The threat simulation theory in light of recent empirical evidence: a review.

Abstract

The recently proposed threat simulation theory (TST) states that dreaming about threatening events has a biological function. In the past few years, the TST has led to several dream content analysis studies that empirically test the theory. The predictions of the TST have been investigated mainly with a new content analysis system, the Dream Threat Scale (DTS), a method developed for identifying and classifying threatening events in dreams. In this article we review the studies that have tested the TST with the DTS. We summarize and reevaluate the results based on the dreams of Finnish and Swedish university students, traumatized and nontraumatized Kurdish, Palestinian, and Finnish children, and special dream samples, namely recurrent dreams and nightmares collected from Canadian participants. We sum up other recent research that has relevance for the TST and discuss the extent to which empirical evidence supports or conflicts with the TST. New evidence and new direct tests of the predictions of the TST yield strong support for the theory, and the TST's strengths seem to outweigh its weaknesses.

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American Journal of Psychology
Spring 2009, Vol. 122, No. 1 pp. 17–38 • © 2009 by the Board of Trustees of the University of Illinois
The threat simulation theory in light
of recent empirical evidence: A review
KATJA VALLI and ANTTI REVONSUO
University of Turku
The recently proposed threat simulation theory (TST) states that dreaming about threatening
events has a biological function. In the past few years, the TST has led to several dream content
analysis studies that empirically test the theory. The predictions of the TST have been investi-
gated mainly with a new content analysis system, the Dream Threat Scale (DTS), a method de-
veloped for identifying and classifying threatening events in dreams. In this article we review the
studies that have tested the TST with the DTS. We summarize and reevaluate the results based
on the dreams of Finnish and Swedish university students, traumatized and nontraumatized
Kurdish, Palestinian, and Finnish children, and special dream samples, namely recurrent dreams
and nightmares collected from Canadian participants. We sum up other recent research that has
relevance for the TST and discuss the extent to which empirical evidence supports or conflicts
with the TST. New evidence and new direct tests of the predictions of the TST yield strong sup-
port for the theory, and the TST’s strengths seem to outweigh its weaknesses.
The threat simulation theory (TST) (Revonsuo,
2000a, 2000b) claims that dreaming originally
evolved as an offline simulation of the real perceptual
world and that its function in the ancestral human
environment was to repeatedly produce simulations
of the real threatening events that had been encoun-
tered during waking hours and had left a mark in
emotional memory. The function of threat simula-
tion in dreams was the repeated nocturnal rehearsal
of the neurocognitive mechanisms that are essential
for threat recognition and avoidance behaviors while
awake. In the environment of evolutionary adapted-
ness (EEA), recurring threat simulations during
dreaming increased the survival and reproductive
success of the individuals possessing an efficient
threat simulation system. Gradually, as the neural
mechanisms underlying dream consciousness were
genetically transmitted from generation to genera-
tion, the system propagated its own existence in the
EEA. Thus, modern humans still possess this ancient
defense mechanism, although the threat simulation
system probably cannot fulfill its original biological
function in the present environment because the
modern environment is remarkably different from
the one where the system evolved.
The TST states that dreaming fulfills a threat
simulation function, and this statement can be tested
directly by investigating whether dreaming shows fea-

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tures that we would predict a threat simulation mech-
anism to have. In contrast, when the TST postulates
an evolutionary origin for the threat simulation func-
tion of dreaming—that is, infers a historical causal re-
lationship between the present form of dreaming and
the origin of dreaming—this is not a directly testable
statement. Regardless, historical causal claims about
evolved psychological mechanisms can be investi-
gated indirectly by study of the traces the proposed
function has left in the psychological architecture
of currently living humans (Tooby & Cosmides,
1992, 1995). Consequently, the decision whether the
mechanism is an evolved adaptation is based on a
probability estimate (Tooby & Cosmides, 1992). In
sum, we can empirically directly test whether dream-
ing shows the features needed for a threat simulation
function, but we can only indirectly estimate whether
threat simulation during dreaming is an adaptation.
Although the TST was published quite recently
(Revonsuo, 2000b), within a few years empirical evi-
dence relevant to the TST has been accumulating
rapidly. The aim of this article is to bring together all
available findings and evaluate the TST in light of the
newly acquired information. First, we briefly review
the TST and its predictions. Second, we present the
method with which the TST is usually tested: the
Dream Threat Scale (DTS). Third, we review and
summarize the findings of dream studies that have
used the DTS, and fourth, based on these results,
we offer estimates for annual threat simulation rates
in healthy young adults. Fifth, we survey a variety of
other recent research, draw together all the currently
available evidence, and discuss the propositions of
the TST in light of the recent evidence.
