Is synaesthesia a dominantly female trait?

Abstract

Synaesthesia is a familial condition that gives rise to unusual secondary percepts. We present a large-scale prevalence study which informs our ideas on whether the condition is more prevalent in men or women. A number of studies over the last 20 years have suggested the condition is found more commonly in women, with up to six times more female synaesthetes than male. Other studies attributed this female bias to merely a recruitment confound: women synaesthetes may be more likely to self-refer for study. We offer two pieces of evidence that there is no extreme female bias in synaesthesia: first we re-analyse previous reports of very large female biases to show again that they likely arose from self-referral or other methodological issues. Second, we present the largest published prevalence study to date on grapheme→colour synaesthesia in which our prevalence (1.39% of the population) replicates our earlier estimates (e.g., Simner et al., 2006) and in which we demonstrate no strong female bias even with sufficient power to detect such a difference.

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Is synaesthesia a dominantly female trait?
Julia Simnerab & Duncan A. Carmichaelbcd
a School of Psychology, Pevensey Building, University of Sussex, Falmer, UK
b Department of Psychology, University of Edinburgh, Edinburgh, UK
c Institute for Adaptive & Neural Computation, University of Edinburgh, Edinburgh, UK
d Division of Psychiatry, University of Edinburgh, Royal Edinburgh Hospital, Edinburgh, UK
Accepted author version posted online: 03 Mar 2015.Published online: 09 Mar 2015.
To cite this article: Julia Simner & Duncan A. Carmichael (2015) Is synaesthesia a dominantly female trait?, Cognitive
Neuroscience, 6:2-3, 68-76, DOI: 10.1080/17588928.2015.1019441
To link to this article: http://dx.doi.org/10.1080/17588928.2015.1019441
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Is synaesthesia a dominantly female trait?
Julia Simner
1,2
and Duncan A. Carmichael
2,3,4
1
School of Psychology, Pevensey Building, University of Sussex, Falmer, UK
2
Department of Psychology, University of Edinburgh, Edinburgh, UK
3
Institute for Adaptive & Neural Computation, University of Edinburgh, Edinburgh, UK
4
Division of Psychiatry, University of Edinburgh, Royal Edinburgh Hospital, Edinburgh, UK
Synaesthesia is a familial condition that gives rise to unusual secondary percepts. We present a large-scale
prevalence study which informs our ideas on whether the condition is more prevalent in men or women. A
number of studies over the last 20 years have suggested the condition is found more commonly in women, with
up to six times more female synaesthetes than male. Other studies attributed this female bias to merely a
recruitment confound: women synaesthetes may be more likely to self-refer for study. We offer two pieces of
evidence that there is no extreme female bias in synaesthesia: first we re-analyse previous reports of very large
female biases to show again that they likely arose from self-referral or other methodological issues. Second, we
present the largest published prevalence study to date on grapheme→colour synaesthesia in which our prevalence
(1.39% of the population) replicates our earlier estimates (and in which we demonstrate no strong female bias
even with sufficient power to detect such a difference.
Keywords: Synaesthesia; Prevalence; Sex ratio; Synesthesia.
For people with synaesthesia, stimuli are experienced
with unusual secondary associations (e.g., hearing
sound triggers colours in the visual field; Ward,
Huckstep & Tsakanikos, 2006). Synaesthesia is a
multi-variant condition with an estimated 65 (Day,
2005) to 150 (Cytowic & Eagleman, 2009) known
sub-types, depending on which modalities are linked
(e.g., sound triggering colours, taste triggering touch
etc.). One key question is how common synaesthesia
is, and whether it affects men and women differently.
Early estimates described the condition as extremely
rare (e.g., 1 in 250,000) and very strongly female
dominant (with a 6:1 ratio; Baron-Cohen, Burt,
Smith-Laittan, Harrison & Bolton, 1996). Later
studies have called into question both these claims
and we examine these issues in the current paper.
Despite a relatively contentious history, the
question of synaesthesia’s prevalence appears to
now be reasonably well understood. Early estimates
of prevalence varied widely at least partly because
researchers were focussing on different sub-types or
using different definitional criteria (Ramachandran &
Hubbard, 2001). However, even in studies that aimed
to report the prevalence of all forms of synaesthesia,
estimates ranged from 1 in 4 (Calkins, 1895; Domino,
1989; Uhlich, 1957), to 1 in 10 (Rose, 1909), 1 in 20
Correspondence should be addressed to: J. Simner, School of Psychology, Pevensey Building, University of Sussex, BN1 9QJ, UK.
