Received: 26 August 2019
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Revised: 1 July 2020
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Accepted: 2 July 2020
DOI: 10.1002/ab.21916
RESEARCH ARTICLE
Twin study of laboratory‐induced aggression
Bojana M. Dinić1
| Snežana Smederevac1 | Selka Sadiković1 | Milan Oljača1 |
Nataša Vučinić2 | Mechthild Prinz3 | Zoran Budimlija4
1
Department of Psychology, Faculty of
Philosophy, University of Novi Sad,
Novi Sad, Serbia
2
Department of Pharmacy, Faculty of
Medicine, University of Novi Sad,
Novi Sad, Serbia
3
Department of Science, John Jay College of
Criminal Justice, City University of New York,
New York City, New York
4
Department of Neurology, NYU School of
Medicine, New York City, New York
Abstract
The aim of this study was to explore genetic and environmental contributions to
laboratory‐induced aggressive behavior. On a sample of 478 adult twins (316
monozygotic), the Competitive Reaction Time Task was used for aggression induction. The results showed that the initial, basic level of aggression could be explained
by both shared (45%) and nonshared environmental factors (55%), while only nonshared environmental factors (100%) had a significant influence on changes in aggression as provocation increased. Genetic factors had no influence on laboratory‐
Correspondence
Bojana M. Dinić, Department of Psychology,
Faculty of Philosophy, University of Novi Sad,
Dr Zorana Đinđića 2, 21000 Novi Sad, Serbia.
Email: bojana.dinic@ff.uns.ac.rs
Funding information
Ministry of Education, Science and
Technological Development of the Republic of
Serbia, Grant/Award Number: ON179006
induced aggression. The results highlight the importance of environmental factors in
shaping situation‐specific aggressive responses to provocation.
KEYWORDS
Competitive Reaction Time Task, laboratory aggression, twin study
1 | INTRODUCTION
each other in reaction time and received “punishments” that represented situational provocation. Thus, aggression is induced by si-
Aggression is a complex phenomenon that could refer to different
tuational provocation, which increases during the procedure. The
types of behavior, with many variations across forms and functions (e.g.,
procedure allows for the measurement of different aspects of ag-
Bushman & Bartholow, 2010). While behavioral manifestations of ag-
gression, such as the initial, basic level (intercept) of aggression and
gression may take different forms, such as physical or verbal, the
changes (slope) of aggression due to increasing provocation. Conse-
functions of aggression indicate the motivation for aggressive behavior,
quently, the behavior induced in the CRTT should correspond to re-
which allows for the distinction between proactive and reactive
active aggression. Previous results have shown that the CRTT is a valid
aggressive patterns. Reactive aggression is a response to a real or
procedure for the assessment of aggressive behavior (e.g., Chester &
perceived provocation or threat, aimed to harm another person, while
Lasko, 2018). Moreover, despite various measures of aggression in
proactive aggression is instrumental, aimed to achieve other goals, such
research using the CRTT and different CRTT analytic approaches,
as money, social status, or justice (e.g., Bushman & Bartholow, 2010).
reasonably similar empirical aggressive profiles have been consistently
Although aggression can be seen as a stable individual difference
confirmed (Hyatt, Chester, Zeichner, & Miller, 2019).
with a high level of cross‐situational consistency (Tremblay &
An increasing number of studies have attempted to explain the
Belchevski, 2004), research has also shown that aggressive behavior is
causes of aggression by examining its biological roots or genetic basis
highly situation‐specific (Campbell, Bibel, & Muncer, 1985).
(e.g., Veroude et al., 2016). Twin studies have offered an insight into
Several paradigms have been developed to examine laboratory‐
heritability of the examined characteristics, comparing the covaria-
induced, situation‐specific aggression (e.g., McCarthy & Elson, 2018).
tion between monozygotic (MZ) and dizygotic (DZ) twins (see Plomin,
In this study, we used the Competitive Reaction Time Task (CRTT).
Chipuer, & Neiderhiser, 1994). MZ twins share 100% of their genetic
The CRTT is an umbrella term for many procedures based on Taylor's
material, while the percentage of genetic material shared by DZ
(1967) methodology (see Warburton & Bushman, 2019). In this pro-
twins amounts to 50%. In a twin design, two aspects of environ-
cedure, participants were led to believe that they competed against
mental influences could be distinguished—shared and nonshared
Aggressive Behavior. 2020;1–9.
wileyonlinelibrary.com/journal/ab
© 2020 Wiley Periodicals LLC
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environmental influences. Both MZ and DZ twins share 100% of their
aggression (e.g., verbal and indirect assault, verbal and indirect hos-
shared environmental factors. Shared environmental factors refer to
tility) can mostly be explained by nonshared environmental influ-
experiences that are the same for all family members and make
ences (e.g., Coccaro, Bergeman, Kavoussi, & Seroczynski, 1997; Dinić,
members of the same family who live together similar to each other.