The TST
The TST was originally presented in the form of six
propositions, each leading to several empirically test-
able predictions (Revonsuo, 2000b). Consequently,
the central claims of the theory are open to support-
ing or falsifying evidence. The first proposition of
the TST is that dream consciousness is an organized
and selective simulation of the perceptual world. To
begin with, the TST accepts, based on previous em-
pirical evidence, that the form of dream experience
is organized along the same lines as perceptual wak-
ing consciousness (Foulkes, 1985; Rechtschaffen &
Buchignani, 1992; Strauch & Meier, 1996) and that
the coherent dream world functions as a credible
multimodal world analog (Foulkes, 1985) that is ex-
perienced, during dreaming, as real life (Revonsuo,
2000b, 2006). Consequently, the TST claims that
the dream experience is functionally constructed to
resemble waking experiences and therefore shows
clear design features for a world simulation function.
Moreover, the TST assumes, also based on currently
available evidence, that dream consciousness is se-
lective in a sense that certain types of contents (e.g.,
social interactions and emotions) (Domhoff, 1996;
Strauch & Meier, 1996) are regularly simulated in
dreams, whereas some others are not (e.g., reading,
writing, using a computer, and many other activities
often practiced during waking hours) (Hartmann,
2000; Schredl & Hofmann, 2003).
Second, the TST claims that dream consciousness
is specialized in the simulation of threatening events,
that is, dream content selectively represents negative
situations. Thus, the TST predicts that threatening
events should occur in dreams more frequently than
in waking life and that the content of dream threats
should reflect the original function of dream con-
sciousness as a threat simulator. Therefore, threat-
ening events in dreams should focus on simulating
dangerous events critical for the future survival of the
individual, and the threats should mainly endanger
the well-being of the dream self and people significant
for the self (close kin and allies). The threats should
be realistic portrayals of real threatening events. The
dream self should be able to participate in the course
of the threatening events and defend itself with appro-
priate and reasonable actions that would be relevant
in a comparable waking situation.
Third, the TST predicts that only real threaten-
ing events can fully activate the threat simulation sys-
tem. Encounters with real threats in the environment
are stored in “hot” emotional memory, and when
asleep, the dream production system automatically
selects the memory traces with the highest salience
for dream construction. Consequently, memory
representations containing the highest negative
emotional charge, encoding fearful and threatening
events, should be selected over and over again for
simulation. Thus, real-life threat experiences acti-
vate the threat simulation system by offering raw
materials to it. Moreover, the activation of the threat
simulation system after exposure to life-threatening
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events should be a universal phenomenon, not de-
pendent on any specific culture, and the intensity of
the simulations (frequency and persistence) should
be directly related to the degree of personal threat
experienced in the original situation.
The fourth proposition of the TST states that
threat simulations are perceptually and behaviorally
realistic rehearsals of real threatening events. During
dreaming, the offline world simulation engages the
same brain mechanisms as perceptual consciousness
and seems real to us because we are unaware that it
is nothing but a hallucination (except very rarely in
lucid dreams). Similarly, the sleeping brain is capable
of simulating the planning and execution of motor
commands, but muscular atonia during rapid eye
movement (REM) sleep prevents our bodies from
carrying out the planned behaviors. If the muscular
atonia during REM sleep is removed, we should ob-
serve that the body of the dreamer carries out isomor-
phic behaviors while they happen in the dream.
Fifth, the TST assumes that perceptually and be-
haviorally realistic simulation and rehearsal of any
skills (in this case threat recognition, avoidance, and
coping skills) should improve performance, regard-
less of whether the training episodes are explicitly
remembered. Simulation training and mental training
have been shown to improve performance; that is, im-
plicit procedural learning occurs when behaviors are
repeatedly practiced in nonreal situations (Lejune,
Decker, & Sanchez, 1994; Yaguez et al., 1998). Fur-
thermore, procedural learning occurs also in amne-
siacs, showing that performance enhancement can be
attained without explicit memory of training episodes
(Schacter, 1996).
Finally, the sixth proposition of the TST states that
the original human EEA included frequent dangerous
events, which imposed severe selection pressures on
ancestral populations, and that the ecologically valid
threat cues in the environment fully activated the threat
simulation system. Thus, we can predict that children
old enough to implement threat recognition and avoid-
ance skills during wakefulness should be capable of
simulating threatening events in their dreams, espe-
cially when exposed to ecologically valid threat cues.
Moreover, ontogenetically early exposure to real threat
cues should lead to earlier, more frequent, and more
intensive threat simulation dreams, whereas lack of
exposure should lead to slower development of the
dream production system and less frequent and milder
threat simulation dreams. (The latter may often be the
case in modern Western world.)