E-mail: j.simner@sussex.ac.uk
Julia Simner and Duncan A. Carmichael contributed equally to this work
DAC was supported in part by grants EP/F500385/1 and BB/F529254/1 for the University of Edinburgh, School of Informatics Doctoral Training
Centre in Neuroinformatics and Computational Neuroscience (www.anc.ac.uk/dtc) from the UK Engineering and Physical Sciences Research Council
(EPSRC), UK Biotechnology and Biological Sciences Research Council (BBSRC), and the UK Medical Research Council (MRC).
The research leading to these results has received funding for author JS from the European Research Council under the European Union’sSeventh
Framework Programme (FP/2007-2013) / ERC Grant Agreement n. [617678].
COGNITIVE NEUROSCIENCE, 2015
Vol. 6, Nos. 2–3, 68–76, http://dx.doi.org/10.1080/17588928.2015.1019441
© 2015 The Author(s). Published by Taylor & Francis.
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the original work is properly cited.
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(Galton, 1883), 1 in 200 (Ramachandran & Hubbard,
2001), 1 in 2000 (Baron-Cohen et al., 1996), and 1 in
25,000–100,000 (Cytowic, 1993,1997). One problem
was that many of these early estimates were
essentially ‘best guesses’. Nonetheless, these early
studies served the important purpose of stimulating
research on synaesthesia’s prevalence and inspired the
first empirical assessments which followed thereafter.
The first prevalence study of its kind in the modern
literature (Baron-Cohen et al., 1996) assessed the
occurrence of synaesthesia by placing adverts in two
local newspapers in Cambridge, UK, calling for
synaesthetes to come forward. The advert described
several types of synaesthesia (sound [including ling-
uistic sounds]→colour, touch/taste/smell→vision/
sound) and identified two types of synaesthesia in
respondents, now known commonly as grapheme-
colour synaesthesia (experiencing colours from letters
and/or digits) and music-colour synaesthesia
(experiencing colours from sounds such as music). By
comparing the number of synaesthetes who came
forward (and who were subsequently verified as
genuine using an objective test; see below) against the
circulation figures of the newspapers, Baron-Cohen and
colleagues concluded that synaesthesia was at least as
common as 1 in 2000 people (i.e., 0.05%). However,
their methods would have greatly underestimated the
true prevalence because they relied on synaesthetes
making the effort to come forward in response to a
newspaper advert. For this reason the authors of that
study were careful to point out that their figure was only
a lower estimate, although their data has almost always
been misrepresented in the following literature as an
absolute estimate. A small number of studies in the
historical literature had avoided the problems of self-
referral by individually questioning every member of a
participant pool, although they established prevalence
only subjectively (at 6.7–23.0%: Calkins, 1895;
Domino, 1989;Rose,1909;Uhlich,1957)byrelying
on self-declaration only, which is an approach known to
over-estimate prevalence (Simner et al., 2006). Hence,
one set of studies tends towards a conservative estimate
and the others towards an overly-liberal one.
Our own study in 2006 addressed these limitations
by individually assessing a large number of people
(n = 1690
1
) and verifying their self-declarations with
an objective test of genuineness (see below). These
improvements in methodology showed the condition
to be far more common than previously thought,
affecting 1 in 23 members of the general population
across the relatively wide range of synaesthesias
tested (Simner et al., 2006). The important element
in this study was that synaesthetes were not required
to make the effort to self-refer in response to an
advert. Instead, a large sample of the general
population was individually assessed to find the
synaesthetes from among them, and this gave a
prevalence of 4.4% of synaesthetes within the
general population, for the variants tested. Within
this figure, one particularly common form was
grapheme-colour synaesthesia, in which colours are
triggered by letters and/or digits (e.g., Amight trigger
the experience of red, Byellow, and so on). The
prevalence of grapheme-colour synaesthesia was 2%
(counting those with coloured letters and/or digits; or
1.1–1.4% for those with both coloured letters and
digits). Since the time of this study, these estimates
for the prevalence of different forms of synaesthesia
have been widely accepted (e.g., Banissy et al., 2012;
Bor, Rothen, Schwartzman, Clayton & Seth, 2014;
Cohen Kadosh & Henik, 2007; Ward, 2013; Weiss
& Fink, 2009).