Nikolašević, Oljača, & Bugarski Ignjatović, 2018; Seroczynski,
Those factors refer not only to family‐related characteristics such as
Bergeman, & Coccaro, 1999; Sluyter et al., 2000; Vernon, McCarthy,
the socioeconomic status, common social rituals, or coping strategies,
Johnson, Jang, & Harris, 1999). Nevertheless, some research has
but also to characteristics outside the family, such as neighborhood
shown higher nonshared environmental contribution (62%) on the
characteristics (e.g., housing density), school‐related characteristics
physical aggression factor (including indicators of fighting and phy-
(e.g., academic track), or any other feature of the environment to
sical assault), compared to genetic contribution (38%), while also
which family members are jointly exposed.
indicating a different pattern of heritability for physical aggression
By contrast, all variance within twin pairs that is not explained
and the general aggression factor (which includes indicators of
by genetic and shared environmental factors is attributed both to
temper tantrums, verbal, and indirect aggression, see Yeh, Coccaro, &
nonshared environmental factors and measurement error (Plomin,
Jacobson, 2010). Nonshared environmental effects have also been
DeFries, Craig, & McGuffin, 2003). Nonshared environment refers
higher in reactive than in proactive aggression (e.g., Baker, Raine, Liu,
to individual experiences and perceptions of events that contribute
& Jacobson, 2008). However, the stability of reactive aggression can
to differences between family members who live together. Just like
be attributed to genetic and nonshared environmental factors, while
shared, nonshared environmental factors encompass experiences
the stability of proactive aggression can primarily be attributed to
either within or outside the family. Thus, twins may be treated
the genetic factors (Tuvblad, Raine, Zheng, & Baker, 2009). In other
differently by their parents or they could belong to different peer
words, although previous findings indicate a strong genetic influence
groups, which, in turn, may be related to differences in twins' be-
on aggressive behavior, the environment has a crucial role in learning
havior. It is important to note that if twins in a pair perceived the
and the manifestation of aggression.
same events differently and were affected by these events in dif-
Twin studies of adult aggression have usually been designed as
ferent ways, the influence would then be attributed to nonshared
self‐assessment of relatively stable behaviors and tendencies.
environmental effects. For example, the family's socioeconomic
However, the pattern of genetic and environmental influences on a
status could be perceived as average from the standpoint of one
specific aggressive reaction, induced in a particular situation, may
twin in the pair, but below‐average from the standpoint of the
be different. To the best of our knowledge, there has been only one
second twin in the pair. In sum, shared environmental factors are all
twin study to date in which aggression was experimentally induced
factors that make twins in a pair more alike, whereas nonshared
and measured (Achterberg, Duijvenvoorde, Meulen, Bakermans‐
environmental factors are all influences that make twins different
Kranenburg, & Crone, 2018). In this study on 7‐ to 9‐year‐old twins,
from each other.
results showed that nonshared environmental factors accounted
Meta‐analyses of twin studies have shown that heritability of
for the largest part of the variance in aggression after negative
aggression among children and adolescents up to 18 years of age is
social feedback (74–90%), while genetic and shared environmental
65% (Burt, 2009) or around 50% when considering a wider age
factors were quite low (up to 20%, see Achterberg et al., 2018).
range (Miles & Carey, 1997). In the twin studies review by
Other studies including laboratory‐induced behaviors (e.g., risk
Veroude et al. (2016), it was shown that heritability of aggression
taking, cooperative behavior) have shown that nonshared en-
ranged from 39 to 60% for children between ages 2 and 6 and from
vironmental factors constitute a much more important source of
46 to 60% for children between ages 6 and 14, while in adults, it
phenotypic variation than genetic factors, while shared environ-
was difficult to compare the results, but overall heritability was
mental factors have nonsignificant or very low contributions (e.g.,
slightly lower and around 30%. Possible reasons for the wide
Cesarini, Dawes, Johannesson, Lichtenstein, & Wallace, 2009;
variation in heritability among studies are the use of different
Cesarini et al., 2008).
