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Journal of Experimental Social Psychology
journal homepage: www.elsevier.com/locate/jesp
Happy to help? A systematic review and meta-analysis of the effects
of performing acts of kindness on the well-being of the actor
☆
Oliver Scott Curry
a,⁎
, Lee A. Rowland
b,e
, Caspar J. Van Lissa
c,f
, Sally Zlotowitz
d
, John McAlaney
b
,
Harvey Whitehouse
a
a
Institute of Cognitive and Evolutionary Anthropology, University of Oxford, United Kingdom
b
Department of Psychology, Bournemouth University, United Kingdom
c
Methodology & Statistics, Social and Behavioural Sciences, Utrecht University, Netherlands
d
Department of Clinical Educational and Health Psychology, University College London, United Kingdom
e
School of Anthropology and Museum Ethnography, University of Oxford, United Kingdom
f
Erasmus School of Social and Behavioural Sciences, Erasmus University Rotterdam, Netherlands
ARTICLE INFO
Handling editor: Elizabeth Page-Gould
Keywords:
Kindness
Well-being
Altruism
Happiness
Positive psychology
ABSTRACT
Do acts of kindness improve the well-being of the actor? Recent advances in the behavioural sciences have
provided a number of explanations of human social, cooperative and altruistic behaviour. These theories predict
that people will be ‘happy to help’family, friends, community members, spouses, and even strangers under some
conditions. Here we conduct a systematic review and meta-analysis of the experimental evidence that kindness
interventions (for example, performing ‘random acts of kindness’) boost subjective well-being. Our initial search
of the literature identified 489 articles; of which 24 (27 studies) met the inclusion criteria (total N= 4045).
These 27 studies, some of which included multiple control conditions and dependent measures, yielded 52 effect
sizes. Multi-level modeling revealed that the overall effect of kindness on the well-being of the actor is small-to-
medium (δ= 0.28). The effect was not moderated by sex, age, type of participant, intervention, control con-
dition or outcome measure. There was no indication of publication bias. We discuss the limitations of the current
literature, and recommend that future research test more specific theories of kindness: taking kindness-specific
individual differences into account; distinguishing between the effects of kindness to specific categories of
people; and considering a wider range of proximal and distal outcomes. Such research will advance our un-
derstanding of the causes and consequences of kindness, and help practitioners to maximise the effectiveness of
kindness interventions to improve well-being.
1. Introduction
Do acts of kindness improve the well-being of the actor? Over the
past few decades, advances in the behavioural sciences have developed
numerous theories of human social, cooperative and altruistic beha-
viour. These theories —kin altruism, mutualism, reciprocal altruism,
and competitive altruism —make it possible to explain a variety of
different types of kindness (for example, love, sympathy, gratitude and
heroism). And they predict that people will be ‘happy to help’family,
friends, community members, spouses, and even strangers under some
conditions.
More recently, there has been growing interest in using kindness as
an intervention to boost subjective well-being. The idea that, for ex-
ample, ‘random acts of kindness’can boost the well-being not only of
the recipient, but also the actor, and could thereby provide a simple,
effective, inexpensive and widely-available means of addressing social
problems ranging from social isolation to more serious mental and
physical health conditions, has been taken up and promoted by a large
number of research groups, charities and government organisations
(Aked, Marks, Cordon, & Thompson, 2008;Aked & Thompson, 2011;
see S1; Huppert, 2009).
Here we outline existing theories of altruism and their relation to
kindness, and consider the predictions these theories make about well-
being. We then conduct a systematic review and meta-analysis of
https://doi.org/10.1016/j.jesp.2018.02.014
Received 12 May 2017; Received in revised form 2 February 2018; Accepted 26 February 2018
☆
The work on this article was supported by kindness.org (R46536/CN001). We also thank the many researchers who generously responded to our requests for unpublished papers and
data. In particular we note the exceedingly helpful co-operation from Lara Aknin, Kate Hannibal, Ashley Whillans, Katherine Nelson, and Kristin Layous. And thanks to Rongqin Yu for
statistical advice, to Rosalind Arden for useful discussions, to Helena Cronin for comments on the manuscript, and to Alexandria Henke, Divia Joseph, Steve Rowland and Emma Seymour
for research assistance.
⁎
Corresponding author at: Institute of Cognitive and Evolutionary Anthropology, University of Oxford, 64 Banbury Road, Oxford, OX2 6PN, United Kingdom.
E-mail address: oliver.curry@anthro.ox.ac.uk (O.S. Curry).
Journal of Experimental Social Psychology xxx (xxxx) xxx–xxx
0022-1031/ © 2018 The Authors. Published by Elsevier Inc. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
Please cite this article as: Curry, O.S., Journal of Experimental Social Psychology (2018), https://doi.org/10.1016/j.jesp.2018.02.014
previous experimental studies of the effects of kind acts on the well-
being of the actor. And we end with a discussion of the limitations of
the existing literature, and make recommendations for future research.
2. The causes of kindness
Kindness refers to actions intended to benefit others. Why and under
what circumstances are people kind to others? Why do people behave in
prosocial, cooperative and altruistic ways? Recent interdisciplinary
research has provided a wealth of answers to these questions (Curry,
2016).