In contrast to prevalence estimates, the sex ratio of
female to male synaesthetes has caused perhaps greater
controversy. Several early studies proposed that there
was a very strong female bias in synaesthesia,
suggesting a possible X-linked dominant mode of
inheritance (e.g., Baron-Cohen et al., 1996; Smilek
et al., 2002). Indeed, the extent of this female bias in
some studies (e.g. 6:1; Baron-Cohen et al., 1996) led
researchers to believe that the trait may even be
associated with male lethality in utero (Bailey &
Johnson, 1997; Baron-Cohen et al., 1996). This
would in turn suggest that synaesthetes’families
should contain more women than men. However,
both these claims were subsequently challenged by
later studies, and we describe this development below.
Until 2006, the most commonly cited synaesthesia
study on prevalence and sex-ratios (Baron-Cohen
et al., 1996) proposed a female: male ratio of 5.5:1,
and this was followed by a second study (Rich,
Bradshaw & Mattingley, 2005) proposing a female
bias of 6.2:1. However, both studies based their
estimates on the number of synaesthete who self-
referred in response to media advertisements (e.g.,
newspaper adverts). Not only will this method
underestimate the total number of synaesthetes in a
population (see above) but it is also likely to over-
estimate the females. This is because females are
known to be more likely than males to come
forward to report atypical experiences, and this is
1
Specifically, 1190 individuals were assessed for grapheme-
colour synaesthesia, and a further 500 individuals were tested for
162 different synaesthesias, one also being grapheme-colour
synaesthesia. Since the estimates of prevalence for grapheme-
colour synaesthesia were approximately equivalent across both
populations, Simner et al. (2006) collapsed both population sizes
to give a grand total of 1690 people tested.
SEX RATIOS IN SYNAESTHESIA 69
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seen across a range of domains (e.g., Dindia & Allen,
1992). Simner et al. (2006) therefore suggested that a
self-referral confound may be responsible for the
previously high rates of female synaesthetes in
prevalence studies. Indeed, when this potential
confound was directly avoided by Simner et al.
(2006), we found that earlier studies had indeed
apparently over-inflated the proportion of females.
As noted above, Simner et al. (2006) specifically did
not rely on self-referred recruitment in their
prevalence estimate, but instead, they individually
assessed every member of a large participant pool
and used an objective test to identify the
synaesthetes from among them. Using this improved
methodology we found that that there was no large
(e.g., 6:1) bias towards female synaesthetes. Instead,
we found a female: male ratio of 1.1:1 when
considering a wide range of synaesthesias in a
population of n = 500, and a female: male ratio of
0.9:1 when considering grapheme-colour
synaesthesia
2
in a population of 1190. Neither of
these comparisons showed any significant sex bias.
On the basis of the above literature we might
conclude that synaesthesia affects around 1 in 23
individuals and has no very strong sex bias.
However, there have been three subsequent
challenges to our position. First, a small number of
studies continue to cite the prevalence and/or sex ratio
from Baron-Cohen et al. (1996) despite the self-
referral confound, and this has propagated in the
literature a low value of prevalence and a high
estimate of female synaesthetes. Second, one
subsequent study (Barnett, Newell, Finucane, Asher,
Corvin, Mitchell, 2008) has pointed out that the sex
differences identified in self-referral more generally
(Dindia & Allen, 1992) only account for a slight
variation (10%) in men and women’s responding,
making it possible that very high early estimates for
female synaesthetes were at least pointing in the right
direction. Third, that same study (Barnett et al., 2008)
presented data that were ostensibly free from the self-
referral confound, but which continued to show a
strong (6:1) ratio of female to male synaesthetes.
For these three reasons we return to the issue of sex
differences in synaesthesia in the current paper. The
position we take is to re-affirm that there is no strong
6:1 ratio of female to male synaesthetes when all self-
referral confounds are removed. We do this below by
presenting our own very large-scale study free of self-
referral, but before then, we also re-evaluate the
findings by Barnett et al. (2008). Their findings had
been reported as evidence of a strong (6:1) ratio of
female to male synaesthetes in data that were
presented as being apparently free from the self-
referral confounds. We re-evaluate this claim below.
Barnett and colleagues conducted a synaesthesia
study of the mode of inheritance, and prevalence of
synaesthesic sub-types within families. In their study
they looked not only at self-referred probands (i.e., 53
synaesthetes who self-referred to the university in
response to a media advert) but also a subset of
their family members who were questioned by the
proband and/or directly contacted by the researchers.
Since family members were tested as well as self-
referred probands, Barnett et al. claim their findings
are free of a self-referral confound, and they report
that “our total sample of 92 confirmed and
unconfirmed synaesthetes includes 78 females and
14 males, yielding a female to male ratio of 6:1 in
the Irish population”(pg. 877). Below we present
several responses to these claims.
