version
IV1. IV2. IV3. Target
Relatedness Age Sex Depicted N
.00 2 Male Male Stranger 20
Female Female Stranger 21
20 Male Male Stranger 22
Female Female Stranger 21
45 Male Male Stranger 21
Female Female Stranger 20
70 Male Male Stranger 21
Female Female Stranger 22
.125 2 Male Male Cousin 20
Female Female Cousin 17
20 Male Male Cousin 25
Female Female Cousin 22
45 Male Male Cousin 20
Female Female Cousin 21
70 Male Great Uncle 17
Female Great Aunt 22
.25 2 Male Nephew 18
Female Niece 20
20 Male Half Brother 18
Female Half Sister 18
45 Male Uncle 19
Female Aunt 21
70 Male Grandfather 21
Female Grandmother 21
.50 2 Male Son 20
Female Daughter 20
20 Male Brother 22
Female Sister 21
45 Male Father 21
Female Mother 21
70 Male Father 21
Female Mother 20
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4(3). 2010.
119
Procedure
We approached individuals as they were passing through a local farmer’s market.
Upon consent, each participant read one of the 32 versions of the Trolley Problem, and
then checked Yes or No in response to the question, “Would you flip the switch in this
situation?” To emulate a real-life situation, we asked participants to answer as quickly as
possible. Participants also reported their age and sex; they were debriefed in writing.
Results
Overall, 53% of participants opted to flip the switch on the lone target (to save
the five people). We conducted multinomial logistic regression analyses to test the main
and interactive effects of participant sex, target sex, target age, and target genetic
relatedness on likelihood of flipping the switch. We treated all variables as categorical,
including our dependent variable, “flip the switch.” Using a forward entry procedure with
hierarchical constraints in place, the best-fitting model included three main effects and no
higher order interactions. Table 2 provides the parameter estimates and fit statistics
(Peng, Lee, & Ingersoll, 2002) for the final reduced model. This model had an overall
correct prediction rate of 67.2%.
The three significant main effects in the final reduced model, displayed in the
three panels of Figure 1, supported our hypotheses. First (upper panel), participants were
increasingly unwilling to flip the switch on targets of increasing levels of genetic
relatedness. Second (middle panel), participants were less likely to flip the switch on a
very young lone target than on the other lone targets. Third (lower panel), women were
less likely than men to flip the switch on the lone target. Figure 2 displays the effects of
target age and target genetic relatedness, across participant sex, on participants’ decision
to sacrifice the target to save the five passengers.
STUDY 2
In Study 2, we focused on the effect of reproductive opportunity on willingness
to sacrifice one life for five lives. We acquired a sample of young adults, and in every
scenario we described passengers and targets as being of the same age as the participant.
We hypothesized that participants would be less likely to sacrifice a target who was likely
to provide reproductive opportunity – specifically, a target depicted as their “current
romantic partner.” We predicted that participants would be especially unwilling to
sacrifice a hypothetical romantic partner if they were actually currently involved in a
romantic relationship because they would be more likely to visualize a specific person.
Method
Participants
Participants were 956 (352 men and 604 women) students from a public
university. They ranged from 16-27 years, with 97% between 18 and 22 (M = 18.92, SD
= 1.36).
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Journal of Social, Evolutionary, and Cultural Psychology – ISSN 1933-5377 – volume
4(3). 2010.
120
Table 2. Study 1: Logistic Regression Analysis of Participants’ Decision to Flip the Switch on a
Lone Target
Predictor
β
SE β
Wald’s χ2
df
p
Constant
.164
.252
.423
1
.515
Target relatedness = 0
2.039
.260
61.551
1
.000
Target relatedness = .125
.769
.235
10.674
1
.001
Target relatedness = .25
.567
.239
5.613
1
.018
Target relatedness = .50
----
----
----
0
----
Target age = 2 years old
1.353
.254
28.414
1
.000
Target age = 20 years old
-.299
.239
1.565
1
.211
Target age = 45 years old
-.198
.241
.673
1
.412
Target age = 70 years old
----
----
----
0
----
Participant sex = female
-.661
.180
13.415
1
.000
Participant sex = male
----
----
----
0
----
Final Reduced Model
χ2
df
p
Overall model evaluation (against intercept-only
model)
Likelihood ratio test
116.85
7
.000
Model fitting criteria
AIC
147.051
NA
NA
BIC
182.891
NA
NA
-2 log likelihood
131.051
NA
NA
Goodness-of-fit test
Pearson chi-square
29.592
24
.199
Note. The last category of each predictor variable served as the reference category. For the
dependent variable “flip the switch,” the reference category is “No.” Cox and Snell R2 = .164.
