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Associations between iris characteristics and personality in adulthood
Mats Larsson
a,
*, Nancy L. Pedersen
b
,Ha
˚kan Stattin
a
a
Department of Behavioral, Social and Legal Sciences, Center for Developmental Research, O
¨rebro University, 701 82 O
¨rebro, Sweden
b
Department of Medical Epidemiology and Biostatistics Karolinska Institutet, Box 281, 171 77 Stockholm, Sweden
Received 10 March 2005; accepted 24 January 2007
Available online 1 February 2007
Abstract
Variable and person-oriented analyses were used to explore the associations between personality and three previously untested general iris
characteristics: crypts, pigment dots and contraction furrows. Personality data, as measured by the NEO PI-R and ratings of iris characteristics from
428 undergraduate students were collected. Crypts were significantly associated with five approach-related behaviors, i.e., feelings, tendermind-
edness, warmth, trust and positive emotions, whereas furrows were associated with impulsiveness. These findings suggest that because Pax6
induces tissue deficiencies in both the iris and the left anterior cingulate cortex, Pax6 may influence the extent people engage in approach-related
behaviors. The results from using a person-oriented analysis suggested that people with different iris configurations tend to develop along different
personality trajectories. Future longitudinal studies, twin-studies and genetic association studies, may benefit from collecting iris data and testing
candidate genes for crypts and furrows.
#2007 Elsevier B.V. All rights reserved.
Keywords: Personality; Iris characteristics/crypts/pigment dots/contraction furrows; Candidate genes/Pax6/Six3/Lmx1b; Anterior cingulate; Hemispheric
asymmetries
1. Introduction
The idea that personality differences are related to iris
characteristics is not new. In 1965, Cattell (1965) observed
differences in cognitive styles between blue and brown eyed
subjects and since then eye color has been found to be related to a
great variety of physiological and behavioral characteristics.
Dark eyed people have on average higher scores on extraversion,
neuroticism (Gentry et al., 1985), ease of emotional arousal
(Markle, 1976) and sociability (Gary and Glover, 1976).
However, there are a number of studies that fail to replicate
the personality findings, typically because the effect tends to fade
after early childhood. For instance, Rubin and Both (1989) found
that blue-eyed children in kindergarten and Grade 2 were
overrepresented in groups of extremely withdrawn youngsters,
whereas no association could be found in Grade 4 or between eye
color and extreme sociability in any grade. Furthermore, in
subjects around 20 years of age, eye color failed to account for
any difference between high versus low scorers on personality
measures (Lester, 1987; Rim, 1983). Consequently, it has been
assumed that the biological mechanisms that may underlie the
association between eye color and social wariness in early
childhood tends to be ‘‘overcome’’ around age 9 when the
increasing influence of social environmental factors has gained
momentum (Rubin and Both, 1989). To our knowledge, no
research has been reported concerning eye color and its relation
to personality since 1998, when Coplan et al. (1998) found a
significant eye color by gender interaction for social wariness in a
sample of 3.5–5.5-year-old toddlers. This finding raised further
questions regarding the phenotypic penetrance of the suggested
biological mechanism involved. Leading researchers in the field
concluded that eye color cannot be a useful marker for
personality in older ages (Coplan et al., 1998; Rosenberg and
Kagan, 1989; Rubin and Both, 1989).
One of the reasons eye color has been considered to be a
useful biomarker is its high heritability (98%), and the fact that
eye color does not change much over time (Bito et al., 1997).
Other key reasons are that eye color can be linked to biological
mechanisms that provide a somewhat speculative, but still
plausible, biological explanation for the associations found. For
instance, Kagan suggested that melanin production in the eyes
(the pigment that colors the eye brown), may be associated with
norepinephrine and cortisol production, which are biological
www.elsevier.com/locate/biopsycho
Biological Psychology 75 (2007) 165–175
* Corresponding author. Tel.: +46 19 303415; fax: +46 19 303484.
E-mail address: mats.larsson@bsr.oru.se (M. Larsson).
0301-0511/$ – see front matter #2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.biopsycho.2007.01.007
indices of behavioral inhibition (Kagan et al., 1988; Reznick,
1989).
High levels of norepinephrine and cortisol can thus
potentially inhibit melanin production in the eye and increase
responsiveness of the limbic system, which has been associated
with higher scores on inhibition (Rosenberg and Kagan, 1989).
Placental cortisol level during the initial stages of pregnancy
and the gene POMC, which are likely to play an important role
in individuals’ responses to varying levels of arousal
(Kupfermann, 1991) and to the synthesis of melanin (Pawelek
and Korner, 1982), have also been suggested to contribute to the
associations found.
However, if one carefully examines people’s irises, it is clear
that there are other iris characteristics than eye color that
potentially could be associated with personality, such as Fuchs’
crypts, contraction furrows and pigment dots. To take one
example, if the biological mechanisms associated with the
frequency of Fuchs’ crypts (a measure characterized by
different degrees of hypoplasia of the anterior border layer
and underlying stroma in the iris), influence behavior, then
biological mechanisms other than those suggested to be
associated with eye color can be responsible for the
associations. This study explores the association between iris
characteristics other than eye color and personality. Three
general iris characteristics that to our knowledge not have been
considered previously will be tested.
The iris characteristics of interest in this study are depicted
in Fig. 1: frequency of Fuchs’ crypts in the main stroma leaf
(which from now on will only be called ‘‘crypts’’); frequency of
pigment dots; the distinction and extension of contraction
furrows. These iris characteristics are moderately to highly
heritable, show no sex differences and are stable over time
(Larsson et al., 2003).