First, in their calculations of the female: male ratio,
Barnett et al. appear to directly compare their 78
female synaesthetes against their 14 male
synaesthetes, concluding that a female: male ratio in
synaesthesia of 6:1 exists in the general population
(more precisely this would be:
78
/
14
:
14
/
14
= 5.57:1).
However, Barnett et al. evaluated twice as many
females than males (118 vs. 61) if we include all 179
participants whose status was somehow appraised
during their study (i.e., excluding all those with an
unknown status). This factor would considerably
reduce the absolute proportion of female synaesthetes
to males in their estimate for the general population.
A second consideration comes in the claim that
Barnett et al.’sfindings were not contaminated by a
self-referral bias, because they looked not only at self-
referred probands but also their families. However,
according to our reading of their report, Barnett and
colleagues were able to objectively verify the
synaesthesia of all their self-referred probands, but
only a small portion of their non-proband
synaesthetes. Indeed, 81 of their 92 synesthetes
overall were either objectively unconfirmed cases
(n = 28), or they self-referred in response to an
advert (n = 53) and were therefore likely to be a
priori female-skewed. Equally, when Barnett et al.
state there was “no difference…between the sex
ratio for probands (46 females and 7 males) and …
relatives who did not contact us directly (30 females
and 5 males)”, we point out that almost 70% of these
synaesthete relatives appear to have received no
objective test for synaesthesia. As such, almost
every member of their cohort were either
2
This particular reported figure related to grapheme-colour
synaesthetes who experience both coloured letters and digits
(rather than coloured letters and/or digits).
70 SIMNER AND CARMICHAEL
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self-referred, or were not verified as synaesthetes by
the usual objective standard.
Finally, Barnett et al. (2008) report that 17 families
were fully explored as far as all first-degree relatives of
the proband and this still gave a “6:1”(pg 885) ratio of
female to male synaesthetes (i.e., 45 female synaesthetes
and 8 male synaesthetes found within these 17 families).
We point out, as above, that 45 female vs. 8 male
synaesthetes cannot be interpreted as 6:1 prevalence in
the general population without first knowing the sex of
each family member tested in those 17 families as a
whole which was not provided. We also point out that
one third of the synaesthetes discovered within those 17
families (i.e., 17 of the 53 synaesthetes discovered)
would have been contaminated by a self-referral
confound that strongly skews towards females,
because these families centred around 17 synaesthete
probands, who self-referred for study. Importantly, 87%
of all (n = 53) probands were female, meaning that
approximately 87% of the 17 probands in the target
families would be females, from what we know is a
skewed sampling method. In summary, target families
were not selected in a way to be free of an apriori
recruitment confound because all contained a proband
recruited by self-referral (see also Ward & Simner, 2005
for a similar problem). Finally, we point out, as above,
that approximately half ofthe 53 synaesthetes within the
17 target families did not receive an objective test (of
consistency) for synaesthesia.
In summary, we conclude that the 6:1 ratio towards
female synaesthetes found by Barnett et al. (2008)didnot
take into account the total number of females categorised
overall, or aprioriconfounds in the recruitment of self-
referred synaesthetes, and it did not categorise
synaesthetes with objective testing throughout. For
these reason we conclude that their 6:1 bias towards
female synaesthetes was affected, at least to some
degree, by self-referral methodology or other issues.
(Nonetheless, we point out that the study by Barnett
and colleagues provided much robust data on a number
of other epidemiological and cognitive factors within
synaesthesia—e.g., transmission of different variants
within families, trends in synaesthetic colours. etc.—
and it therefore represents a valuable step towards
understanding how synaesthesia might manifest itself,
beyond this sex issue.) Below we test whether there is a
6:1 female bias empirically when self-referral is removed,
but we first conduct a power analysis to confirm the
numbersthatwouldneedtobetestedinorderto
determine whether such a difference were statistically
significant. This is important because previous
epidemiological studies of synaesthesia aiming to
remove the self-referral confound (e.g., Simner,
Harrold, Creed, Monro & Foulkes, 2009;Simneretal.,
2006) have tested too few people to provide sufficient
power for a statistical comparison of the sexes.
POWER ANALYSIS
The female bias in synaesthesia estimated by Baron-
Cohen et al. (1996) was 5.5:1, and by Rich et al.
(2005) it was 6.2:1, and by Barnett et al. (2008)it
was 5.6:1. These values, repeatedly circling around a
6:1 ratio of female to male synaesthetes, can be tested
empirically if there is sufficient power in the number
of individuals tested. In order to calculate this we first
need to estimate what the individual prevalences of
synaesthesia would be for males versus females,
given a hypothesised 6:1 difference.