Humans evolved from a long line of social primates, who have been
living in social groups for over 50 million years (Shultz, Opie, &
Atkinson, 2011). Group living affords numerous opportunities for var-
ious different types of mutually beneficial cooperative interaction
(Lehmann & Keller, 2006;Nunn & Lewis, 2001;Sachs, Mueller, Wilcox,
& Bull, 2004). Natural selection has favoured a range of evolved psy-
chological mechanisms for taking advantages of these opportunities,
and realising the benefits of cooperation. These mechanisms –kin al-
truism, mutualism, reciprocal altruism, and competitive altruism –
make it possible to identify and explain several different types of
kindness.
2.1. Kin Altruism: people will be kind to their families
Natural selection favours kindness to genetic relatives, to family
members (Hamilton, 1964). Examples of such ‘kin altruism’are wide-
spread in nature (Gardner & West, 2014), most obviously in cases of
parental care (Royle, Smiseth, & Kölliker, 2012). Humans too possess
adaptations for detecting and delivering benefits to kin (Lieberman,
Tooby, & Cosmides, 2007;Mateo, 2015), especially to offspring (Geary
& Flinn, 2001). Kin altruism can explain kindness in the form of love,
care, sympathy and compassion. And the theory predicts that these
tendencies will be elicited by others who exhibit cues of genetic relat-
edness, especially vulnerable children (Platek, Burch, Panyavin,
Wasserman, & Gallup Jr, 2002).
2.2. Mutualism: people will be kind to members of their communities
Natural selection favours the tendency to coordinate, collaborate
and be kind to others with whom the actor shares a common interest –
team mates, group members, coalition partners. Such ‘mutualisms’–for
the purpose of collective defence, or collaborative hunting –are
widespread in nature (Bissonnette et al., 2015;Boinski & Garber, 2000;
Boos, Kolbe, Kappeler, & Ellwart, 2011;Harcourt & Waal, 1992), and
are an ancient and recurrent feature of human social life (Alvard, 2001;
Wrangham, 1999). This process has led, in humans, to a psychology
that forms and maintains groups (clubs, gangs, clans, sects, nations, and
so on), and acts to promote their interests (sometimes at the expense of
rival groups) (Balliet, Wu, & De Dreu, 2014). Mutualism can explain
kindness in the form of loyalty, solidarity, camaraderie, civic-mind-
edness, community spirit, and commitment to a cause ‘greater than
oneself’. The theory predicts that these tendencies will be elicited by
other members of the groups with which one identifies (including
strangers) (Whitehouse & Lanman, 2014).
2.3. Reciprocal Altruism: people will be kind to those they might meet again
Natural selection favours kindness to those who might return the
favour at a later date (Axelrod, 1984;Trivers, 1971).
1
Surprisingly, few
if any examples of such ‘reciprocal altruism’have been found in non-
human species (Amici et al., 2014;Clutton-Brock, 2009). But in
humans, reciprocal altruism is implemented by psychological me-
chanisms that: detect those in need of help, initiate cooperation, signal
recognition of favours received, keep track of who has returned the
favour and who has not, make amends for favours not returned, and
accept repentant cheats back into the fold (Cosmides & Tooby, 2005;
McCullough, Kurzban, & Tabak, 2013;Trivers, 1971). Thus, reciprocal
altruism can explain kindness in the form of sympathy (for those in
need), trust (initiating cooperation), returning favours, gratitude (for
favours yet to be returned), forgiveness and friendship. Reciprocal al-
truism predicts that these tendencies will most likely be elicited in re-
peated interactions where individuals expect to meet again, where one's
cooperative (or uncooperative) behaviour can be observed by others,
and towards others who have helped them in the past, or will be able to
help them in the future (Kraft-Todd, Yoeli, Bhanot, & Rand, 2015). This
can includes kindness to strangers: a kind act may be a way of making a
new friend; after all, ‘a stranger is just a friend you haven't met yet’
(Delton, Krasnow, Cosmides, & Tooby, 2011;Krasnow, Delton, Tooby,
& Cosmides, 2013).
2.4. Competitive Altruism: people will be kind to others when it enhances
their status
Natural selection also favours kindness that impresses peers and
attracts mates (Gintis, Smith, & Bowles, 2001;Maynard Smith & Price,
1973). Many animals resolve status competition by engaging in costly
displays of prowess (Hardy & Briffa, 2013;Riechert, 1998). In humans,
and perhaps some other species (Zahavi & Zahavi, 1997), these displays
includes altruistic acts that benefit the audience (Hardy & Van Vugt,
2006;Hawkes, 1991;Hawkes, O'Connell, & Blurton Jones, 2001;
Mazur, 2005;Miller, 2000;Smith & Bleige Bird, 2000). This ‘competi-
tive altruism’can explain kindness in the form of generosity, bravery,
heroism, chivalry, magnanimity and public service. The theory predicts
that these tendencies will be elicited in the presence of rivals, or po-
tential mates, where acting altruistically may enhance one's status
(Raihani & Smith, 2015). This includes acts of kindness to strangers:
helping a stranger may improve your status whether the recipient is in a
position to return the favour or not (Barclay, 2011;Raihani & Bshary,
2015).