Crypts and contraction furrows are measures that are related
to the thickness and density of the iris. Crypt frequency is
characterized by different degrees of hypoplasia in the two top
cell layers in the iris (i.e., anterior border layer and underlying
stroma), whereas contraction furrows are characterized by
different degrees of hypoplasia or density conditions in any of
the five cell layers present in the iris (Oyster, 1999). What is
intriguing about these cell layers is that genes that influence the
growth of precursors to these cell layers during the
embryological development also cause tissue loss in the brain.
For example, the genes Pax6, Six3 and Lmx1b have been
recognized as candidate genes for crypts and contraction
furrows (Hsieh et al., 2002; Pressman et al., 2000; Sale et al.,
2002) and the embryological events that make these genes
plausible candidates for crypts and contraction furrows have
been reviewed (Larsson and Pedersen, 2004). In addition, the
most recent studies available confirm that Pax6 is a likely
candidate gene for tissue differences in the iris. Pax6 dosage
influences the muscle-cell differentiation in the iris (Jensen,
2005), and Davis-Silberman et al. (2005) conclude that (p.
2474) iris stroma defects are likely to be due to cell-non-
autonomous events, where Pax6 regulates the expression of the
cells that are required for the adhesion of the iris stroma. Thus,
it is possible that Pax6, Six3 and Lmx1b, which influence the
growth of the precursors to the cell layers in the iris, could
influence the variability of crypts and contraction furrows
typically present in people’s irises (Hsieh et al., 2002; Pressman
et al., 2000; Sale et al., 2002; Simpson and Price, 2002).
Furthermore, the expression pattern of thesegenes in the brain
supports the notion that these iris characteristics can be
associated with personality. In the first study published that
has demonstrated a major genetic contribution to frontal lobe
dysfunction, family members with a mutation in Pax6 showed
high rates of unusual behavior including disinhibition, impulsive
behavior, impaired social understanding and impaired verbal
inhibition (Heyman et al., 1999). In addition, magnetic resonance
imaging (MRI) in this family (Ellison-Wright et al., 2004)
demonstrated local gray and white matter changes in anterior
cingulate cortex (approximate Brodmann areas 24 and 32), as
well as posterior white matter abnormalities in corpus callosum
extending posteriorly into cingulate cortex, which are brain areas
that have been associated with personality (Davidson, 2001;
Johnson et al., 1999; Posner and Rothbart, 1998). Healthy
university subjects with larger right anterior cingulate gyrus
(Brodmann’s areas 24a, 24b and 25), score higher on harm
avoidance, which is associated with emotional responses that
regulate withdrawal-related behaviors, whereas subjects with
larger left cingulate gyrus, score higher on Novelty seeking,
which is associated with emotional responses that regulate
approach-related behaviors (Pujol et al., 2002). Indeed,
anatomical data revealed that hemispheric asymmetry in the
anterior cingulate gyrus was very common (83% of
cases, n= 100) and that surface measures of the right anterior
Fig. 1. The numbered arrows point toward the iris characteristics of interest. (1) Furchs’ crypts in the main stroma leaf; (2) pigment dots; (3) contraction furrows.
M. Larsson et al. / Biological Psychology 75 (2007) 165–175166
cingulate gyrus accounted for 24% of the variance in harm
avoidance. Overall, these findings are consistent with the
assumed specialization of each hemisphere in the control of
individual differences in withdrawal- and approach-related
behaviors (Davidson, 1992, 2001; Davidson and Irwin, 1999;
Pujol et al., 2002).
Moreover, the fact that the production of dopamine (Kohwi
et al., 2005) and noradrenalin neurons (Jaworski et al., 1997)
has been associated with Pax6 functioning provides another
link to personality (Reif and Lesch, 2003; Rosenberg and
Kagan, 1989). But Pax6 is not the only candidate gene for crypt
frequency and contraction furrows that potentially could
influence personality. Lmx1b is required for the survival of
the dopaminergic neurons in the mesolimbic system (Burbach
et al., 2003). It is also essential for the production of
serotoninergic neurons (Ding et al., 2003), and Six3, which
is a downstream target to Pax6 (Simpson and Price, 2002) and
which controls the division of the brain into two hemispheres
(Wallis and Muenke, 2000). Thus, based on these findings we
assume that tissue differences in the iris, such as crypts and
contraction furrows, could be associated with Pax6, Six3 and
Lmx1b, which in turn could be associated with personality.
The links between pigment dots and personality are less well
documented, but nevertheless, pigment dots could potentially be
associated with personality, as the development of pigment dots
is in part regulated by neurotransmitters produced by the
autonomic nervous system (Hu, 2000; Hu et al., 2000; Mukuno
and Witmer, 1977), which in turn is influenced by the extent the
individual engages in withdrawal- and approach-related beha-
viors (Davidson and Irwin, 1999). Thus, through the expression
patterns of Pax6, Six3 and Lmx1b in the iris and the brain, crypts,
contraction furrows and pigment dots may be associated with
both brain structures and neurotransmitters that influence
personality. However, there is no evidence available that shows
that these genes influence crypts and contractionfurrows directly.
In addition, other candidate genes for crypts and contraction
furrows have been recognized (Larsson and Pedersen, 2004).