measures of aggression, different research designs (e.g., parent/
The objective of this study was to determine genetic and en-
teacher report, observation, and self‐report), and different sample
vironmental effects on situationally provoked aggressive responses.
characteristics and size. However, all previous results point to the
Aggression was induced by employing the CRTT procedure. Previous
fact that the environment is important for shaping aggressive
studies have shown robust effects of genetic and environmental
behavior. While a nonshared environment has had an impact on
factors on aggression (e.g., Tuvbald & Baker, 2011). However, the
aggression in every study, the contribution of the shared en-
fact that they were mainly based on self‐assessment questionnaires
vironment has been less common (see Tuvbald & Baker, 2011),
could contribute to the neglect of the role of situational character-
detected mostly on samples of children and adolescents (e.g.,
istics. In line with the results from previous twin studies (e.g., Tuvbald
Veroude et al., 2016).
& Baker, 2011) and of laboratory‐induced behavior (e.g., Achterberg
Similar results have been obtained in twin studies of specific
et al., 2018), the assumption was that genetic and environmental
forms of aggression conducted on adult samples. For example, phy-
influences would have significant contributions in shaping a specific
sical aggression and direct assault can be explained by both genetic
aggressive response in a provocative situation. Since this is the first
and nonshared environmental influences, while other forms of
study to combine twin and experimental design in examining adult
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aggressive behavior, predictions based on the specifics of the used
3
2.3 | Competitive Reaction Time Task
procedure were necessary.
Namely, in the CRTT procedure, opponents have usually been
Aggression was induced through the CRTT in which twins were led to
unfamiliar. In this study, twins competed against each other. It is
believe they were competing with each other in the reaction time in
possible that the interpersonal dynamic between twins could con-
visual perception tasks in separate rooms. Before each trial in the
tribute to their expectations based on previous shared experiences.
CRTT, each twin had an opportunity to set the “punishment” for his/
Previous studies have shown greater closeness, familiarity (Segal,
her twin pair. The punishment comprised of the settled intensity (on a
Hershberger, & Arad, 2003), and cooperation between MZ twins
scale from 0 = no punishment, 1 = 60 db to 10 = 105 db, as in Bertsch,
compared to DZ twins (Segal & Hershberger, 1999), supporting as-
Böhnke, Kruk, Richter, & Naumann, 2011) and duration of an aversive
sociations between genetic relatedness and prosocial behavior. To
noise (on a scale from 1 = 0.5 s to 10 = 5 s). After each twin settled the
test whether CRTT aggression has different patterns in MZ and DZ
punishment, they started competing in the reaction time task. The trial
twins compared to participants who compete with an unknown op-
consisted of a square that could appear in one of the five positions on
ponent, we included a control group of nontwins from the study by
the screen (left, right, up, down, and center) with a different inter-
Dinić and Smederevac (2019) where the same procedure was used.
stimulus interval for each trial. The task of each twin was to press the
bar as quickly as possible when the square appeared on the screen.
The following two outcomes were possible for each twin: win (faster
2 | METHOD
reaction time than the other twin) and lose (slower reaction time than
the other twin). Then, the slower twin would receive the punishment
2.1 | Participants and data collection
determined by the faster, winning twin. Before the experiment, each
twin heard the minimum and the maximum punishment noises to gain
The Serbian Twin Registry includes data on 1,654 participants. The
insight into the range of punishments. Thus, twins did not receive
procedure for collecting data from twins is described elsewhere
information about the exact punishment setting of their opponent, but
(Smederevac et al., 2019). Participants in this study were 316 MZ and
only the punishment itself. There were four blocks in the procedure
162 DZ twins raised together (478 in total; see Table 1). Data col-
(each contained 10 trials), with the first block designed as practice. In
lection was mostly done at the faculty premises in the period from
this block, twins only administered the punishment and did not receive
2011 to 2018. Participation was voluntary and each participant
it if they lost. In blocks 2 through 4, twins received predetermined
signed an informed consent form before the research session. The
punishments, which increased during the procedure: in the second
study was approved by the Institutional Ethics Committee. The ap-
block, Mintensity = 70 db (60–75) and Mduration = 0.75 s (0.5–1); in the
proval certificate can be found at the following link: http://
third block, Mintensity = 85 db (80–90) and Mduration = 2 s (1.5–3); and in
psihologija.ff.uns.ac.rs/etika/?odobreno=20111020000004_e1b8.