The DTS
Because the TST makes claims about the content of
dreams, dream content analysis can be used in the di-
rect testing of the TST. Content analysis is a method
of transforming verbal material into numbers, that is,
a tool that allows objective quantification of qualita-
tive data (Hall & Van de Castle, 1966). The aim is to
describe qualitative material in terms of frequencies
and rates, so that statistically significant features can
be revealed from large samples of individual cases.
In order to test the predictions of the TST,
Revonsuo and Valli (2000) devised a content analy-
sis system to specifically measure the frequency and
quality of threatening events in dreams, the DTS.1
The aim was to develop a method that would help
in extracting all the information in dream reports
that describe threatening situations and to further
categorize them according to the specific content.
The threat identification criteria used in the DTS
include objective threats, that is, events in which the
physical or mental well-being of any person, or the
physical resources or territory of any person, is en-
dangered, and subjective threats, that is, events that
are interpreted or emotionally experienced by the
dreamer to be somehow dangerous or threatening,
even if no objective threat is reported to accompany
this feeling.2 In contrast, situations that are experi-
enced as fictional in the context of the dream (e.g.,
the dreamer reports watching a movie or playing a
computer game that includes threats) are not con-
sidered as threatening events, nor are situations in
which the dream self is deliberately trying to harm
himself or herself.
As to coding the specific quality of dream threats,
the DTS accounts for the nature of threats (escapes or
pursuits, accidents or misfortunes, failures, catastro-
phes, disease or illness, nonphysical aggression, and
direct physical aggression), the targets of threats (self,
significant others or resources, insignificant people
or resources), the severity of threat to self (life-threat-
ening, physically dangerous, nonphysically severe,
minor), the participation of self in the event (does par-
ticipate, does not or cannot participate), the reaction
of self to the event (relevant or adequate, physically
tst aNd receNt emPIrIcal evIdeNce  •  19

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impossible, irrelevant, no reaction or reaction not
possible), the resolution of threat (resolved, realized,
resolution unclear due to discontinuity, resolution
not reported), the consequences of threat to self (no
losses, minor losses, severe losses, consequences not
reported), and the realism of threat (realistic, realistic
but unlikely, fictitious). In sum, the DTS takes into
account all types of harmful events that either directly
or indirectly had a significant effect on the survival
and reproductive success of an individual in EEA.
Including all kinds of threatening events in the DTS
and separately investigating their specific quality al-
lows us to test hypotheses about the activity level of
the threat simulation system in different populations,
as the TST predicts that waking life experiences have
a significant effect on the frequency and quality of
threats in dreams.
Studies conducted using the DTS
After the development of the DTS, the next goal in
testing the TST was to collect dream samples from
various participant populations, with varying expo-
sure levels to real threatening events, from different
cultures, and from children as well as adults. It was
assumed that if the TST correctly predicts the fre-
quency and content of threatening events in many
different dream samples, the theory would receive
support, whereas if the predictions proved to be false,
the theory should be rejected or at least thoroughly
modified. Consequently, all the studies using the
DTS were designed to test a specific hypothesis or
hypotheses of the TST, and the participant popula-
tions varied accordingly.
UNIVERSITY STUDENT SAMPLES.
In the very first study conducted to test the TST
(Revonsuo & Valli, 2000, Study I), the aim was to test
the predictions derived from the second proposition
of the TST, that is, that threatening events should be
frequent in dreams, targeted against the dream self,
severe, and realistic and should enable the dreamer
to participate in and react to the situation. The par-
ticipants were 52 Finnish university students who
kept systematic dream diaries for 4 weeks. Detailed
instructions on how to report dreams were provided,
and altogether, the participants produced 592 dream
reports. The data were collected in 1995, well before
the TST existed. Thus, the participants and the re-
searchers involved in the data collection were blind
to the predictions of the TST. The second study un-
dertaken to test the TST (Study II) was a replication
of the original Study I in a sample of 248 dreams
collected from 50 Swedish university students (Valli,
Lenasdotter, MacGregor, & Revonsuo, 2007). The
purpose of the study was to investigate whether the
findings of the original Revonsuo and Valli (2000)
study would be replicated in an independent but
comparable participant sample, collected in different
language area, and analyzed by different judges.
Study III (Valli, Strandholm, Sillanmäki, &
Revonsuo, 2008) tested the hypotheses of the TST
that dream threats are more frequently experienced
than comparable real-life threats and that in threat
simulations the severity of threats is exaggerated. Al-
though the frequency and quality of dream threats
were described in Studies I and II, there was no base-
line of the frequency and quality of real-life threat
experiences to compare the frequency and quality of
dream threats to. The participants were 39 Finnish
university students who wrote down their dreams
(N = 419) and simultaneously recorded their real-life
experiences in daily logs (N = 490). The participants
were also interviewed about all the threatening events
they had ever experienced and could recall from their
long-term autobiographical memory (N = 714). Then,
the frequency and quality of dream threats, most
recent threats, and threats encoded into long-term
episodic memory were compared.