In order to discriminate between the biological pathways
that theoretically could influence the presumed association
between iris characteristics and personality, and to more fully
evaluate a potential role of Pax6, we test three hypotheses. They
draw heavily on the Pax6 expression pattern in the anterior
cingulate cortex as documented by Ellison-Wright et al. (2004),
in conjunction with the findings made by Pujol et al. (2002),
Davidson (1992, 2001) and Davidson and Irwin (1999). These
hypotheses increase in their specificity. If all three hypotheses
are supported, we believe there is a high likelihood that Pax6,
rather than other candidates, is involved. Thus, based on the
findings reviewed above, we expect, first, that crypts more often
than other iris characteristics will be associated with
personality. Crypts are related to greater tissue loss in the
iris than the other two iris characteristics, which in turn may
reflect greater tissue loss in the brain (which makes it likely that
more measures of personality are associated with crypts than
the other iris characteristics). Second, because the tissue
damage that was due to Pax6 mutations in anterior cingulate
cortex primarily affected the left hemisphere (Ellison-Wright
et al., 2004), and this hemisphere primarily relates to approach-
related behaviors (Davidson, 1992, 2001; Davidson and Irwin,
1999), we expect that crypt frequency in most cases will be
related approach-related behaviors. Third, because greater
tissue loss in the iris, may reflect greater tissue loss in the left
anterior cingulate cortex, we expect individuals with an open
crypt structure (those with more tissue loss in the iris) to score
lower on traits that relate to approach-related behaviors than
those with a dense crypt structure. Furthermore, in order to
deepen the analysis, the present study combines variable- and
person-oriented approaches. As has been shown repeatedly
within a holistic interactionistic framework, person-oriented
approaches are able to answer different questions than more
traditional variable-oriented approaches (Bergman and Mag-
nusson, 1997; Magnusson and Stattin, 1998; von Eye and
Bergman, 2003). For example, in variable-oriented analysis the
relationship among variables and their way of functioning in the
totality of an individual is assumed to be the same for all
individuals. However, this prerequisite may not be fulfilled in
the present study since most of the genetic variance for each iris
characteristic is independent of the other iris characteristics
(Larsson and Pedersen, 2004). In other words, the behavioral
meaning of having a specific crypt frequency may depend on
which other iris characteristics are present in any one iris
(Lander and Schork, 1994). Person-oriented analysis takes
these possibilities into consideration and is based on the
perspective that the individual organism can be thoroughly
understood in its totality (Bergman and Magnusson, 1997).
Furthermore, this totality is assumed to result from self-
organization. This means that although there are, theoretically,
an infinite variety of differences with regard to process
characteristics and observed states at a detailed level, at a more
global level there will often be a small number of more
frequently observed patterns (Bergman and Magnusson, 1997;
Bergman et al., 2003). This principle of self-organization has
been demonstrated in many human biological systems and has
been applied to the development of the neural, sensory and
cognitive systems (Post and Weiss, 1997) as well as personality
(Bosma and Kunnen, 2002; Carver and Scheier, 2002).
Consequently, we expect that from the embryological events
that form the adult iris (detailed level), distinct configurations
of iris characteristics will occur in the population studied.
Furthermore, if the iris characteristics that define these
naturally occurring configurations are associated with person-
ality, then we expect that these configurations will define
subgroups of individuals that differ from each other on
personality in a systematic manner. Developmentally, the
individuals in these subgroups may represent different
trajectories with regard to personality, and subgroups with
more tissue loss in the iris are, just as in the variable-oriented
analysis, expected to score lower on approach-related
behaviors. Moreover, because the relationship among iris
variables and their way of functioning in the totality of an
individual is not likely to be the same for all individuals (Lander
and Schork, 1994; Larsson and Pedersen, 2004), the effect size
in the variable-oriented analysis is expected to be lower than in
the person-oriented analyses.
M. Larsson et al. / Biological Psychology 75 (2007) 165–175 167
To sum up, the purpose of this study is to explore the
association between iris characteristics other than eye color and
personality, using both variable- and person-oriented analyses,
as well as highlight genes expressed in the iris that potentially
could be candidate genes for personality. In performing the
variable-oriented analysis, we test three hypotheses, which
together explore the likelihood that the neurodevelopmental
control gene Pax6 influences personality. In performing the
person-oriented analyses, we test the extent to which self-
organizing properties of the developmental system create
configurations of iris characteristics that can identify subgroups
of individuals who differ in their personality. In comparing the
effect size in the variable- and person-oriented analyses, we
explore how efficiently the self-organizing properties of the
developmental system can identify individuals who differ in
their personality.
2. Method
2.1. Participants and recruitment procedure
The participants were all undergraduate student volunteers at O
¨rebro
University, Sweden, recruited from programs in psychology (47%), social
studies (30%), economics (10%), automatic data processing (5%), political
science (4%) and engineering (4%). Most participants were female (80% of 428
participants). The mean age of the sample was 26.6 years (19–52 years,
S.D. = 7.1). The participants were recruited after a regular lecture by the first
author, who was given an opportunity to describe the study. Consenting
participants filled out the NEO PI-R personality questionnaire in a location
where they were not disturbed (Costa and McCrea, 1992). However, psychology
and social studies students responded to a computerized version of NEO PI-R in
a group setting with 15 students. Close-up iris color photography was taken
individually in a lab setting at the university.
There were two waves of data collection. In the first wave of data collection,
all students that volunteered were accepted as participants. In the second wave,
only those individuals with irises that were most similar to scale-step 1 and 3–5
on the crypt frequency scale (see below) were accepted. This was done in order
to arrive at approximately the same number of participants with each level of
crypt frequency.