the fourth block, Mintensity = 100 db (95–105) and Mduration = 4.2 s
To determine a possible difference in the aggressive response
(3.5–5), based on the study by Bertsch et al. (2011). In each block,
between twins and respondents competing with unknown same‐sex
twins randomly won in 50% of the trials. The most common measures
opponent, the sample from the study by Dinić and Smederevac
of aggression—intensity and duration of determined punishments—
(2019) was used as a control group. In this study, the same CRTT
were used. Thus, it was possible to test a biometric latent growth
procedure was used. This sample included 60 university students
model (LGM) to explore genetic and environmental factors on both the
(71.7% females), the majority of whom were in the second year of
basic level of aggression (intercept) and changes, that is, the increase
studies (about 19–20 years old).
or decrease of aggression during the trials as the provocation increased (slope). After the CRTT, participants were debriefed to check
whether they believed the experimental manipulation and to explain
2.2 | Zygosity
the purpose of this procedure. One twin did not believe the cover
story. Thus, the data for this twin pair were excluded from the sample
Zygosity was determined by a DNA analysis of the buccal swab. DNA
description and analysis. In the case of the control group, out of the
was extracted using standard DNA extraction techniques (QIAgen®,
total sample used in Dinić and Smederevac (2019), three participants
Hilden, Germany) and zygosity was determined using the Global Filer
did not believe the experimental manipulation and their data were
kit (Applied Biosystems; Thermo Fisher Scientific, Waltham, MA).
excluded from the sample description and analysis.
T A B L E 1 Twin sample characteristics
Sex
Age
Twins
Total
Male
Female
Different sex
Age range
M
SD
Monozygotic
316
76 (24.1%)
240 (75.9%)
–
16–58
24.89
7.64
Dizygotic
162
62 (38.3%)
100 (61.7%)
76 (46.9%)
16–48
22.89
5.92
Abbreviations: M, mean; SD, standard deviation.
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2.4 | Data analysis
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covariate, but results were the same, thus we kept the analysis
without the age effect. Additionally, differences between same‐sex
First, descriptive statistics and univariate cross‐twin correlations (both
and opposite‐sex pairs were tested compared the following groups:
Pearson's and intraclass coefficients of correlation [ICC] with corre-
(a) MZ twins, (b) DZ same‐sex, (c) DZ opposite‐sex, and (d) control
sponding 95% confidence interval [CI]) were calculated for measures
group with the same‐sex unknown opponent. For posthoc compar-
of aggression. The ICCs were calculated using a random‐effects one‐
isons, the Bonferroni test was used. Effect size was calculated as ηp2,
way analysis of variance (ICC 1.1; Shrout & Fleiss, 1979), by the for-
which should be up to 0.06 for small, 0.07–0.14 for medium, and
mula ICC = (MSbetween − MSwithin)/(MSbetween + MSwithin), in which the
>0.14 for large effects (Cohen, 1988).
expected values of the within‐pair mean‐square (MSwithin) and the
Fourth, the LGM (McArdle & Hamagami, 2003) was applied to
between‐pair mean‐square (MSbetween) in MZ were compared to those
estimate genetic and environmental influences of the initial, basic
in DZ twin pairs. A higher and positive ICC value indicates higher
level of aggression (intercept) and the rate of aggression change
concordance of variables in the twin pair. For multiple Pearson's
during the four blocks of the CRTT procedure (slope). An LGM is
correlation coefficients, Bonferroni correction of p‐value was used.
presented in Figure 1. The top part of the model represents the
Second, the effect of manipulation in the CRTT procedure was
standard biometric model. The total variation of the phenotype could
tested by the combined repeated measures general linear model
be explained by additive genetic variance (A), shared environmental
(GLM). Since previous studies have shown sex (e.g., Archer, 2004)
variance (C), and nonshared environmental variance (E). A reflects
and age effects (e.g., Piquero, Carriaga, Diamond, Kazemian, &
the component of the total genetic variance that can be explained by
Farrington, 2012) on aggression, these effects were tested to de-
genes within genotypes, C defines common environmental effects for
termine which characteristics should be controlled, that is, partialized
both twins, whereas E reflects individually specific factors of en-
out from aggression for the biometric models. Thus, sex was entered
vironmental and individual experiences for each individual and
as a between‐subject factor, age as a covariate, and the block of the
measurement error. Four models were tested—full ACE and reduced
CRTT procedure as a within‐subject factor or repeated measure,
AE, CE, and E models. An important specificity of the biometric model
while aggression in the CRTT procedure was a dependent variable.