Thus, multiple theories –kin altruism, mutualism, reciprocal al-
truism, competitive altruism –explain multiple types of kindness. And
the human capacity for culture—the ability to invent and share new
ways of living (Boyd, Richerson, & Henrich, 2011;Pinker, 2010)—has
allowed us to build and elaborate upon this benevolent biological
foundation, with rules, norms and other social institutions that further
inculcate and amplify cooperation and altruism (Hammerstein, 2003).
These theories predict that people will be motivated to be kind to fa-
mily, friends, colleagues, spouses, and even strangers under some
conditions.
2
And the possession of such motivational systems leads us to
expect that helping others might make people happy.
3. The consequences of kindness
Subjective well-being –including happiness, life-satisfaction and
positive affect –refers to a range positively valenced psychological
states (Dolan & Metcalfe, 2012;OECD, 2013). Why would performing
kind acts improve well-being? Why would helping make you happy?
Broadly speaking, happiness can be seen as an internal reward system
for acting in ways that promote survival and reproduction (Buss, 2000).
Happiness is: “a psychological reward, an internal signaling device that
tells us that an adaptive problem has been, or is in the process of being,
1
For further discussion of various subtypes of reciprocity, such as indirect and network
reciprocity, see (Roberts, 2008;Tanimoto, 2015)
2
Note that the argument here is that biology and culture have equipped us to help
automatically, intuitively, innocently –there is no suggestion that people are necessarily
aware of the causes of their benevolent behaviour, or are acting from any ulterior motive.
“The heart has its reasons of which reason knows nothing”(Pascal, 1669).
O.S. Curry et al. Journal of Experimental Social Psychology xxx (xxxx) xxx–xxx
2
solved successfully”(Hill, DelPriore, & Major, 2013). From this per-
spective, it is no problem to explain why ‘eating’or ‘having sex’makes
people happy; these behaviours meet important adaptive goals. And, for
the reasons outlined above, it is equally straightforward to explain why
performing acts of kindness might make people happy: it is because
caring for family, maintaining coalitions, trading favours and increasing
status are also important adaptive goals (Schulkin, 2011). Indeed, we
might even expect helping others to produce more happiness than
helping yourself: it is precisely because helping others can sometimes
give a better return on investment than helping yourself that evolution
has favoured kindness in the first place.
Thus, the evolutionary behavioural science approach to altruism
predicts that people will be happy to help family, friends, community
members, spouses, and even strangers under some conditions. This
prediction has received some support from the existing literature. A
large body of research has established an association between kindness
and well-being (Anik, Aknin, Norton, & Dunn, 2009;Konrath & Brown,
2013). However, much of this research has been correlational —
showing, for example, that people who spend more money on others are
happier (Aknin, Barrington-Leigh, et al., 2013), or people who volun-
teer to help others are healthier (Jenkinson et al., 2013).
3
While such
correlational evidence is consistent with the prediction that people will
be happy to help others, it is not sufficient to establish a causal re-
lationship between kindness and well-being. After all, it's possible that
helping makes you happy; but it could also be that happiness makes you
helpful, or it could be that some third variable –health, income, or
personality –makes you both happy and helpful. The distinction be-
tween correlation and cause is not a mere philosophical nicety; it is a
genuine difference with important practical implications. In the ab-
sence of a clear causal connection, kindness interventions may not
work. They may waste time and money, or displace other more effective
interventions. Worse, they may be counter-productive. If happiness
causes helping (rather than the other way around), then forcing un-
happy people to help may make them less happy still.
In order to establish whether performing acts of kindness can cause
happiness, it is necessary to focus on the experimental literature. And so
we undertook a systematic review and meta-analysis of research that met
the following inclusion criteria: (a) experiments that randomly allocated
participants to (b) interventions involving kind behaviour and controls and
(c) subsequently measured and compared participant well-being.
4. Methods
In order to identify suitable experimental studies of the effects of
altruism on the altruist's well-being, we conducted searches of the sci-
entific databases Web of Science and PsychInfo for academic articles.
The most recent search was conducted on 16th November 2017. The
process is summarised in the flow diagram in Fig. 1. Searching topic,
abstracts and keywords, we used the search string: (kindness OR al-
truis* OR prosocial OR co-operat* OR cooperat*) AND (wellbeing OR
well-being OR happiness OR life satisfaction OR positive affect OR
negative affect OR PANAS) AND (experiment* OR control OR condition
OR random* OR empirical OR trial) NOT (mindfulness OR meditation
OR loving-kindness). This search identified 712 articles. To this we
added 36 articles identified by other means (following references in
books and journal articles, Google scholar searches, viewing academic
researchers' web-pages, reviewers' suggestions, and contacting authors
to request unpublished data). After removing duplicates, we were left
with 489 articles.
This initial set of 489 articles was screened. Two researchers (LAR
and OSC) read the titles and abstracts. Subsequently 432 articles were
excluded for not meeting the inclusion criteria. These articles were ei-
ther: (a) not experimental (for example, were qualitative or correla-
tional studies, or review papers); (b) did not involve kind behaviour (for
example, they involved hypothetical or recalled kindness); (c) did not
measure participant well-being (for example, they measured sub-
sequent kindness, or the happiness of the recipient); or were otherwise
offtopic (for example, kindness in animal husbandry, climate change
and planetary wellbeing). Cases in which the researchers disagreed
were given greater scrutiny and discussed, and where no consensus was
reached, the articles were included in the next stage of the analysis.