2.2. Measures
2.2.1. Personality
Personality was assessed with the Swedish version of the NEO PI-R (Costa
and McCrea, 1992), which is considered to be a reliable and valid assessment of
personality based on the Five-Factor Model of Personality. Eight-item scales
measured six specific facets for each of the five broad factors. The internal
consistencies (Cronbach’s alpha) were high overall, ranging from .69 to .85
(mean = .72) and did not differ substantially from what has been found for other
samples with NEO PI-R measured in Sweden (Costa and McCrae, 1985), and
other countries.
2.2.2. Iris characteristics
Close-up photos, where the diameter of the iris on the 36 mm 24 mm
Fujicolor, Professional 100 ASA slide film was about 22 mm, were taken of
both irises from all subjects. An Olympus OM-4T camera with a double sided
flash device from Lena Medical Photo Design Systems, and an Olympus 50 mm
macro lens with a 1:2 converter was used. The shutter speed was 60 and the
aperture was set to 16. To assist focus adjustment and to standardize the extent
the iris was dilated, a lamp with standard brightness was shown into the
subjects’ irises during focus and photography. To prevent blurring caused by
involuntary movements from the subjects, all participants rested their cheeks on
a stand during the procedure. The iris photos were digitized using Minoltas
35 mm Dimage Scan Dual film scanner. The resolution of the digitized color
pictures was 768 512 pixels (96 pixels/in.). A high contrast color computer
screen (Brand: Eizo; Model: FlexScan F55) with 1024 768/85 Hz resolution
(.28 mm Dot Pitch CRT; fH: 27–70 kHz/fV: 50–120 Hz) and the software
program Photo Shop 7.0 was used during the rating process.
Three scales, one for each iris characteristic of interest, were constructed:
frequency of crypts, pigment dots and extension and distinction of contraction
furrows. Specially trained raters independently reviewed the photographs of the
right iris and judged which scale step to which each photo was most similar. The
raters’ judgments were scored on an ordinal scale. Scale construction, the rating
procedure and the reliability of the procedures are reported in detail in Larsson
et al. (2003). The pictures used for the scales may be seen at: http://www.mol-
vis.org/molvis/v10/a98/ (or obtained directly from the first author).
2.2.3. Analysis
The first set of analyses was variable-oriented in nature and tested whether
our three expectations derived from Pax6 expression pattern in the anterior
cingulate cortex were correct. This was done by computing Pearson correlations
between the three iris characteristics and the personality measures. Due to the
fact that the facet level traits in NEO PI-R have their own biological basis (Jang
et al., 1998) and the necessity to include as specific measures as possible has
been emphasized in studies, such as ours (Heyman et al., 1999), both facet level
traits and the main domains of personality were included in the analysis. The
distributions of the iris characteristics used in these analyses are presented in
Table 1.
In the second step, we performed the person-oriented analyses. They tested
if the iris configurations that typically exist in early adults can identify
subgroups of individuals that differ in their personality in a systematic manner.
First a cluster analysis based on WARD’s method (Peck and Williams, 2002)
was performed. Only the iris characteristics that were found to be significantly
associated with personality in the correlation analysis were included. A four
cluster solution was chosen which explained 77% of the total variance in the
sample. The variance explanation was higher than the minimum criterion of
67%, recommended by Bergman et al. (2003). One-way analyses of variance
were used to investigate if any mean differences in personality existed between
the clusters (Tukey HSD were used for post hoc tests). Both facet level traits and
the main domains of personality were included in the variance analysis.
Finally, to explore how efficiently the self-organizing properties of the
developmental system could pick out subgroups of individuals that differed in
their personalities, we calculated the effect size d (Cohen, 1988) for all
associations that were found to be significant in the variable- and person-
oriented analysis.
3. Results
3.1. Variable-oriented analysis
The results of the variable-oriented analyses are presented in
Table 2. We found that two of the five domains were
Table 1
Distribution of iris characteristics
Iris characteristic and scale description Number of
observations
Fuchs’ crypt frequency
(1) Only parallel and/or densely packed curly fibres 129
(2) Spots of wavy fibres and 1–3 two shallow crypts 169
(3) Four shallow crypts or more 130
Pigment dots
(1) Absence of, or only minute grains of pigment dots 280
(2) One pigment dots or more 148
Contraction furrows
(1) Extending less than 1/4 circle 106
(2) Extending between 1/4 and 8/10 of a circle 142
(3) More distinct, extending 8/10 of one circle or more 180
M. Larsson et al. / Biological Psychology 75 (2007) 165–175168
significantly associated with crypt frequency: openness to
experience and agreeableness. In contrast, furrows were only
associated with neuroticism, and pigment dots were not
associated with any of the main domains of personality. Not
surprisingly, most of the correlations between the specific
facets of personality and crypt frequency were found for facets
belonging to the two domains with significant associations.
Thus, our first hypothesis – i.e., that most association should
involve crypt frequency – was confirmed. Furthermore, our
second and third hypotheses – i.e., that crypt frequency in most
cases should be related to approach-related behaviors and that
individuals with a dense crypt structure should score higher on
such behaviors – was also confirmed. Most of the significant
associations with crypt pinpointed behaviors that measure
approach-related behaviors and individuals with a dense crypt
structure scored higher on such behaviors (i.e., warmth, positive
emotions, openness to experience, feelings, agreeableness, trust
and tendermindedness).
In addition, the magnitude of the associations with the iris
characteristics was, as expected, quite modest and ranged from
.10 to .15. No associations between eye color and personality
could be found (results not shown).