The most robust and widely cited synaesthesia
prevalence study to date (Simner et al., 2006), report
an overall prevalence of synaesthesia of 4.4% of the
population, when testing for 162 different variants.
However, there are considerable challenges to
identifying so many different types of sub-variants
within a single study (see Simner et al., 2006 for
discussion) so we instead chose to test for just one
variant of synaesthesia in the current study. We chose
grapheme-colour synaesthesia since this variant is very
well understood, relatively prevalent, and can be tested
for using a single standardised computerised method
(see below). Below we therefore conduct a power
analysis to reveal the number of individuals required
for screening in order to identify any 6:1 bias of female
synaesthetes with grapheme-colour synaesthesia.
Simner et al. (2006) report the prevalence of
grapheme-colour synaesthesia to be 2% (where
“grapheme-colour synaesthetes”are those with either
coloured letters, coloured digits, or with both). With
an assumed sex ratio of 6 female synaesthetes to
every male synaesthete, we would expect to find
1.71 female synaesthetes and 0.29 male synaesthetes
if we tested 100 members of the population. If we
carry out a sample size calculation for a chi-squared
test, with standard levels of power at 0.80 and alpha
at 0.05, in order to detect a difference in proportion of
this magnitude (1.71% versus 0.29%, or proportions
of 0.0171 and 0.0029 respectively) a sample of 1810
participants is required for screening (905 females and
905 males). In our empirical study below, we meet—
and indeed exceed—this sample size.
EMPIRICAL ASSESSMENT
We individually assessed a very large number of
individuals from the general population for
SEX RATIOS IN SYNAESTHESIA 71
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grapheme-colour synaesthesia, avoiding a self-referral
bias. Every person was assessed using the behavioural
“gold-standard”test which identifies synaesthetes by
detecting the most widely accepted core characteristic
of synaesthesia. This characteristic is the consistency
in the reporting of synaesthetic sensations over time.
In grapheme-colour synaesthesia for example, a given
letter tends to elicit a consistent synaesthetic colour
for any given synaesthete in repeated testing (e.g., A
might be consistently red, Bconsistently blue, etc.).
This consistency-over-time is taken as the behavioural
hallmark of synaesthesia in standard diagnostic tests
for synaesthesia (see Johnson, Allison & Baron-
Cohen, 2013 for review). The mostly widely used
version of this test for grapheme-colour synaesthesia
is available at an online interface known as the
Synesthesia Battery (Eagleman, Kagan, Nelson,
Sagaram & Sarma, 2007). In this test, participants
are required to repeatedly report their synaesthetic
associations for the letters A-Z and/or the digits 0–9,
each shown three times in a random order. In order
for people to be diagnosed as synaesthetes, they must
achieve high enough consistency in their colour-
choices to show they are significantly better than
non-synaesthete controls, who previously performed
the same test to provide a robust base-line. This task
was used in our own study, and more details are given
in Eagleman et al. (2007) and in our methods below.
Participants
We individually screened 3893 participants for
grapheme-colour synaesthesia using The Synesthesia
Battery (2135 female; 1758 male). Their mean age
was 28.3 years (SD = 14.2). A further 65 participants
were excluded from study because they had entered
an obviously false date of birth (e.g., 2013; n = 48) or
because they reported too few coloured graphemes for
their synaesthesia to be meaningfully evaluated
(n = 17; see Eagleman et al., 2007). Participants
were unpaid, and our study was approved by the
local university ethics board.
Participants were recruited as part of a large-scale,
centrally co-ordinated undergraduate research project,
described in detail in Carmichael, Down, Shillcock,
Eagleman and Simner (2015). In this, every student
registered on the 2nd year of the Psychology
undergraduate course at the University of Edinburgh
between September 2012 and May 2015 acted as a
research assistant (RA), each recruiting approximately
8 participants (4 male and 4 female) over 16 years of
age. In recruiting participants, we took a number of steps
to ensure as random a sample as possible: RAs were
required to pre-select their sample, and then approach
participants in a targeted way, rather than sending out an
advert for self-referrals. Indeed, RAs were required to
refrained from recruiting participants via any advert or
open calls at all. For example, they could not post the
testing URL on social media websites or internet
forums. Furthermore, RAs were instructed not to
deliberately seek out, nor to avoid, people they knew
to be synesthetes and were also instructed not to apriori
inform participants that the study investigated
synesthesia. Instead, they pre-selected their samples to
create a pre-determined, non-referred testing cohort, and
then individually tested every member of that cohort.