Nagelkerke R2 = .219. For model with intercept only, AIC = 249.916, BIC = 254.396, -2 log
likelihood = 247.916. NA = not applicable
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Evolution and the Trolley Problem
Journal of Social, Evolutionary, and Cultural Psychology – ISSN 1933-5377 – volume
4(3). 2010.
121
Figure 1. Study 1: Effects of target genetic relatedness (upper panel), target age (middle
panel), and participant sex (lower panel) on likelihood of flipping the switch to sacrifice
the lone target.
77.4
51.8
47.4
34.3
0
20
40
60
80
100
0
0.125
0.25
0.5
Percent that Flipped the Switch
on the Lone Target
Genetic Relatedness of Lone Target on Alternate
Track
34.0
56.2
58.5
62.0
0
20
40
60
80
100
2
20
45
70
Percent that Flipped the
Switch on the Lone Target
Age of Lone Target on Alternate Track
61.5
47.9
0
20
40
60
80
100
Male Participants
Female Participants
Percent that Flipped the
Switch on the Lone Target
Sex of Participant
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4(3). 2010.
122
Measures
As shown in Table 3, we created 12 versions of the original Trolley
Problem by manipulating the sex of the five people originally in danger (unstated,
same-sex, and opposite-sex) and the sex of the target who could be sacrificed to
save them (unstated, same-sex, opposite-sex, and romantic partner). We refined
the general dilemma to include more detail as well as clarification of the
consequences of each decision.
Figure 2. Study 1: Effects of target genetic relatedness and target age, across participant sex, on
likelihood of flipping the switch to sacrifice the lone target.
An example of the refined scenario is as follows (key manipulated areas in
italics):
It's a lovely day out, and you decide to go for a walk along the trolley tracks that
crisscross your town. As you walk, you hear a trolley behind you, and you step
away from the tracks. But as the trolley gets closer, you hear sounds of panic --
the five men on board, who are about your age, are shouting for help. The
trolley's brakes have gone out, and it's gathering speed. It is going to crash and
kill the passengers.
0
10
20
30
40
50
60
70
80
90
100
2 Years Old
20 Years Old
45 Years Old
70 Years Old
Percent that Flipped the Switch on the Lone Target
Age of LoneTarget on Alternate Track
0 Relatedness
0.125 Relatedness
0.25 Relatedness
0.5 Relatedness
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4(3). 2010.
123
You find that you just happen to be standing next to a side track that veers into a
sand pit, which would provide safety for the trolley's five passengers. All you
have to do is pull a hand lever to switch the tracks, and you'll save the five men.
But there's a problem. Along this offshoot of track leading to the sandpit stands a
woman about your age who is totally unaware of the trolley's problem and the
action you're considering. There's no time to warn her. So by pulling the lever
and guiding the trolley to safety, you'll save the five men. But you'll kill the
woman.
The experiment included two subject variables (participant sex and participant
relationship involvement) and two independent variables (passenger sex and target sex)
for a 2x2x3x4 between-subjects design.
Procedure
We approached students on orientation day on a university campus. Upon
consent, each participant read one of the 12 versions of the Trolley Problem. Then,
participants checked their decision to either Pull the hand lever or Don’t pull the hand
lever. Again, we asked participants to answer as quickly as possible. They then reported
their age, sex, and whether or not they were currently involved in a romantic relationship.
They were debriefed in writing.
Table 3. Study 2: Independent Variables Combined to Form 12 Versions of the Trolley
Problem, and Number of Participants (N) Exposed to Each Version
______
IV1. IV2.
Sex of Lone Target Sex of Passengers N
______
Unstated Unstated 79
Same-sex 88
Opposite-sex 71
Same-sex Unstated 81
Same-sex 75
Opposite-sex 86
Opposite-sex Unstated 82
Same-sex 72
Opposite-sex 81
Romantic Partner Unstated 81
Same-sex 82
Opposite-sex 76
_____
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Evolution and the Trolley Problem
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4(3). 2010.
124
Results
Overall, 66% of participants opted to pull the lever to sacrifice the target and
save the five passengers. We conducted multinomial logistic regression analyses to test
the main and interactive effects of participant sex, participant involvement, target sex,
and passenger sex on likelihood of pulling the lever to sacrifice the target. Using a
forward entry procedure with hierarchical constraints in place, the best-fitting model
included two main effects and one higher order interaction (this pattern replicated with a
general loglinear analysis). Table 4 provides the parameter estimates and fit statistics for
the final reduced model, which had an overall correct prediction rate of 72.7%.