We are aware that many correlations are reported in Table 2.If
we were to apply a conservative Bonferroni correction, no
significant correlation would remain. Nevertheless, for two of the
three measures of iris characteristics, the number of significant
correlations obtained is significantly above chance. As for the
associations between crypts and the Big Five measures, we
conducted 35 separate analyses. By chance, wewould expect 1.75
analyses with p-values of .05 or lower (35 analyses .05 = 1.75)
and .35 analyses with p-values of .01 or lower (35 analyses
.01 = .35). For crypts, we obtain eight significant correlations at
the .05 level and five at the .01. For furrows, we obtain three
correlations with p-values of .05 or lower and 2 at the p-value of
.01 or lower. Hence, for crypts and furrows, the number of signi-
ficant correlations is well above the chance. For pigment dots, on
the other hand, the findings are not much different from chance
expectations. Moreover, the fact that most of the significant
associations in Table 2 are consistent with our hypotheses
decreases the likelihood that these findings are due to random
effects. The significant results in the person-oriented analysis,
which pinpoint the same behaviors, also support this notion.
In the person-oriented analysis, a four cluster solution was
chosen based on the criteria that each group should differ as
much as possible in their iris morphologically, as well as define
subgroups with a sufficient number of members with a view to
the subsequent analyses of variance. Only the iris measures that
were significantly associated with personality in the variable-
oriented analysis were included in the cluster analysis. The
result of the analysis, which represents the configurations of iris
characteristic that typically tends to appear in early adults, is
presented in Table 3.
3.2. Person-oriented analysis
Two different types of configurations emerged. The
individuals in clusters 1 and 2 had dense crypt structures,
whereas clusters 3 and 4 contained individuals with open crypt
structures. The amount of furrows that existed in each of these
clusters further differentiated between the clusters. Clusters 1
and 4 individuals had few contraction furrows, whereas subjects
in clusters 2 and 3 had many.
The one-way analyses of variance that tested if any mean
differences emerged between these four clusters, produced
results which were in agreement with our expectations, mean
differences in regards to personality emerged (Table 4). Overall,
individuals in the clusters with more tissue loss in the iris scored
lower on approach-related behaviors, compared with those with a
dense iris structure. For example, the individuals in cluster 3
scored significantly lower than the individuals in cluster 1 on all
approach-related behaviors that in the variable-oriented analysis
were found to be significantly associated to crypts. Hence, tissue
loss in the iris was in the person-oriented analysis, just as in the
variable-oriented analyses, associated with lower scores on
approach-related behaviors.
Table 2
Pearson correlation between iris characteristics and the personality measures
Personality measure Iris characteristics
Crypts Pigment dots Furrows
Neuroticism .04 .06 .11
*
Anxiety .02 .01 .06
Angry-hostility .04 .07 .04
Depression .01 .07 .09
Self-consciousness .05 .08 .09
Impulsiveness .06 .02 .15
**
Vulnerability .08 .04 .07
Extraversion .09 .02 .04
Warmth .15
**
.04 .04
Gregariousness .05 .06 .01
Assertiveness .01 .02 .00
Activity .05 .00 .05
Excitement seeking .01 .01 .07
Positive emotions .13
**
.01 .03
Openness to experience .10
*
.01 .06
Fantasy .01 .02 .03
Aesthetics .13
**
.01 .06
Feelings .14
**
.05 .03
Actions .03 .02 .06
Ideas .01 .03 .02
Values .07 .03 .08
Agreeableness .10
*
.06 .00
Trust .11
*
.05 .01
Straightforwardness .04 .10
*
.03
Altruism .06 .01 .03
Compliance .04 .01 .02
Modesty .04 .05 .01
Tendermindedness .13
**
.03 .03
Conscientiousness .07 .05 .09
Competence .02 .07 .09
Order .00 .03 .08
Dutifulness .06 .09 .01
Achievement striving .08 .03 .06
Self-discipline .08 .03 .12
**
Deliberation .04 .03 .04
*
p<.05.
**
p<.01.
M. Larsson et al. / Biological Psychology 75 (2007) 165–175 169
But the results of the analyses of variance also demonstrated
that furrows were associated with mean differences between
clusters that had the same crypt structure. For example, subjects
in clusters 1 and 2, both of which contained individuals with
dense crypt structures, scored differently on the neuroticism
domain as well as the facet scale Impulsiveness in this domain.
The individuals in cluster 1 who had few furrows, scored
significantly lower than individuals in cluster 2 who had many
furrows. Likewise, subjects in clusters 3 and 4, which both
contained individuals with open crypt structures, scored
differently on the Conscientiousness domain as well as the
facet scales achievement-striving and self-discipline in this
domain. The individuals in cluster 3 who had many furrows,
scored significantly lower than individuals in cluster 4 who had
few furrows. Furrows were thereby associated with mean
differences between clusters that had the same crypt structure,
and the direction of these associations was consistent with the
significant association found between furrows and neuroticism,
impulsiveness and self-discipline in the variable-oriented
analyses.
Especially noteworthy was that mean differences emerged
on behaviors that were not significantly associated with crypts
in the variable-oriented analysis. Mean differences between
clusters 1 and 3 were observed for values in the openness to
experience domain, as well as the conscientiousness domain
and the facet scales achievement-striving and self-discipline in
this domain. These traits were not significantly associated with
crypts in the variable-oriented analysis.
The effect sizes for all associations that were significant in
the variable- and person-oriented analyses are presented in
Table 5. Overall, the effect sizes in the person-oriented analysis
were about twice as large as in the variable-oriented analysis.