The remaining 57 articles were then read in full, and assessed for
appropriateness for the meta-analysis (see S2 for the full list). This
process excluded a further 33 records (and several studies from in-
cluded articles) for reasons summarised in Table S1.
4
At the end of this
process we were left with 24 articles, containing a total of relevant 27
studies that had experimentally tested the hypothesis that kindness
causes well-being.
For each of these studies we coded the following characteristics:
•mean age of sample
•sex of participants
•location of study
•type of participant (for example, whether participants were ‘typi-
cally developed individuals’, as opposed to having been diagnosed
with some psychopathology)
•type of intervention (for example, ‘random act of kindness’, proso-
cial purchase, charitable donation)
•type of recipient (for example, whether family, friend, stranger)
•type of control condition(s) (for example, no treatment, self-kind-
ness, other activity)
•dependent measure(s) (for example, happiness, life-satisfaction)
•size of the intervention group(s)
•size of the control group(s)
•effect size(s) (Cohen's d)
Effect sizes were either taken directly from the paper, or computed
from reported inferential or descriptive statistics (Lenhard & Lenhard,
2016). For the handful of studies that reported outcomes at multiple
time-points, we coded the effect closest in time to the intervention.
5. Results
5.1. Study characteristics
The characteristics of the 27 studies are presented in Table 1. These
27 studies included a total of 4045 participants (mean proportion
male = 35%, mean age = 25.04, SD = 11.05).
5
The majority of participants came from Canada, USA and Europe,
although there were also studies conducted in South Africa, Korea and
Vanuatu. Most participants were university students, although there
were also two studies with children, one study of Vanuatu villagers, and
one with elderly participants. Most were ‘typically developed in-
dividuals’, although two studies involved participants who scored
3
Even then the effects are modest. This meta-analysis of the relationship between
volunteering and health in the elderly found that volunteers were 22% less likely that
non-volunteers to die during the follow-up period of the studies (Jenkinson et al., 2013).
However, the import of this finding depends on the base-rate. By way of illustration, if on
average 10 out of 1000 (1%) non-volunteers die during the follow-up period, then a 22%
percent decrease means that 7.8 out of 1000 (0.78%) volunteers would die during the
same period. Moreover, as this review goes on to say: “These findings were not confirmed
by experimental studies.”
4
The most highly cited paper in the kindness literature (with 597 citations at the time
of the last search) purports to provide evidence that kind acts boosts the well-being of the
actor (Lyubomirsky, Sheldon, & Schkade, 2005). However, the article does not report the
size of the sample, the dependent measure, or any inferential statistics (for example, effect
size or significance). Email correspondence with the author revealed that the data are no
longer available.
5
These averages are approximate (~), because the age and sex ratio of the samples
were not available for some studies.
O.S. Curry et al. Journal of Experimental Social Psychology xxx (xxxx) xxx–xxx
3
highly on measures of social anxiety.
The two most common interventions were ‘acts of kindness’and
‘prosocial purchasing’. Typical instructions for the ‘acts of kindness’
intervention were as follows:
“During the coming week, please perform at least five acts of
kindness per day and report on them in the evening, including the
responses of others that you received. Examples of acts of kindness
are: holding a door for someone at university, greeting strangers in
the hallway, helping other students in preparing for an exam, et-
cetera. It does not matter whether you address your acts of kindness
to people you know or not”.
(Ouweneel, Le Blanc, & Schaufeli, 2014)
Prosocial purchasing interventions involved giving participants a
sum of money, and instructing them to spend it on someone else. Most
‘acts of kindness’involved a cost; but, the ‘prosocial spending’studies
that involved a windfall payment to the participant did not.
The recipients of kindness included colleagues and charities, but
were for the most part left unspecified, and could be ‘anyone’–familiar
or unfamiliar, family, friend, community member of stranger.
Control conditions also varied. Some studies compared acting kindly
with doing nothing (thus possibly confounding the effects of kindness
with the effects of performing any novel fun activity), whereas others
compared acting kindly with some other non-social activity, or with
helping oneself.
Most studies used a self-report measure of subjective well-being,
happiness, life-satisfaction, or positive and negative affect. These in-
cluded the Subjective Happiness Scale (SHS; Lyubomirsky & Lepper,
1999), the Steen Happiness Index (SHI; Seligman, Steen, Park, &
Peterson, 2005), the Satisfaction With Life Scale (SWLS; Diener,
Emmons, Larsen, & Griffin, 1985), the Positive Affect and Negative
Affect Scale (PANAS; Watson, Clark, & Tellegen, 1988) and Psycholo-
gical Flourishing (Lamers, Westerhof, Bohlmeijer, ten Klooster, &
Keyes, 2011). Three studies used more objective measures: two used
other-rated smiling, and one used ‘other rated happiness’.
Some studies had multiple control conditions, and/or multiple
outcome measures, and hence provided more than one effect size; there
were 52 in total.
5.2. Descriptive statistics
The effect size estimates ranged from −0.46 to 1.25 (M=0.25,
SD = 0.32). Sample sizes ranged from 26 to 474 participants
(M= 158.57, SD = 132.05). Several studies reported multiple effect
sizes (1–6, with most reporting one or two effect sizes).