is fixing the values of certain parameters. Parameter A was fixed at
Third, since twins competed with each other, to explore the
1.00 for MZ, since they shared 100% of the genes, while this para-
possible differences in aggression between twins and participants
meter was fixed at 0.50 for DZ, since they shared about 50% of their
who compete with unknown opponents, the GLM was also applied to
genes on average. Parameter C was fixed at 1.00 in both MZ and DZ,
group membership (MZ twins, DZ twins, control group—competition
due to the assumption that twins shared 100% of the shared en-
with an unknown opponent) and sex as between‐subject factors, the
vironmental variance. Path coefficients for the intercept were fixed
block of the CRTT as a within‐subject factor, and aggression in the
at 1 because the intercept is constant across time and it is assumed
CRTT as a dependent variable. Preliminary, age was entered as a
to be the baseline level of certain trait/state, independent of changes
F I G U R E 1 A biometric latent growth model. Note: Ac = common additive genetic factor, Acs = unique additive genetic factor, Ec = common
nonshared environmental factor, Ecs = unique nonshared environmental factor, Cc = common shared environmental variance, Ccs = unique
shared environmental variance, λ1, λ2 = freely estimated path coefficients, e1–8 = errors
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5
due to experimental procedure or time (McArdle, 2006). The slope
0.87, rblock2 = 0.91, rblock3 = 0.91, and rblock4 = 0.91 for dizygotic twin
path coefficient was fixed at 0 for the first block (as the first block is
pairs), composite scores (average scores) combining intensity and
basically the baseline level of the trait or the phenomenon of inter-
duration were used in further analysis, as in previous studies (e.g.,
est) and was fixed at 1 for the last block, while for the second and the
Dinić & Smederevac, 2019). Descriptives for the average aggression
third blocks it was freely estimated. This allowed for the monitoring
scores across blocks for MZ and DZ twins are presented in Table 2.
of the shape of the change through the blocks of the CRTT. Model fit
was evaluated by using several fit indices: the quotient χ2/df, which
should be lower than 2, the comparative fit index, the Tucker–Lewis
index, which should be higher than 0.90, and the root mean square
3.2 | Manipulation check, sex, and age effects on
laboratory‐induced aggression
error of approximation, which should be lower than 0.08. Comparisons between nonnested models were tested via the change in the
The results of the combined repeated measures GLM showed that
Akaike information criterion (AIC), with lower values indicating
the block of the CRTT procedure had a significant effect on aggres-
better fit (e.g., Kline, 2010). Statistical analyses were performed in
sion (F[3,465] = 7.25, p < .001; ηp2 = 0.045), as well as the interaction
Amos 23 by using the maximum likelihood estimator and in SPSS 24.
between block and sex (F[3,465] = 2.68, p = .046; ηp2 = 0.017), while
The data that support the findings on this study are openly available
the interaction between block and age was not significant
in Open Science Framework (OSF) at https://osf.io/j4vqx/.
(F[3,465] = 1–71, p = .164; ηp2 = 0.011). In the case of main effect of
block, posthoc Bonferroni tests showed that there were significant
differences in aggression between the fourth block and the rest of
3 | RESULTS
the blocks (all p < .001), as well as between the third block and the
rest of the blocks (all p < .001), while the only nonsignificant differ-
3.1 | Descriptive statistics and univariate cross‐
twin correlations
ence was between the first and the second block (p = .35; see Table
SA). Thus, there was increment of aggresssion from the second to the
fourth block of the CRTT procedure (see Table 2 and Figure 2).
Univariate cross‐twin correlations in aggression measures between
Regarding interaction effect between the block and sex, posthoc
DZ twins were almost equal or slightly higher compared to MZ twins
Bonferroni tests showed that although males had higher aggression,
(Table 2). Because correlations among punishment intensity and
the increment of aggression among females was higher from the
duration for the same blocks were very high (rblock1 = 0.86, rblock2 =
second through the fourth block (see Table SB and Figure SA). Thus,
0.84, rblock3 = 0.86, and rblock4 = 0.91 for monozygotic and rblock1 =
the effect of sex was partialized in biometric LGM.