Table 3
The configurations of iris characteristic that typically exists in early adults
Iris characteristic and scale description Cluster 1
(N= 143)
1
Cluster 2
(N= 155)
2
Cluster 3
(N= 73)
3
Cluster 4
(N= 57)
4
Crypt frequency
(1) Only parallel and/or densely packed curly fibres Yes Yes – –
(2) Spots of wavy fibres and 1–3 two shallow crypts Yes Yes – –
(3) Four shallow crypts or more – – Yes Yes
Contraction furrows
(1) Extending less than 1/4 circle Yes – – Yes
(2) Extending between 1/4 and 8/10 of a circle Yes – Yes –
(3) More distinct, extending 8/10 of one circle or more – Yes Yes –
1
Mean
crypts
= 1.61/mean
furrows
= 1.65.
2
Mean
crypts
= 1.52/mean
furrows
= 3.00.
3
Mean
crypts
= 3.00/mean
furrows
= 2.34.
4
Mean
crypts
= 3.00/mean
furrows
= 1.00.
Table 4
Means of the personality measures with standard deviations within parentheses in each of the four clusters, with test for mean differences between the clusters (one-
way ANOVA, post hoc = Tukey HSD)
Personality measure Cluster 1
(N= 143)
Cluster 2
(N= 155)
Cluster 3
(N= 73)
Cluster 4
(N= 57)
d.f. F
Neuroticism .15 (.89) .16 (1.01)
1
.01 (1.08) .11 (1.08) 3; 423 2.63
*
Impulsiveness .14 (.95) .19 (.96)
1
.04 (1.14) .18 (1.11) 3; 423 3.52
*
Extraversion – – – – – –
Warmth .13 (.97)
3
.03 (.95) .26 (1.13) .13 (.99) 3; 423 2.90
*
Positive emotions .13 (.92)
3
.05 (1.02) .26 (1.09) .11 (.97) 3; 423 2.86
*
Openness to experience .17 (.99)
3
.00 (.98) .30 (1.07) .06 (.88) 3; 423 3.82
**
Aesthetics .21 (.96)
3
.01 (.97) .35 (1.09) .06 (.90) 3; 423 5.42
**
Feelings .22 (.94)
3
.01 (1.01)
3
.38 (1.00) .15 (.96) 3; 423 6.49
**
Values .12 (.97)
3
.00 (.99) .34 (1.10) .07 (.87) 3; 423 3.67
**
Agreeableness .18 (.97)
3
.04 (1.01) .18 (1.09) .14 (.88) 3; 423 2.92
*
Trust .20 (.93)
3
.03 (.98) .22 (1.17) .09 (.90) 3; 423 3.48
**
Tendermindedness .20 (.93)
3
.07 (1.02) .20 (1.08) .12 (.91) 3; 423 3.57
**
Conscientiousness .11 (.94)
3
.02 (.94) .31 (1.04) .17 (1.14)
3
3; 423 3.53
*
Achievement striving .04 (.93)
3
.04 (.97)
3
.33 (.99) .16 (1.15)
3
3; 423 3.39
*
Self-discipline .20 (.95)
3
.08 (.98) .39 (1.00) .19 (1.02)
3
3; 423 6.84
**
The table show Z-transformed values. Elevated numbers indicates between which clusters a significant mean difference was observed (post hoc = Tukey HSD).
*
p<.05.
**
p<.01.
M. Larsson et al. / Biological Psychology 75 (2007) 165–175170
4. Discussion
The introduction in this study reviews several possible
biological pathways that theoretically could explain an
association between iris characteristics and personality. In
order to more fully evaluate a potential role of the
neurodevelopmental control gene Pax6 we tested three
hypotheses by performing both variable- and person-oriented
analysis. These hypotheses draw heavily on the Pax6
expression pattern in the human iris (Davis-Silberman et al.,
2005; Jensen, 2005; Sale et al., 2002) as well as the anterior
cingulate cortex as documented by Ellison-Wright et al. (2004)
in conjunction with the findings made by Pujol et al. (2002),
Davidson (1992, 2001) and Davidson and Irwin (1999).The
specificity of the hypotheses increased. We believed that if all
predictions were supported the likelihood that Pax6 would be
involved would have to be considered high.
Our first hypothesis, that most of the associations with
personality would be found for crypt frequency, was confirmed.
Furthermore, our second and third hypotheses were also
confirmed; crypts were primarily associated with approach-
related behaviors and individuals with a dense crypt structure
scored higher than those with an open crypt structure on such
behaviors.
Thus, the findings in the variable-oriented analyses show
that crypt frequency is associated with the extent in which
individuals engage in approach-related behavior. Furthermore,
since our hypotheses were confirmed, crypt frequency may
likely be caused by subtle Pax6 differences. In turn, these Pax6
differences may also act as a mediator to approach-related
behavior since Pax6 potentially can induce tissue deficiencies
in the left anterior cingulate cortex.
The results in the person-oriented analysis supported this
notion, as the findings were essentially the same. Clusters with
less tissue loss in the iris were associated with higher scores on
approach-related behaviors and the significant mean differ-
ences between clusters 1 and 3 were systematically pinpointing
such traits (i.e., warmth, positive emotion, feelings, agreeable-
ness, trust and tendermindedness) (see Table 4).
But maybe the most intriguing finding was that smaller
tissue differences than crypts were also of major importance.