5.3. Meta-analysis
Meta-analysis was conducted in R (R Core Team, 2017) and the R-
packages metafor (Viechtbauer, 2010), and metaforest (Van Lissa,
Full-text articles excluded
(n = 33)
1) No kind acts, just
recall n=3
2) Counting kind acts, no
new ones n=4
3) Expected or imaginary
kindness n=3
4) No control n=2
5) Comparing kindness
on other IV n=4
6) Kind acts embedded
with other positive
activities n=8
7) Incomplete description
of experiment n=1
8) Review or meta-
analysis n=3
9) Correlational n=3
10)Irrelevant DV n=2
Records after duplicates removed
(n = 489)
Studies included in
quantitative synthesis
(meta-analysis)
(n = 27)
Full-text articles
assessed for eligibility
(n = 57)
Records excluded
(n = 432)
Records screened
(n = 489)
Additional records identified
through other sources
(n = 36)
Identification
Eligibility
Included Screening
Records identified through
database searching
(n = 712)
Fig. 1. Flow diagram of the search and selection procedure of studies.
O.S. Curry et al. Journal of Experimental Social Psychology xxx (xxxx) xxx–xxx
4
Table 1
Main characteristics of studies included in the meta-analysis.
Study Sex Age Location Donor Intervention (IV) Control Recipient(s) Outcome (DV) n1 (I) n2 (C) d
Aknin, Barrington-Leigh, et al. (2013) Study 3 38 21.00 Canada/South
Africa
Typical Prosocial purchase Personal purchase Anonymous sick
children
PA 100 100 0.46
Aknin, Barrington-Leigh, et al. (2013) Study 3 38 21.00 Canada/South
Africa
Typical Prosocial purchase Personal purchase Anonymous sick
children
SWLS 100 100 0.13
Aknin, Broesch, Hamlin, & Van de
Vondervoort (2015)
Study 1 42 45.00 Vanuatu Typical Prosocial purchase Personal purchase Family/friends PA 13 13 0.93
Aknin et al. (2015) Study 2 70 ~3 Vanuatu Typical Donate own sweets Donate other
sweets
Puppet Smiling 20 20 0.30
Aknin, Dunn, Sandstrom, & Norton (2013) Study 3 34 21.00 Canada Typical Prosocial purchase Personal purchase Someone WB 25 25 0.24
Aknin, Fleerackers, & Hamlin (2014) 41 19.90 USA Typical Prosocial purchase Personal purchase Anonymous sick
children
PANAS 60 59 0.38
Aknin et al. (2014) 41 19.90 USA Typical Prosocial purchase Personal purchase Anonymous sick
children
ORH 60 59 0.44
Aknin, Hamlin, & Dunn (2012) 55 1.90 Canada Typical Donate own sweets Donate other
sweets
Puppet Smiling 20 20 0.46
Alden & Trew (2013) 28 19.56 Canada Socially anxious AK BE Anyone PA 43 40 0.59
Alden & Trew (2013) 28 19.56 Canada Socially anxious AK BE Anyone NA 43 40 0.16
Alden & Trew (2013) 28 19.56 Canada Socially anxious AK LD Anyone PA 43 43 0.54
Alden & Trew (2013) 28 19.56 Canada Socially anxious AK LD Anyone NA 43 43 −0.42
Anik, Aknin, Norton, Dunn, & Quoidbach
(2013)
Study 1 41 37.28 Australia Typical Prosocial purchase ($25) None Charity PANAS 41 48 −0.15
Anik et al. (2013) Study 1 41 37.28 Australia Typical Prosocial purchase ($50) None Charity PANAS 44 48 0.49
Buchanan & Bardi (2010) 26 38.00 UK? Typical AK New acts Anyone SWLS 28 28 0.41
Buchanan & Bardi (2010) 26 38.00 UK? Typical AK No acts Anyone SWLS 28 28 0.62
Chancellor, Margolis, Jacobs Bao, &
Lyubomirsky (2017)
27 35.60 Spain Typical AK Receiver Co-worker SHS 16 34 *
Chancellor et al. (2017) 27 35.60 Spain Typical AK Receiver Co-worker SWLS 16 34 *
Chancellor et al. (2017) 27 35.60 Spain Typical AK None Co-worker SHS 16 33 *
Chancellor et al. (2017) 27 35.60 Spain Typical AK None Co-worker SWLS 16 33 *
Donnelly, Grant, Lamberton, Walker
Reczek, & Norton (2017)
Study 1 52 22.57 USA Typical Social recycling Trash/recycling Unknown lab
workers
H 59 56 0.77
Donnelly et al. (2017) Study 1 52 22.57 USA Typical Social recycling Take item Unknown lab
workers
H 59 59 0.85
Donnelly et al. (2017) Study 2b 50 37.77 USA Typical Social recycling Trash Unknown lab
workers
PA 107 108 1.25
Dunn, Aknin, & Norton (2008) Study 3 26 College Canada Typical Prosocial purchase Personal purchase Anyone/charity H 23 23 0.67
Geenen, Hoheluchter, Langholf, & Walther
(2014)
21 College Germany Typical Prosocial purchase Personal purchase Friends PANAS 34 34 0.70
Hanniball & Aknin (2016) 43 19.37 Canada Typical Helping behaviour: mapping out
course schedule
Helping self Colleague PANAS 51 56 −0.46
Layous, Kurtz, Margolis, Chancellor,
Lyubomirsky (2017)
Study 1 16 18.55 USA Typical AK Track daily activity Someone known SHS 70 69 0.08
