Norscia et al. [1] recently reported
the first evidence for a sex bias in contagious
yawning among humans. Based on previous research
showing an indirect connection between
contagious yawning and empathy (e.g.
[2&endash;4], but see [5,6]) and
that levels of empathy appear to be higher in
women compared with men (e.g.
[7&endash;9]), the authors investigated
whether there is also a sex difference in the
expression of contagious yawning through an
observational study on humans. By examining 92
dyads already showing evidence of contagious
yawning, which was a subset of a much larger
sample of individuals, Norscia et al. report
that both social bond and sex were significant
predictors of this response. In particular,
contagious yawning was more common if the dyad
was strongly bonded and when the individual
catching the yawn was female.
Here, we call into question these
conclusions about a sex difference in contagious
yawning. First, we draw attention to more than a
dozen previous publications on contagious
yawning in humans, many of which were not cited
by Norscia et al. showing no difference in
susceptibility to yawn contagiously and/or
contagious yawning frequency between men and
women. Similarly, we do the same for nearly a
dozen unacknowledged publications in the
comparative literature. Lastly, we challenge the
validity of the reported sex difference within
the Norscia et al. study itself, based on their
analysis of only a restricted subset of
documented contagious yawners.
A close examination of the empirical
literature clearly shows that the sex difference
reported by Norscia et al. stands in stark
contrast to previous findings on contagious
yawning in humans. While Norscia et al. only
reference a single publication that did not find
a difference in the frequency of contagious
yawning between men and women [5], we
provide references to an additional 14 papers
showing no effect (two of which were in fact
written by some of the Norscia et al. authors in
question; table 1). Furthermore, within the 15
total publications showing no effect, two
include two independent studies and samples.
Therefore of the 16 papers (18 samples) that
have addressed the question of a sex difference
in contagious yawning, only Norscia et al. found
a statistically significant effect between men
and women.
Given the logic of statistical probability,
the most parsimonious explanation for this
pattern of results is that the null effect is
real and the sex difference reported by Norscia
et al. represents a false positive. In fact, if
the false positive rate is 5% (p=0.05), then the
cumulative percentage chance of discovering less
than or equal to 1 significant effects out of
the 18 total samples is greater than 77%
(binomial test). The odds of finding one or more
significant effects are greater than 60%. If the
recently reported sex bias is indeed real,
however, the chance of 17 independent samples
showing no effect is highly improbable. Even if
there was an equal likelihood of expecting a sex
bias or a null effect (much like flipping a
coin), which would be highly conservative, the
odds of discovering 17 null effects out of 18
samples is less than 0.01% (p=0.00007). Although
a variety of methods and measurements were used
across these studies for both eliciting and
measuring contagious yawning (table 1), there is
no a priori reason to believe this would alter
the expression of yawns in men versus women
consistently in one direction or the other
across these samples. In fact, these differences
in methodology across studies suggest that the
failure to find a sex difference is a robust and
easily replicable effect.
Norscia et al. draw on comparative/animal
literature to support the sex difference they
found in humans, by citing other non-human
examples in which there is a 'female skew' for
contagious yawning. However, just as in the case
of their examination of the human literature,
there are papers that Norscia et al. do not cite
in which no effect of sex is reported (one of
which was written by the authors in question).
For example, no main effect of sex has been
observed for chimpanzees
[4,23&endash;25], bonobos [18],
domesticated dogs [26&endash;29] and
budgerigars [30]. There is also no sex
difference for video-induced yawning in
stump-tailed macaques, although it is not clear
whether this response represents a mechanism of
contagion [31]. Moreover, the findings
supporting a female bias in nonhumans do not
actually describe a female bias that is
comparable to what Norscia et al. report for
humans; i.e. a main effect of the sex of the
receiver on the frequency of contagious yawning.
For example, female wolves show a shorter
reaction time to yawn than males but do not
differ in frequency [32], bonobos are
more likely to yawn to a female model but the
sex of the receiver is not a significant
predictor [33], and for gelada baboons
female&endash;female dyads showed more
contagious yawning than female&endash;male but
no comparison is made for this response in
male&endash;male pairs [2]. Arguing
along those lines, Massen et al. [24]
actually found opposing results in chimpanzees,
whereby male yawns were more contagious than
female yawns and male&endash;male dyads showed
most contagious yawning. Whereas there have been
interesting hypotheses posed to explain the
differences in yawn contagion between species
with regard to the compositions of the trigger
and responder, it is interesting that Norscia et
al. do not test for this dyadic aspect (i.e. the
interaction effect of the trigger's and
receiver's sex) in their study. What we can
conclude, however, is that the effects described
in the comparative literature are far more mixed
than what Norscia et al. originally
reported.
Lastly, we question the validity of the
conclusions derived by Norscia et al. about a
sex bias in yawn contagion among humans. The
analytic strategy they employed did not actually
assess whether there was a difference in
susceptibility to yawn contagiously between men
and women, and consequently also did not assess
whether there was a difference in contagious
yawning frequency in the total sample of men and
women in their study. Instead, the authors used
a particular set of exclusion criteria and only
analysed data from dyads in which (i) yawn
contagion was present and (ii) at least three
independent occasions of contagious yawning were
available. Although the authors rightfully argue
that this ensures that yawn contagion is
correctly detected, these methods omit relevant
zeros, especially when taking into account that
the number of occasions to show yawn contagion
varied across individuals. In general (table 1),
contagious yawning is at first analysed by
assessing what affects whether an individual
will or will not yawn (0/1) after witnessing a
yawn. In fact, the authors reported in a
previous paper [15], on a subsample of
the study in question, that no sex difference
was found regarding the probability to yawn
contagiously. In addition, the authors did not
assess contagious yawning among strangers. All
these (and additional) decisions restricted the
analysed sample to 34.5% of the total dataset.
What Norscia et al. can conclude from their
analysis is that of the men and women thatmet
these criteria for contagious yawning, females
yawned more than men in response to yawning
stimuli. They do not show that females aremore
susceptible to contagious yawning, but only that
among those who are susceptible within their
restricted sample women are more likely to do
so. Thus, the results reported by Norscia et al.
are inconclusive if not misleading.
In summary, the available evidence on
contagious yawning in humans shows that there is
no difference between men and women. We have
identified a total of 15 other publications
(consisting of 17 study samples) that have
addressed this question, and all report no
effect of sex in terms of the susceptibility to
contagious yawning and/or the frequency of
contagious yawning. Furthermore, we have
provided additional evidence for a much more
mixed picture for the comparative literature on
this topic. Therefore, until the sex difference
reported by Norscia et al. is replicated
independently by other investigators, it should
be considered a false positive. Furthermore, the
conclusions reached by Norscia et al. are
misleading in that they imply a global effect of
sex on the susceptibility to yawn contagiously,
when in fact their analyses were restricted to a
subset of their sample that already showed
abundant evidence of contagious yawning. As for
contagious yawning as an indirect marker for
empathic processing, we are left to conclude
that the consistent lack of a female bias in the
literature fails to support this
connection.