T A B L E 2 Descriptive statistics and univariate cross‐twin correlations for laboratory‐induced aggression
Monozygotic twins
Dizygotic twins
Control group
Aggression
Block
M
SD
r
ICC (95% CI)
M
SD
r
ICC (95% CI)
M
SD
Punishment intensity
1
4.22
2.93
.41
0.41 (0.27–0.53)
4.61
2.55
.37
0.37 (0.16–0.54)
3.77
2.42
2
4.10
2.94
.39
0.38 (0.24–0.51)
4.80
2.91
.37
0.37 (0.16–0.54)
3.56
2.55
3
4.32
2.95
.37
0.36 (0.21–0.49)
5.19
3.03
.51
0.51 (0.33–0.65)
3.98
2.83
4
4.66
3.08
.40
0.38 (0.24–0.51)
5.46
3.00
.46
0.46 (0.27–0.62)
4.51
3.07
1
3.76
2.77
.33
0.30 (0.16–0.44)
4.15
2.54
.36
0.35 (0.14–0.53)
3.43
2.04
2
3.60
2.62
.35
0.28 (0.14–0.42)
4.28
2.81
.41
0.38 (0.18–0.55)
3.29
2.24
3
4.02
2.79
.41
0.36 (0.22–0.49)
4.71
2.86
.47
0.44 (0.25–0.60)
3.78
2.61
4
4.54
2.98
.41
0.38 (0.24–0.51)
5.21
2.85
.49
0.48 (0.29–63)
4.53
2.88
1
4.13
2.65
.41
0.40 (0.26–0.52)
4.51
2.57
.41
0.42 (0.22–0.58)
3.60
2.13
2
4.08
2.72
.40
0.37 (0.23–0.50)
4.72
2.87
.44
0.44 (0.25–0.60)
3.43
2.30
3
4.43
2.82
.43
0.40 (0.26–0.52)
5.14
3.00
.53
0.52 (0.35–0.67)
3.88
2.63
4
4.85
2.94
.44
0.42 (0.28–0.54)
5.47
3.00
.50
0.50 (0.32–0.65)
4.53
2.92
Punishment duration
Average scores for intensity and
duration of punishments
Note: Control group – participants who competed with an unknown same‐sex person. Descriptives for the control group were calculated from the data
used in Dinić and Smederevac (2019). All Pearson's correlations were statistically significant at p < .001 after applying the Bonferroni correction for
p‐value (for 24 Pearson's correlation coefficients, with default critical p‐value set on .05, the corrected critical value was p = .00208).
Abbreviations: CI, confidence interval; ICC, intraclass coefficient of correlation; M, mean; SD, standard deviation.
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indicated that females from DZ opposite‐sex twin pairs showed
somewhat higher aggression scores compared to females from the
control group (p = .065; see Figure SB), while the same was not observed in males.1 Thus, it seems that the differences between DZ
twins and the control group are due to the differences in
aggression between females from opposite‐sex DZ twin pairs and
females from the control group.
3.4 | Biometric latent growth model of
laboratory‐induced aggression
Although all proposed biometric models showed good model fit, according to the AIC, the most optimal was the CE model (Table 3). In
F I G U R E 2 Laboratory‐induced aggression across blocks of the
CRTT procedure among monozygotic twins, dizygotic twins, and
control group (bars represent 95% CI of the standard errors of mean)
this model, nonshared environment effects were stronger for the
intercept (baseline) level (β = −0.74, p < .001; E = 0.55) than the effects of shared environment (β = 0.67, p < .001; C = 0.45). The slope
variance, or the variance of change, was fully explained by the nonshared environment effects (β = 1.00, p = .051; E = 1.00).
3.3 | Comparison between twins and nontwins in
laboratory‐induced aggression
4 | D IS C U S S I O N
First, the three groups of participants (MZ twins, DZ twins, and the
control group) were compared in aggression, including sex as the
The objective of this twin study was to determine genetic and en-
second between‐subject factor. The results of the combined repeated
vironmental effects on laboratory‐induced aggression. First, a ma-
measures GLM showed that group membership had a marginally
nipulation check of the used CRTT procedure for aggression
significant small effect on aggression (F[2,532] = 2.88, p = .057; ηp2 =
induction showed that increasing provocation results in a stronger
0.011), while none of the interactions including group membership
aggressive response, which is in line with previous findings (e.g.,
were significant (all p > .05; see Table SC). Bonferroni posthoc tests
Bettencourt & Miller, 1996; Chester & Lasko, 2018; Weidler
showed that a significant difference was obtained only between DZ
et al., 2019). Such “tit for tat” strategy evolves as a common reaction
twins and the control group (mean difference = 1.08; p = .021), with
within the CRTT paradigm (e.g., Lindsay & Anderson, 2000) and
DZ twins obtaining higher scores on aggression (see Figure 2). There
corresponds to reactive aggression, shaped by specific situational
were no significant differences in aggression between MZ and DZ
triggers or provocation. Although the CRTT paradigm could en-
twins (p = .326), nor between MZ twins and controls (p = .219).