Layous et al. (2017) Study 1 16 18.55 USA Typical AK Track daily activity Someone known WB 70 69 0.20
Layous et al. (2017) Study 1 16 18.55 USA Typical AK Track daily activity Someone known EWB 70 69 0.26
Layous et al. (2017) Study 2 19 18.93 USA Typical AK Make self happier Other/one SHS 178 81 0.30
Layous et al. (2017) Study 2 19 18.93 USA Typical AK Make self happier Other/one WB 178 81 0.12
Layous, Lee, Choi, & Lyubomirsky (2013) 38 College
students
USA/Korea Typical AK Track locations Anyone WB 213 104 0.18
Layous, Nelson, Oberle, Schonert-Reichl,
& Lyubomirsky (2012)
na 10.60 Canada Typical AK Whereabouts Anyone SHSc 208 208 −0.05
Layous et al. (2012) na 10.60 Canada Typical AK Whereabouts Anyone PAc 208 208 −0.12
Layous et al. (2012) na 10.60 Canada Typical AK Whereabouts Anyone SWLSc 208 208 0.07
Martela & Ryan (2016) 36 20.40 USA Typical Benevolence Neutral activity Charity PA 34 42 0.55
Martela & Ryan (2016) 36 20.40 USA Typical Benevolence Neutral activity Charity NA 34 42 0.42
Mongrain, Chin, & Shapira (2011) 16 33.63 Canada Typical AK Memory Anyone SHI 237 237 0.08
(continued on next page)
O.S. Curry et al. Journal of Experimental Social Psychology xxx (xxxx) xxx–xxx
5
2017), following the recommendations summarised in (Field & Gillett,
2010). We used three-level meta-analysis to account for dependent ef-
fect sizes within studies (Van den Noortgate, López-López, Marín-
Martínez, & Sánchez-Meca, 2015). Let y
jk
denote the jobserved effect
sizes y, originating from kstudies. The multi-level model is then given
by the following equations:
=+ ∼
=+ ∼
=+ ∼
⎫
⎬
⎪
⎭
⎪
()
yβ Nσ
βθw w Nσ
θδb bNσ
ϵwhereϵ 0,
where (0, )
where (0, )
jk jk jk jk
jk kjk jk w
kkk b
ϵ
2
2
2
jk
The first equation indicates that observed effect sizes are equal to
the underlying population effect size, plus sampling error ϵ
jk
. The
second equation indicates that population effect sizes within studies are
a function of a study-specific true effect size, plus within-study residuals
w
jk
. The third equation indicates that the distribution of study-specific
true effect sizes are distributed around an overall mean effect, with
between-study residuals b
k
. Results revealed that the overall effect size
estimate was δ= 0.28, 95% CI [0.16, 0.41], Z= 4.36, p< .001 (see
Fig. 2). This is a small-to-medium effect, approximately equivalent to an
increase of 0.6 on a standard 0–10 happiness scale (Helliwell, Layard, &
Sachs, 2016). The within-studies variance component σ
w2
was negli-
gible, 0.00, 95% CI [ < 0.01, 0.02]. The between-studies variance σ
b2
,
on the other hand, differed significantly from zero, 0.08, 95% CI [0.04,
0.18]. The fact that the between-studies component was larger than the
within-studies component indicates that the variation in effect sizes was
primarily accounted for by differences between studies, whereas dif-
ferences between effect sizes within the same studies were negligible.
Likelihood ratio tests also indicated that constraining the within-studies
variance to zero would not worsen model fit, whereas constraining ei-
ther the between-studies variance or both variance components to zero
did lead to significant deteriorations in model fit (see Table 2). This
again indicates that there was substantial heterogeneity between
average effect sizes across studies, but not between effect sizes pub-
lished within the same studies.
File drawer analysis (Rosenthal, 1979) revealed that 1919 un-
published, filed, or unretrieved studies averaging null results would be
required to bring the average unweighted effect size to nonsignificance.
Visual inspection of the Funnel plot (Fig. 3) did not clearly indicate
asymmetry, which could be a sign of publication bias. Begg's test of
funnel asymmetry (based on random-effects meta-analysis) similarly
did not indicate significant bias, Z= 1.07, p= 0.28.
5.4. Moderation
We coded several potential theoretical and methodological moderators:
proportion of male participants, average age of the sample, type of parti-
cipant (typical, socially anxious), type of intervention (acts of kindness,
prosocial spending, other), type of control condition (nothing, neutral ac-
tivity, self-help, other), and outcome measure (happiness, life satisfaction,
positive or negative affect or emotion, other).