This was apparent in three different ways. First, the association
between furrows and the facet scale impulsiveness in the
variable-oriented analyses were of the same magnitude as the
strongest association found for crypts. Second, in the person-
oriented analyses furrows could differentiate between indivi-
duals who scored high and low on the neuroticism and
conscientiousness domain, even when the individuals in these
clusters had the same crypt structure. Third, when the combined
effect of having specific configurations of crypts and furrows
was taken into consideration in the person-oriented analyses,
the effect sizes were on average about twice the magnitude of
what was found in the variable-oriented analyses. Thus, it is
apparent that even smaller tissue differences than crypts were of
importance. Naturally emerging configurations of both crypts
and furrows were, as expected, pinpointing subgroups of
individuals that were especially likely to differ in their
personalities. Indeed, when both crypts and furrows were
Table 5
The effect size for all associations that was significant in the variable- and person-oriented analysis
Personality measure Cohen’s d
Variable-oriented analysis
a
Person-oriented analysis
b
Pearson correlations Clusters
1 and 2
Clusters
1 and 3
Clusters
2 and 3
Clusters
3 and 4
Crypts furrows Crypts furrows
Neuroticism – .22
*
.33
*
–––
Impulsiveness – .30
**
.36
*
–––
Extraversion – – – – – –
Warmth .30
**
– – .37
*
––
Positive emotions .26
**
– – .39
*
––
Openness to experience .20
*
– – .47
**
––
Aesthetics .26
**
– – .55
**
––
Feelings .28
**
– – .62
**
.40
*
–
Values – – .44
**
––
Agreeableness .20
*
– – .36
*
––
Trust .22
*
– – .39
**
––
Tendermindedness .26
**
– – .41
**
––
Conscientiousness – – – .42
*
– .44
*
Achievement striving – – – .38
*
.38
*
.49
*
Self-discipline – .24
**
– .61
**
– .57
**
a
Cohen’s d=2r/H(1 r
2
), where ris the Pearson correlation.
b
Cohen’s d=M
1
M
2
/s
pooled
, where M
1
is the mean in one of the cluster and M
2
is the mean in the other cluster. s
pooled
is the pooled standard deviation for the two
clusters (spooled ¼p½ðs2
1þs2
2Þ=2).
*
p<.05.
**
p<.01.
M. Larsson et al. / Biological Psychology 75 (2007) 165–175 171
taken into consideration in the person-oriented analyses, mean
differences in all main domains of personality emerged.
Overall, these findings support the notion that people with
different iris configurations tend to develop along different
trajectories in regards to personality. Furthermore, the self-
organizational properties of the developmental system that are
reflected in the iris appear to be quite efficient in pinpointing the
individuals who differ in their personalities. The effect sizes
between the clusters in the person-oriented analyses ranged
from d= .33 to .62, which is much larger than, for example,
women’s tendency to be more emotional than men, d= .05 to
.30 (Brebner, 2003).
When considering which approach-related behaviors were
most salient in the variable-oriented analyses, it was apparent
that a group of five approach-related traits (i.e., feelings,
tendermindedness, warmth, trust and positive emotions) was
driving the significant associations in the main domains. Both
the significant associations with the facet scales in the openness
to experience and agreeableness domain suggest that.
People with dense structures were more receptive to their
inner feelings and tended to sympathize and feel concern for
other people’s needs more than people with open crypt
structures. Furthermore, the most robust results for crypt
frequency suggest that people with dense structures form
warmer and more trustful attachments to others and experience
and express positive emotions, such as joy, happiness, and
excitement more often than those with open crypt structures.
Crypt frequency was thereby predominately associated with
facet scales that tap the emotional space in the extraversion,
openness to experience, and agreeableness domains. Thus,
tissue loss in the two top cell layers in the iris, as assessed by our
crypt measure, is in this general sense most strongly associated
with the emotional component of personality.
However, contraction furrows, which measure the thickness
and density differences in all five-cell layers in the iris, also
appear to be associated with emotional functioning. Furrows
were primarily associated with impulsiveness, and people with
many contraction furrows were less able than people with few
contraction furrows to control their cravings and urges. Thus,
the variable-oriented analyses revealed that the behavioral
correlates of the two iris characteristics differ. Crypt frequency
is most strongly related to the emotional space in the
extraversion, agreeable and openness to experience domains,
whereas the effect of contraction furrows is more specific and
relates directly to impulse control.
It is, therefore, interesting to note that when the principles of
self-organization were applied in the person-oriented analysis
(Bergman and Magnusson, 1997), the configurations of iris
characteristics that naturally emerge pinpointed clusters of
individuals that scored significantly different on the con-
scientiousness domain—a domain that was unrelated to the iris
characteristics in the variable-oriented analysis. At first sight
this may not be surprising since the inability to resist impulses,
or poor self-control, generally also influences the active process
of planning, organizing and carrying out tasks, which are basic
tendencies in the conscientiousness domain (Costa and
McCrea, 1992). However, the fact that the relationship with
the conscientiousness domain was only possible to identify
when both crypts and furrows were taken in consideration,
suggests two things that previously have not been recognized.
First, small differences in the extent to which people engage in
approach-related behaviors – as assessed by crypt frequency –
matters for how poor impulse control influences the con-
scientiousness domain. Second, crypt frequency identifies for
whom poor impulse control most likely also results in a
tendency to score low on the conscientiousness domain. Thus,
given that it has been difficult to replicate associations between
single genes and personality traits influenced by impulsive
behavior (Persson et al., 2000; Reif and Lesch, 2003), and that
single genes generally explain less than 1.5% of the variance for
a personality trait (Comings et al., 2000), our crypt frequency
scale may be helpful to use in future association studies. If not
considered, crypt frequency may mask potential associations
between candidate genes that potentially could influence the
conscientiousness domain, and potentially many other beha-
vioral traits that in part are dependent on the individual’s ability
to exercise self-control (Schmeichel and Baumeister, 2004).