Contrary to our findings in Study 1, male and female participants in Study 2 did
not differ in the frequency with which they opted to pull the lever, p =.61. As displayed in
Figure 3, however, our hypothesis that participants would be unwilling to sacrifice their
romantic partner in order to save five others was supported. And, as implicated in the
interaction between target sex and participant involvement, the negative effect of
romantic involvement on willingness to pull the lever was apparent only when the lone
target was depicted as the participant’s romantic partner.
Figure 3. Study 2: Interactive effects of target sex and participant relationship
involvement on likelihood of pulling the lever to sacrifice the lone target.
77.3
80.3
73.7
50.0
72.6
77.1
74.5
24.8
0
10
20
30
40
50
60
70
80
90
100
Unstated
Same-sex
Opposite-sex
Romantic Partner
Percent that Pulled the Lever on the Lone Target
Target Sex
Non-Involved Participants
Involved Participants
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4(3). 2010.
125
Table 4. Study 2: Logistic Regression Analysis of Participants’ Decision to Pull the Lever
on a Lone Target
Predictor
β
SE β
Wald’s χ2
df
p
e β (odds ratio)
Constant
-1.110
.211
27.782
1
.000
NA
Target sex = unstated
2.086
.303
47.413
1
.000
8.054
Target sex =
same-sex
2.326
.314
55.022
1
.000
10.238
Target sex = opposite-sex
2.182
.310
49.639
1
.000
8.867
Target sex = romantic
partner
----
----
----
0
----
----
Participant =
not involved
1.110
.280
15.742
1
.000
3.033
Participant = involved
----
----
----
0
----
----
Target unstated, participant
not inv.
-.862
.411
4.405
1
.036
.422
Target unstated, participant
involved
----
----
----
0
----
----
Target same-sex, participant
not inv.
-.921
.422
4.760
1
.029
.398
Target same-sex, participant
involved
----
----
----
0
----
----
Target opp-sex, participant
not inv.
-1.153
.411
7.880
1
.005
.316
Target opp-sex, participant
involved
----
----
----
0
----
----
Target partner, participant
not inv.
----
----
----
0
----
----
Target partner, participant
involved
----
----
----
0
----
----
Final Reduced Model
χ2
df
p
Overall model evaluation (against intercept-only model)
Likelihood ratio test
135.591
7
.000
Model fitting criteria
AIC
191.678
NA
NA
BIC
230.564
-2 log likelihood
175.678
Goodness-of-fit test
Pearson chi-square
27.353
40
.936
Note. The last category of each predictor variable served as the reference category. For the
dependent variable “pull the lever,” the reference category is “No.” Cox and Snell R2 = .132.
Nagelkerke R2 = .184. For model with intercept only, AIC = 313.270, BIC = 318.130, and -2 log
likelihood = 311.270. NA = not applicable.
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Journal of Social, Evolutionary, and Cultural Psychology – ISSN 1933-5377 – volume
4(3). 2010.
126
Discussion
In two studies, we documented that people’s hypothetical decisions follow the
laws of Inclusive Fitness Theory. Participants appeared unwilling to sacrifice the life of a
family member or reproductive partner, even when the alternative was to let five innocent
people die. Our findings are particularly notable in light of our conservative between-
subjects design and use of two distinct samples.
Although some might suggest that the use of hypothetical scenarios is a
limitation of the present research, other researchers (e.g., Hauser, 2006) have noted that
hypothetical moral dilemmas allow researchers to manipulate the very parameters they
propose have an impact on moral decisions. Burning buildings and impending death of
innocent victims do occur, but not predictably enough or close enough in time and space
for researchers to chart their contexts and outcomes. Further, human ancestral
environments involved warfare and raids, harsh environmental conditions, and tragic
accidents with behavioral choices whose outcomes most certainly mirrored the Trolley
Problem in its abstract form. Indeed, our participants told us they easily imagined
themselves in the scenarios they read, and they frequently wanted to tell us about the
exact person they had been asked to imagine. In future research we aim to test the
hypothesis that the effects of target’s genetic relatedness and reproductive opportunity are
heightened when participants are placed under cognitive load, and minimized when
emotional closeness to target is included as a covariate.
We conclude that the complex rules governing humans’ moral intuitions surely
lie in the interplay of our human evolutionary heritage, our individual dispositions, and
the specific contextual factors of a given situation. Hypothetical dilemmas like the
Trolley Problem provide an opportunity for psychologists and moral philosophers to
collaborate in delineating how these factors interact to guide our moral attitudes and
behaviors.
Received July 29, 2009, Accepted March 20, 2010
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