The small sample size limits our ability to include these moderators in
mixed-effects meta-analysis without risking overfitting (modeling random
noise in the data, rather than true moderating effects). We therefore used
metaforest (Van Lissa, 2017) to screen for relevant moderators. This tech-
nique uses the machine learning algorithm “random forests”to prevent
overfitting, and to assess the importance of several potential moderators. An
added benefit is that metaforest can capture non-linear relationships be-
tween moderators and effect size, and higher-order interactions. To this
end, many (in this case, 10,000) bootstrap samples are drawn from the
original data, and a models is estimated on each bootstrap sample. Then,
each model's performance is evaluatedoncasesnotpartofitsbootstrap
sample, yielding an estimate of explained variance in new data, R
oob2
.We
conducted random-effects weighted metaforest, with clustered boot-
strapping to account for the multilevel structure of the data (n
tree
= 10000,
Table 1 (continued)
Study Sex Age Location Donor Intervention (IV) Control Recipient(s) Outcome (DV) n1 (I) n2 (C) d
Mongrain et al. (2011) 16 33.63 Canada Typical AK Memory Anyone CES-D 237 237 0.15
Nelson et al. (2015) 53 19.98 USA/Korea Typical AK Work activity Anyone SHS 54.5 54.5 0.23
Nelson et al. (2015) 53 19.98 USA/Korea Typical AK Work activity Anyone SWLS 54.5 54.5 0.27
Nelson et al. (2015) 53 19.98 USA/Korea Typical AK Work activity Anyone PE 55 55 0.09
Nelson et al. (2015) 53 19.98 USA/Korea Typical AK Work activity Anyone NE 55 55 0.06
Nelson, Layous, Cole, & Lyubomirsky
(2016)
40 29.95 USA Typical AK Track activities Other/world PE 238 116 0.30
Nelson et al. (2016) 40 29.95 USA Typical AK Track activities Other/world NE 238 116 0.36
Nelson et al. (2016) 40 29.95 USA Typical AK Track activities Other/world PF 238 116 0.15
Nelson et al. (2016) 40 29.95 USA Typical AK Self Other/world PE 238 116 0.20
Nelson et al. (2016) 40 29.95 USA Typical AK Self Other/world NE 238 116 0.16
Nelson et al. (2016) 40 29.95 USA Typical AK Self Other/world PF 238 116 0.19
O'Connell, O'Shea, & Gallagher (2016) 43 34.17 USA Typical AK List activities Social network SHS 28 12 0.02
O'Connell et al. (2016) 43 34.17 USA Typical AK Self Social network SHS 28 31 0.12
Ouweneel et al. (2014) Study 2 16 20.88 Netherlands Typical AK Neutral activity Anyone PE 25 24 0.27
Ouweneel et al. (2014) Study 2 16 20.88 Netherlands Typical AK Neutral activity Anyone NE 25 24 0.27
Trew & Alden (2015) 26 20.47 Canada Socially anxious AK Social exposure Anyone PA 38 41 −0.05
Trew & Alden (2015) 26 20.47 Canada Socially anxious AK List activities Anyone PA 36 41 −0.33
Whillans, Dunn, Sandstrom, Dickerson, &
Madden (2016)
Study 2 50 72.02 USA Hypertense Prosocial purchase Personal purchase Anyone WB 36 37 0.19
Note. AK = acts of kindness; CES-D = Center for Epidemiological Studies Depression Scale; EWB = Eudaimonic Well-Being; H = happiness; NA = negative affect; NE = negative emotions; ORH = Other-Report Happiness; PA = positive affect;
PANAS = Positive and Negative Affect Scale; PF = Psychological Flourishing; SHI= Steen Happiness Index; Sex = % men in sample; SHS = Subjective Happiness Scale; SWLS = Satisfaction With Life Scale; WB = well-being. * = Statistics needed
to calculate effect size were not reported in the paper, nor available from the authors.
O.S. Curry et al. Journal of Experimental Social Psychology xxx (xxxx) xxx–xxx
6
Fig. 2. Forest plot for the effect of kindness acts on actor's well-being.
O.S. Curry et al. Journal of Experimental Social Psychology xxx (xxxx) xxx–xxx
7
m
try
= 2). We replicated the analysis 100 times to ensure the reliability of
findings. The median estimated explained variance in out-of-bootstrap cases
was negative (R
oob2
=−0.11), with a large standard deviation across re-
plications (SD = 0.19). When R
oob2
is negative, this means that the average
effect size is a better predictor of out-of-bootstrap cases than the model-
implied predictions. In other words, the model did not capture general-
izable relationships between the moderators and effect size, and we did not
find evidence for associations between the moderators and effect size.
6. Discussion
The results of this systematic review and meta-analysis of the experi-
mental kindness literature suggests that performing acts of kindness im-
proves the well-being of the actor (δ=0.28). The effect of kindness is
small-to-medium –comparable to other positive psychology interventions
(such as ‘mindfulness’,‘positive thinking’and ‘counting your blessings’;
d=0.34, Bolier et al., 2013; d = 0.31, Sin & Lyubomirsky, 2009;
d=0.44, Weiss, Westerhof, & Bohlmeijer, 2016).
6
The effect was not
moderated by sex, age, type of participant, intervention, control condition
or outcome measure. And there was no evidence of publication bias. To-
gether, these results suggest that policy-makers and practitioners are cor-
rect to see kindness interventions as effective ways of improving well-
being. And they support the general claim that, as social animals, humans
possess a range of psychological mechanisms that motivate them to help
others, and that they derive satisfaction from doing so.
However, in interpreting these results, a number of limitations
should be kept in mind.
First, most of the reviewed studies were under-powered. The average
sample size per condition was N= 79; this gives power of only β=0.42 to
detect a typical effect size of d = 0.28. In order to detect such an effect with
power β= 0.80, future researchers should use a sample size of at least 202
per group (Faul, Erdfelder, Lang, & Buchner, 2007).