SPECIAL POPULATIONS.
Study IV and Study V were designed to test the
third proposition of the TST, that exposure to real
threatening events fully activates the threat simulation
system. If this is the case, the frequency and quality
of threat simulation dreams should vary significantly
between participants who have been and participants
who have not been exposed to real life-threatening
events. In Study IV, the participants were 64 severely
traumatized and 53 less traumatized Kurdish chil-
dren and 70 nontraumatized Finnish children (Valli
et al., 2005). Altogether, 763 dreams were acquired,
331 from severely traumatized children and 216 from
both less traumatized and nontraumatized children.
Comparably, in Study V the participants were 235
traumatized and 121 nontraumatized Palestinian chil-
dren, who reported 986 and 362 dreams, respectively
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(Valli, Revonsuo, Pälkäs, & Punamäki, 2006). In both
studies dreams were collected on 7 consecutive nights
using a semistructured dream and sleep diary. The
data were collected for different purposes and before
the publication of the TST, and consequently the par-
ticipants and the researchers who collected the data
were unaware of the TST.
SPECIAL DREAM SAMPLES.
Zadra, Desjardins, and Marcotte (2006) (Study
VI) used the DTS in a Canadian sample to explore
threatening events in recurrent dreams. They aimed
to clarify whether recurrent dreams could be seen
as exemplary cases of threat simulation dreams, as
suggested by Revonsuo (2000a). The data were col-
lected with a retrospective questionnaire from 212
participants recruited to studies on dreams and per-
sonality. One dream report was selected from each
participant, although some of them reported more
than one recurrent dream. In another study, Desjar-
dins and Zadra (2004) used the DTS to explore the
extent to which nightmares could be interpreted as
extreme threat simulation dreams (Study VII). The
participants were 118 healthy Canadian adults. The
data were collected using a 2- to 4-week dream diary,
and from the diaries, a maximum of two nightmares
per participant were randomly selected (N = 174). The
data for Studies VI and VII were not originally col-
lected to test the TST, and therefore experimental
demands did not bias the samples.
RELIABILITY OF THE DTS.
In Study I, the dreams were content analyzed by three
independent raters, two of whom had devised the
DTS. The agreement levels between raters varied
from fair to strong, rendering the results reliable, and
were highest between the two developers. In Study II,
two raters new to the DTS content analyzed the data,
reaching strong interrater agreement levels. This indi-
cates that the DTS can be used reliably by research-
ers other than the developers of the scale. Again, in
Study III two raters analyzed the data; one judge
was new to the DTS, and the other was a developer.
They reached strong interrater agreement levels in
analyzing dream reports, daily log descriptions, and
interview transcripts, indicating that the DTS can be
used reliably not only in analyzing dream reports but
also in investigating other verbal material.

classified the threats, and one of the two had pre-
vious experience with the DTS, whereas the other
was newly trained. In addition, the analyzed mate-
rial differed significantly from that in previous stud-
ies; it was composed of children’s dreams that were
translated from two Kurdish dialects (Study IV) and
Arabic (Study V) into English. The agreement levels
ranged from good to strong, indicating that the DTS
can be used reliably in analyzing children’s dream
content and in samples consisting of participants
with different cultural backgrounds. In Studies VI
and VII, two judges who had not participated in the
development of the scale coded the data. In Study
VI, they reached a level of agreement ranging from
fair to strong. Information on the agreement levels in
Study VII was not available.
To summarize, the DTS has been used in several
studies, and in all these studies the DTS has proven
to be a reliable tool for identifying and classifying
threatening events in written reports, independent
of data collection method (questionnaire vs. dream
diary), source of data (dreams vs. real-life experi-
ences), age of participants (children vs. adults), life
experiences of participants (traumatized vs. nontrau-
matized), and cultural background of participants
(Finnish, Swedish, Canadian, Kurdish, or Palestin-
ian). We therefore recommend the use of the DTS
in future studies on threatening dream events or
empirical tests of the TST.
In Studies IV and V two judges identified and
Results of the reviewed studies
In this section we review the main results of the afore-
mentioned studies. We start by presenting the find-
ings on the frequency of threats in dreams and then
review the more detailed content of these threats.
FREqUENCY OF THREATENING EVENTS.
In Study I, Revonsuo and Valli (2000) found that
threatening events are common in the dreams of or-
dinary young adults. A total of 672 threatening events
were identified in 592 dream reports, amounting to
1.2 threats per report. Two thirds of all dream reports
(66.4%) included a description of a threat, and many
dreams included several threatening events. There
were no participants whose dream reports did not in-
clude threatening events, even though the frequency
of threats per dream report varied widely between
tst aNd receNt emPIrIcal evIdeNce  •  21