compass several levels of provocation, such as no‐provocation, con-
To further test this effect, the distinction between same‐sex and
stant low‐provocation, or increasing provocation, which corresponds
opposite‐sex DZ twins was also included in the second GLM analysis.
to the high‐provocation level (see Dinić & Smederevac, 2019), only
Thus, differences between four groups were tested. Again, sex was
the high‐provocation level was applied in this study. This fact is im-
entered as a between‐subject factor. The results did not show a
portant since previous results have only shown an increase in ag-
significant effect of group membership (F[3,530] = 2.32, p = .074;
gression in high‐provocation conditions (e.g., Dinić & Smederevac,
ηp2 = 0.013) nor the rest of the interactions with group membership
2019), implying that such an aggressive response is influenced by the
(all p > .05; see Table SC).
strength of the provocation, not its repetition. Furthermore, con-
However, since we were interested in differences between DZ
sistent with the results of previous studies, sex had a significant
twins and the control group, we tested the differences between three
effect on laboratory‐induced aggression, since men provided longer
groups—same‐sex DZ twins, opposite‐sex DZ twins, and the control
and more intense punishments (e.g. Zeichner, Parrott, & Frey, 2003).
group. Sex was entered as a between‐subject factor, as in previous
Thus, to control the effect of sex, sex was partialized out from the
analyses. The results showed a significant small effect of group
aggression scores in the main biometric analysis.
membership (F[2,216] = 3.56, p = .030; ηp2 = 0.032), while the rest of
The most important result was that laboratory‐induced aggres-
the interactions with group memberships were not significant (all
sion was best explained by both shared and nonshared environ-
p > .05, see Table SC). Posthoc Bonferroni tests showed that there
mental factors. This result is in line with a previous longitudinal twin
were significant differences only between opposite‐sex DZ twins and
study that showed that environmental factors are important for the
controls (p = .012), with opposite‐sex DZ twins showing higher aggression scores (see Table SD). Additional posthoc Bonferroni tests
when only opposite‐sex DZ twins and controls were included
1
In an additional analysis, a differentiation between female and male MZ pairs was also
included, but there were no significant differences in group membership.
DINIĆ
|
ET AL.
T A B L E 3 Fit indices for the tested biometric latent growth models
and for the intercept and slope of laboratory‐induced aggression
Model
χ
ACE
939.40
2
df
χ /df
CFI
TLI
RMSEA
AIC
68
13.82
0.65
0.71
0.23
979.40
2
7
opposite‐sex DZ twin pairs obtaining higher scores. Since there are
no differences between females from same‐sex DZ and opposite‐sex
DZ twin pairs, this result could indicate a specific interpersonal dynamic between opposite‐sex DZ twins. A previous study on school‐
aged children showed that opposite‐sex DZ pairs had a direct influ-
AE
158.83
70
2.27
0.97
0.97
0.07
194.83
CE
153.81
70
2.20
0.97
0.97
0.07
189.81
et al., 2020). In this study, a higher level of aggression in the male
E
206.31
72
2.87
0.95
0.96
0.09
238.31
twin resulted in less aggression in the female co‐twin, but females
Note: All χ2 tests were significant at p < .001.
Abbreviations: AIC, Akaike information criterion; CFI, comparative fit
index; RMSEA, root mean square error of approximation; TLI,
Tucker–Lewis index.
ence on each other that differed from same‐sex pairs (Luningham
with higher aggression tended to interact with their male co‐twin by
promoting higher aggression, although heritability of aggression was
consistent for males and females. In another study likewise conducted on school‐aged children, it was shown that females from
opposite‐sex DZ twin pairs reached the male level in hyperactivity‐
stability of reactive aggression, in contrast to proactive aggression
impulsivity and aggression and they were more aggressive compared
(Tuvblad et al., 2009). Although the higher contribution of environ-
to singletons (Pulkkinen, Vaalamo, Hietala, Kaprio, & Rose, 2003).
mental factors was expected, the lack of genetic influence was not in
Furthermore, they showed higher leadership compared to other
line with predictions. A possible explanation is that the CRTT pro-
twins and singletons.
cedure induces aggression in a highly contextual situation, which
Thus, we could conclude that the higher aggression in females
leads to significant contributions of environmental influences only. In
from opposite‐sex DZ twin pairs was rather the result of a specific
other words, it is possible that specific reactions to provocation are
social learning context and experiences from the interaction with
generally determined by environmental factors and that the triggers
male co‐twins, which provided opportunities for shaping, imitating,
for the appearance of these reactions are in the environment. A
and practicing various behavioral patterns related to both typical
previous study exploring laboratory‐induced aggression in children
female and typical male behaviors. This result also indicates the need
showed that nonshared environmental factors had a high contribu-
for future exploration of the impact of relationship quality and dy-
tion (74–90%), with a small proportion of genetic (10–20%) and
namic between the twins on aggressive behavior.
shared environmental factors (0–8%; see Achterberg et al., 2018).