Second, most of the reviewed studies used non-clinical samples of
students, from Western, Educated, Industrial, Rich, Democratic
societies (W.E.I.R.D.; Henrich, Heine, & Norenzayan, 2010). Thus it
remains unclear whether the current findings would generalise to
clinical samples of participants diagnosed with specific mental health
problems, in non-WEIRD societies. Future research should employ more
representative community samples (perhaps focussing on social dis-
orders; Qualter et al., 2015), in a wider variety of cultures.
Third, earlier we defined kindness as ‘actions intended to benefit
others’. The studies reviewed here varied whether actions benefitting
others were performed, they did not vary whether the benefits were
intended or not –in other words, they did not manipulate the motive
behind the action. Previous research has found an association between
motive and outcome; one longitudinal study found that volunteers
motivated by a desire to help others lived longer than non-volunteers,
but that volunteers motivated by a desire to help themselves did not
(Konrath, Fuhrel-Forbis, Lou, & Brown, 2012). If this relationship is
causal, then policy-makers should be aware that encouraging people to
help others because of the benefits to themselves may be counter-pro-
ductive –it may, somewhat paradoxically, mitigate or eliminate the
effect. Further experimental research will be needed to investigate the
role of intention on the benefits of helping others.
Fourth, although the finding is consistent with the general evolu-
tionary account of altruism outlined above, existing research has not
tested the more fine-grained predictions that arise from the more spe-
cific theories of helping (kin altruism, mutualism, reciprocal altruism
and competitive altruism). For example, there has been little systematic
investigation of whether different people benefit more from performing
acts of kindness under different conditions. And studies have not sys-
tematically varied the type of recipient, for example family, colleague,
friend, stranger. In fact, in most cases the recipient was left unspecified
–that is, they could be ‘anyone’. And so we do not know whether:
people who have lost touch with their families derive more pleasure
from acts of kin altruism; or whether people are happier giving to
children as opposed to adults. We do not know whether, as mutualism
predicts, people are be happier giving to in-group as opposed to out-
groups; or whether, as reciprocal altruism predicts, people are happier
giving to unlucky, as opposed to lazy, recipients (Petersen, 2012). Nor
do we know whether ambitious people (with more resources to spare)
seeking status are happier engaging in acts of competitive altruism,
whether single people who are courting are happier helping help po-
tential mates, or whether there are any sex differences in the satisfac-
tion derived from different kinds of helping (Balliet, Li, Macfarlan, &
Van Vugt, 2011). Thus future work should seek to fill these gaps in our
understanding. There is already a large literature on whether people
behave more or less altruistically to specific types of people; it would be
fairly straightforward to add measures of subjective well-being to re-
plications and extensions of these designs.
Fifth and finally, existing research has tended to look at the im-
mediate effects of kindness well-being. Hence it is not clear what the
longer-term effects of the intervention, on well-being or more distal
measures, may be. After all, previous research suggests that such effects
are likely to be short-lived –‘happiness’provides an immediate reward
for behaviour that has long-term benefits, and research on the ‘hedonic
treadmill’suggests that people might have a ‘set point’that they return
to whatever happens to them, good or bad (Ryan & Deci, 2001). If the
function of altruistic behaviour is to help families, make new friends,
improve communities, increase status, or find a mate, then it would be
instructive for future experiments to measure these hypothesised long-
term benefits. Do people allocated to the kindness condition report
better relations with their families? More identification with their
communities? More friends? More recognition and honours? More
pride in one's achievements (Sznycer et al., 2017)? More sexual part-
ners (Arnocky, Piché, Albert, Ouellette, & Barclay, 2016)? More com-
mitted relationships (Kogan et al., 2010)? More resilient marriages? If
so, then future research might be able to finally connect the two types
of happiness –short-term hedonic pleasure, and long-term eudaemonic
components of the good life –that have hitherto remained apart.
Table 2
Model fit indices.
df AIC BIC II LRT p
Three-level model 3 22.14 28.16 −8.07
Within-studies variance constrained 2 20.14 24.15 −8.07 0 1
Between-studies variance constrained 2 33 36.76 −14.37 12.61 0
Both variance components constrained 1 78 80.25 −38.12 60.1 0
Note. Significance of variance components is assessed by constraining them to zero, and
examining the significance of a log-likelihood (ll) ratio test (LRT) comparing the con-
strained model to the full three-level model.
Fig. 3. Funnel plot for the effect of acts of kindness on actor's well-being.
6
Although see Coyne (2014a, 2014b) for critical commentary on (Sin & Lyubomirsky,
2009) and (Bolier et al., 2013).
O.S. Curry et al. Journal of Experimental Social Psychology xxx (xxxx) xxx–xxx
8
7. Conclusion
Helping others makes you happy, but the effect is relatively modest.
Further empirical work testing the implications of more specific the-
ories of social, cooperative and altruistic behaviour is needed to de-
termine whether the effect might be larger for some types of helpers,
when helping some types of recipients. This research will advance our
understanding of the causes and consequences of kindness, and help
practitioners to maximise the effectiveness of kindness interventions.
Open practices
The data and analysis script for this study are available on its Open
Science Framework page (https://osf.io/sey6x/).
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://
doi.org/10.1016/j.jesp.2018.02.014.
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