Methods
To screen for grapheme-colour synaesthesia, we used
the consistency test from the Synesthesia Battery on-
line interface (Eagleman et al., 2007), which we
cloned with permission from the authors (see
Carmichael et al., 2015 for details). Participants
were provided with the URL of our online interface
and completed the test in their own time.
Our replication of the Synesthesia Battery first
obtains consent for testing and then records
demographic information about participants including
age and sex. Participants are then asked whether they
experience grapheme-colour synesthesia with the
question “Do numbers or letters cause you to have a
colour experience?”A checkbox is provided for
participants to record separately whether these
colours are triggered automatically by numbers and/
or digits. If participants indicated that they saw neither
letters nor numbers in colour, they advanced to an exit
page thanking them for their participation.
The consistency test was completed by participants
who answered in the affirmative to having coloured
letters/digits. This test displays individually on-screen
the letters A-Z and/or the digits 0–9(accordingtohow
participants responded to the checkboxes described
above). Each grapheme is shown three times in a
random order, and on each display, participants must
indicate their synaesthetic colour by selecting it from an
on-screen palette of 256x256x256 colours.The program
compares the colour selected each time the same
grapheme was presented (e.g., it compares the three
colours for the letter A). It then produces a
standardised score to reflect how far away in colour
space those three colours were, averaged across all
graphemes. A small standardised score reflects
consistent colours (i.e., selections for the same
grapheme were close in colour-space). A score less
than 1 indicates the high level of consistency typical of
72 SIMNER AND CARMICHAEL
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a synaesthete and this is the diagnostic threshold in this
test. For full details regarding how this test is designed
and implemented, please refer to Eagleman et al. (2007).
Results
In our study, we classified as non-synaesthetes all
those who were directed to the early-exit page (i.e.,
those who said they did not experience coloured
letters and/or digits) and all those who continued but
scored 1 or higher. The remainder were classified as
synaesthetes (i.e., those who scored <1).
From 3893 participants, we identified 54 grapheme-
colour synaesthetes with coloured letters and/or digits
(n = 5 with coloured letters; n = 26 with coloured
digits; n = 23 with both coloured letters and digits),
giving an overall prevalence of 1.39%. Of these 54
synaesthetes, 33 were female and 21 were male.
3
Calculating the overall prevalence of grapheme-
colour synaesthesia for each sex separately taking
into account the total number of men and women
tested (2135 and 1758 respectively) gives us a female
prevalence of 1.55% and a male prevalence of 1.19%,
producing a female: male ratio of 1.3:1. This difference
in the ratio of female versus male synaesthetes is not
significant (χ
2
= 0.63, df =1,p= 0.43).
4
Bayesian analysis
To further investigate our null result, we performed
two types of Bayesian analyses below. Together these
suggest that our sufficiently powered investigation of
whether there is a 6:1 ratio gave strong support for the
null hypothesis. However, they also provide an
estimate of how small any possible female bias
might yet be.
First, our Bayes factors analysis allows us to evaluate
to what extent the data supports the hypothesis under
investigation against the null hypothesis (Rouder,
Speckman, Sun, Morey & Iverson, 2009). Following
Jeffreys (1961), a Bayes factor of less than 0.33 provides
strong support for the null hypothesis, a Bayes factor >3
provides support for the alternative hypothesis and
values in between indicate no firm conclusions should
be drawn. Our Bayes factor was 0.014, indicating strong
support for the null hypothesis that sex does not
significantly influence the prevalence of grapheme-
colour synaesthesia.
Exploring our data further, a second analysis suggests
that although there was no large significant difference
across the sexes, there may yet be small difference, and
we can calculate its size. We constructed a beta-binomial
model of our acquired data which shows that any
difference between the numbers of male and female
synaesthetes in the general population is likely to be
very small. Calculating a 95% confidence interval of
the difference in prevalence, we see any difference in
prevalence between females and males is likely to fall in
the range −0.4% to 1.1%. Theoretically speaking,
therefore, if we were confident that—say—our male
prevalence of 1.19% were correct, we would therefore
be 95% sure that the true female prevalence is in the small
range between 0.79% (1.19%–0.4%) to 2.29% (1.19%
+1.1%). Indeed, if there were a difference between men
and women, our beta-binomial model also shows there is
an 82% chance that the prevalence would be higher for
females—albeit to this very marginal degree.