There are several limitations to this study that prevent a reliable
Moreover, the results showed that shared and nonshared en-
generalization of the results. First, females seemed to be over-
vironmental factors almost equally contributed to the explanation of
represented in the study sample. Since aggression is a phenomenon
the initial level of aggression, but the change of aggression under
that contributes to significant sex differences, it is possible that the
provocation depended solely on nonshared environmental factors. It
results were influenced by sex‐specific biases, despite controlling for
is possible that the initial level of aggression is more influenced by
the effect of sex. Second, it remains unclear whether the results were
the learned patterns developed in a shared, family environment,
influenced by specific expectations of the situation due to the ac-
while a strong provocation causes reactions determined by concrete
quaintance between participants, sibling closeness, or twin‐specific
circumstances and idiosyncratic experience.
interaction. In future studies of laboratory‐induced aggression, it
The specificity of the research design of this study suggests the
would be particularly important to vary the degree of acquaintance
need for alternative explanations of the results. It is possible that the
between opponents to determine the possible impact of prior ex-
interpersonal dynamic between the twins and the previous experi-
perience on the characteristics of an aggressive response. Third, re-
ence of possible competition and mutual comparison contributed to
sults in this study are inconsistent with previous studies that have
their expectations and reactions in this specific situation. Namely, in
shown a significant proportion of genetic factors to specific behavior
twin studies of laboratory‐induced cooperative or risk‐taking beha-
responses (Achterberg et al., 2018; Cesarini et al., 2008), leaving the
vior, nonshared environmental factors have been the most important
possibility that it could be due to the used CRTT procedure in which
source of phenotypic variation (Cesarini et al., 2008, 2009), implying
twins competed with each other or due to the small sample size.
the possibility that the specific interpersonal dynamics between ac-
Thus, future research would need to explore aggression in a CRTT
quaintances can significantly define the nature of reactions.
procedure in which twins would compete against unknown persons.
The results showed that MZ and DZ twins did not differ in ag-
Finally, while the sample size could be considered as appropriate for
gression, despite the hypothesis about the connection between ge-
the experimental design, it could be considered as moderate for
netic relatedness and prosocial behavior (Segal & Hershberger,
multivariate biometric models (e.g., Verhulst, 2017). A larger sample
1999). However, there was a significant difference between DZ twins
could certainly provide the basis for further exploration of the po-
and members of the control group who competed with unknown
tential moderation effect of sex, some important environmental
same‐sex opponents, with DZ obtaining higher aggression scores.
variables, or stable individual differences.
Additional analyses showed that those differences probably stem
Despite these limitations, the results highlight the importance of
from the differences between females from opposite‐sex DZ twin
both shared and nonshared environmental factors in the explanation
pairs and females from the control group, with females from
of aggressive responses in provocative situations. In other words,
8
|
DINIĆ
while there is compelling evidence to suggest the genetic basis of
aggressive behavior (Burt, 2009; Veroude et al., 2016), it is possible
that there are highly provocative situations that elicit aggressive
responses fully determined by contextual factors.
A C K N O W L E D GM E N T S
The authors thank Prof. Rainer Reiman for suggestions regarding
statistical analysis, Bojan Branovački for maintenance of the data
matrix, and volunteers in the Centre for Behavioral Genetics for help
in collecting the data. This study was supported by the Ministry of
Education, Science and Technological Development of the Republic
of Serbia (Grant ON179006).
CO NFLICT OF I NTERE STS
The authors declare that there are no conflict of interests.
D A T A A V A I L A B I LI TY S TA T E ME N T
The data that support the findings on this study are openly available
in the OSF at https://osf.io/j4vqx/.
OR CID
Bojana M. Dinić
http://orcid.org/0000-0002-5492-2188
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How to cite this article: Dinić BM, Smederevac S, Sadiković S,
et al. Twin study of laboratory‐induced aggression. Aggressive
Behavior. 2020;1–9. https://doi.org/10.1002/ab.21916