Discussion
We investigated the prevalence of grapheme-colour
synaesthesia in males and females to challenge the
suggestion that there are six times more female
synaesthetes than male in the general population
(Barnett et al., 2008; Baron-Cohen et al., 1996; Rich
et al., 2005). First we pointed out that two previous
studies showing this level of strong bias reported that
their methodology relied on self-referral (e.g., Baron-
Cohen et al., 1996; Rich et al., 2005). This method
likely encouraged female synaesthetes to reply more
than males (Simner et al., 2006; following Dindia &
Allen, 1992). Second, we described how previous
studies not liable to this confound (e.g., Simner
et al., 2009,2006) had not found a strong 6:1 bias
towards females, and indeed had found no significant
difference across the sexes at all. Third we examined
an additional study showing a 6:1 bias of females
which claimed not to rely on self-referral (Barnett
et al., 2008). Using their published data and
descriptions of study, we suggested that they may
not have taken into account the total number of
males/females tested overall or may not have used
objective tests to verify synaesthesia in all
3
Of the 33 female synaesthetes, 12 reported both coloured
letters and digits, 5 reported experiencing coloured letters only
and 16 reported coloured digits only. Of the 21 male, 11 reported
both coloured letters and digits, 0 reported coloured letters only and
10 reported coloured digits only.
4
If we examine the sex ratio for only synaesthetes who have
both coloured letters AND numbers (n = 23; 12 female) we find a
prevalence of 0.59%. Calculating the synaesthesia prevalence of
each sex separately gives us a female prevalence of 0.56% and a
male prevalence of 0.63%, and a ratio of 0.89: 1. Using a chi-
squared test, we again determined the difference in ratio of female
versus male synaesthetes is not significant (χ2 = 0.002, df = 1,
p = 0.962).
SEX RATIOS IN SYNAESTHESIA 73
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participants, and that their methods did not appear to
be entirely free of the self-referral confound.
In our empirical investigation, we screened 3893
individuals for grapheme-colour synaesthesia following
a power analysis. We took care to avoid self-referral
confounds and we individually tested every member of
a pre-determined cohort with an objective test for
synaesthesia. We found that 33 out of the 2135 females
tested had grapheme-colour synaesthesia (for coloured
letters and/or digits; female prevalence 1.55%) as well as
21 out of 1758 males (male prevalence 1.19%). This
ratio of 1.3: 1, female to male synaesthetes, was non-
significant. Further Bayes analyses suggest support for
the null result in our data, but that there remains the
possibility of a very small sex differences, in the range
of −0.4% to 1.1%, with a female bias being more likely
than a male bias.
Our results largely corroborate the findings of our
previous comparison study, Simner et al. (2006)
which reported an overall prevalence of grapheme-
colour synaesthesia of 2%, compared to 1.39% in our
own study (and this difference is non-significant;
χ2 = 1.16, p = .28). This was for synaesthetes with
coloured letters and/or digits, but it is also possible to
directly compare our findings in the female: male
ratio if we consider synaesthetes with both coloured
letters and digits (since this is the type of sex data
reported in Simner et al., 2006). In this comparison
we find a female: male ratio of 0.9:1 in the current
study compared to an identical ratio (0.9:1) found in
Simner et al. (2006; their female prevalence was
1.03% and their male prevalence was 1.15%).
In our calculations we point out that we classified
participants as synaesthetes according to the
conventional cut-off, as stated within the test we used
by Eagleman et al. (2007). This conventional cut-off
for synaesthesia is a score <1. Two recent studies
however have suggested that a more accurate
approach might be a cut-off centred on 1.43 rather
than 1 (for details see Carmichael et al., 2015;
Rothen, Seth, Witzel & Ward, 2013). For this reason,
we also re-calculate our prevalence and female/male
ratio according to the 1.43 cut-off and find a yet-closer
female: male ratio in synaesthesia. For clarity to aid the
reader, we have presented this data along with our
other prevalence/ratios in Table 1.
Of course we point out that our findings relate only
to the sex ratio and prevalence of the population we
sampled, and the type of synaesthesia we
investigated. We note that our average sampled
participant was 28 years old, which is younger than
the national average (median = 40.5 years; Central
Intelligence Agency, 2014), and this might have
influenced the prevalence we generated.
Furthermore, we looked only at grapheme-colour
synaesthesia, which is just one of many variants of
the condition (see Cytowic & Eagleman, 2009; Day,
2005). A recent study of a very large number of self-
referred synaesthetes by Novich, Cheng and
Eagleman (2011) revealed that groups of variants
clusters into synaesthetic subtypes (e.g., people with
grapheme-colour synaesthesia are likely to have a
second form involving colour, but not taste). This
suggests there may be multiple forms of the
condition, and indicates in turn that what is true of
grapheme-colour synaesthesia (e.g., its sex ratio) may
not be representative of all synaesthesias.
One curiosity not yet understood is the apparent
extent of the female bias in self-referral studies for
synaesthesia. We have shown there are roughly
equivalent numbers of female to male grapheme-colour
synaesthesia in the general population—or at the very
most, that there are only 1.3 women for every man.
However, six times more female synaesthetes are
detected in self-referral studies (e.g., Baron-Cohen et al.
1996). We attribute this difference in part to the known
confound that promotes responses from women over
men in self-referral (e.g., Dindia & Allen, 1992;
TABLE 1
Shows the number of confirmed male (M) and female (F) grapheme-color synaesthetes found in our total sample of 3893 subjects
(F = 2135; M = 1758). The prevalences are shown in brackets, with the female: male ratio beneath. This is done twice according to
two different cut-off for synaesthesia (a score of 1 vs. 1.43 in The Synesthesia Battery) and twice according to two different
definitions of grapheme-color synaesthesia (having colored letters AND/OR digits, vs. colored letters AND digits).
Coloured triggers Sex & ratio Battery cut-off at 1 Battery cut-off at 1.43
Letters AND/OR digits F 33 (1.55%) 55 (2.58%)
M 21 (1.19%) 39 (2.22%
F + M =54 (1.39%) =94 (2.42%)
Ratio F:M 1.3:1 1.2:1
Letters AND digits F 12 (0.56%) 23 (1.08%)
M 11 (0.63%) 19 (1.08%)
F + M =23 (0.59%) =42 (1.08%)
Ratio F:M 0.89: 1 1:1
74 SIMNER AND CARMICHAEL
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Rosenthal & Rosnow, 1975). However, Barnett et al.
(2008) point out that this bias usually gives just a slight
variation of around 10% (Dindia & Allen, 1992). Why
then might the female bias be so exaggerated in studies
of synaesthesia—and indeed, why are the rates so
consistent across self-referral studies? It could be, for
example, that female synaesthetes—although not
greatly more common—have perhaps more intense
experiences or are more aware of their synaesthesia or
attend to it more in daily life. This might make them
more likely to self-refer. However, we have no data to
support any specific supposition in this area, so leave this
question for future investigations.
Understanding sex ratios are important in
understanding the origins of synaesthesia. Initial
findings that synaesthesia appeared more common in
females led to suggestions that synaesthesia was
either not fully expressed in males or that it was
linked to the X-chromosome in some way (Bailey &
Johnson, 1997; Baron-Cohen et al., 1996; Ward &
Simner, 2005). Indeed, the extent of the female bias
led researchers to propose that synaesthesia might
causes lethality in males in utero (Baron-Cohen
et al., 1996). Subsequent research, including our
own study here, suggests this is not the case. In
combination with previous studies from our own lab
and elsewhere, we conclude there no very strong
female bias (Simner et al., 2009,2006), that families
containing synaesthetes are equally likely to produce
female or male offspring (Barnett et al., 2008; Ward
& Simner, 2005), that there are confirmed cases of
male-to-male transmission (Asher et al., 2009), and
one case of monozygotic male twins who are
discordant for synaesthesia (Smilek, Dixon &
Merikle, 2005). Finally, neither Asher et al. (2009)
nor Tomson et al. (2011) found evidence for a major
locus on the X chromosome in their genome-wide
studies.
5
This suggests a need to revisit our early
understanding of the mode of inheritance of
synesthesia (see Asher & Carmichael, 2013, for
review) and we provide our data for future studies
to do so.
We finally point out that our own studies have
shown relatively flat distributions of synaesthesia in
men and women, with a slight male bias when
considering grapheme-colour synaesthetes with
coloured letters and digits (female: male ratio of 0.9:
1 both here and in Simner et al., 2006) and a slight
female bias when considering grapheme-colour
synaesthetes with coloured letters and/or digits
(here, female: male ratio of 1.3: 1). It may yet be
possible to estimate the numbers required to test this
much reduced difference across the sexes (e.g., power
analyses in the ratio of 1.3:1 suggest we would need
to screen 47516 participants) but for the current study
we have shown that there is no 6:1 ratio of female to
male synaesthetes, even with sufficient power to test
for such a difference.
Original manuscript received 1 December 2014
Revised manuscript received 12 January 2015
First published online 13 March 2015
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