However, if the crypt frequency scale is considered, or even
better, if the genes that determine crypt frequency can be
identified, this may help to find genes that add up in their effect,
which eventually can account for a greater proportion of the
genetic variance that influences personality as well as other
behaviors (Comings et al., 2000; Schmeichel and Baumeister,
2004). Thus, future association studies may be more fruitful,
and their result more applicable on the individual level, if iris
data is considered. The fact that people with different iris
configurations tend to develop along different trajectories, and
that the self-organizational properties of the developmental
system that are reflected in the iris are quite efficient in
pinpointing the individuals who differ more in their person-
alities, also supports this notion.
The behavioral findings in this study suggest that because
Pax6 induces tissue deficiencies in both the iris (Davis-
Silberman et al., 2005) and the left anterior cingulate cortex
(Ellison-Wright et al., 2004), it may influence the extent to
which people engage in approach-related behaviors (Pujol
et al., 2002). Indeed, there is a substantial body of research that
supports this notion. For example, positive emotions experi-
enced by monkeys increased the left-sided activation of their
brains (Kosslyn et al., 2002). People with positive dispositions
show greater relative left frontal EEG asymmetry at baseline
and respond more strongly to positive than to negative stimuli
(Sutton and Davidson, 1997; Wheeler et al., 1993). Ten-month-
old infants who did not cry when they were separated from their
mothers exhibited more left-brain activation (Fox and
Davidson, 1987). The thresholds for experiencing positive
emotions is lower for people who display left-sided activation
patterns (Fox and Carlkins, 1993). In addition, these left-sided
patterns are stable in both early childhood (Fox et al., 1992) and
adulthood (Davidson, 2003). Thus, the association between
frontal brain asymmetries and approach-related behaviors
exhibits enough stability and consistency to be considered an
underlying biological disposition for personality (Buss and
Larsen, 2005). Overall, these findings correspond well with our
M. Larsson et al. / Biological Psychology 75 (2007) 165–175172
finding as well as the fact that the principal distribution and
depth of crypts are present at birth (Oyster, 1999).
Furthermore, our conscientiousness and Impulsiveness
findings are also consistent with previously published results.
The anterior cingulate function at large is to integrate
attentional and affective information that is critical for self-
regulation and adaptability, and it plays a crucial role in
initiation, motivation and goal-directed behaviors (Devinsky
et al., 1995; Thayer and Lane, 2000). The ventral region (BA
24a–c, 25, 32, 33) is primarily involved in assessing the salience
of emotions, hurtful rejections and romantic love, for example,
as well as the regulation of emotional responses involved in
social interactions which assess one’s own and other people’s
emotions and states of minds (Bartels and Zeki, 2000;
Eisenberger and Lieberman, 2004; Simmons et al., 2006).
The dorsal region (BA 24b0–c0,32
0) is primarily involved in
response selection and the processing of cognitively demanding
information. For example, the cognitions that are activated in
reward-related conflicts and the evaluation of different
outcomes are processed in the dorsal anterior cingulate (Bush
et al., 2000). Furthermore, damage to the anterior cingulate is
associated with a host of social affective impairments including
blunted affect, impulsivity, disinhibition and poor social
judgments (Devinsky et al., 1995; Heyman et al., 1999).
Thus, since processes of planning, organizing and carrying
out tasks as well as resisting impulses are so central for anterior
cingulate functioning at large, as well as for the conscientious-
ness domain, it is not surprising – given the expression pattern
of Pax6 in the iris and brain – that individuals with different
configurations of crypts and furrows scored differently on this
domain.
These findings in conjunction with our findings indicate that
there are sizeable bodies of research that support the notion that
tissue loss in the iris potentially could be associated with tissue
loss in the brain. However, there are also other biological
pathways that may contribute to the associations found. As
mentioned in Section 1, tissue differences in the iris may be
associated with both brain structures and neurotransmitters that
influence personality. The fact that even small differences in the
monoamine systems during the embryological development of
the brain has the strongest influence on the neural development of
the anterior cingulate cortex, effects that persist into adulthood,
further underscores this possibility (Levitt et al., 1997).
Thus, to elucidate these genetic developmental complexities
additional studies are needed. However, given that the genetic
complexities that underlie the appearances of crypts and
furrows probably are difficult to dissect (Aota et al., 2003;
Davis-Silberman et al., 2005). It may be fruitful to investigate
to what extent crypts and furrows can be useful biomarkers in
their own right. The following highlights three areas were it
may be especially rewarding to further investigate the
usefulness of the iris as a biomarker.
4.1. MRI-studies
MRI-studies can identify the extent crypts and furrows are
associated with structural differences in the anterior cingulate
cortex. Thus, crypts and furrows could potentially be
biomarkers for developmental brain differences in large
samples, where it would be too expensive and time consuming
to collect MRI data of the brain. In contrast to MRI data, it takes
about 3 min per person to take digital photos, which readily can
be rated automatically by an iris scanner if the algorithm
currently used for personal identification (Daugman, 2003)
were to be modified to fit this purpose (Dougman, personal
communication, July, 2, 1999). Such efforts would make it
possible to study many neurodevelopmental questions that
today are impractical to study in large samples. For example, do
genes that influence the dopamine, serotonin and noradrenalin
systems and their assumed behavioral associations to specific
personality traits (Comings et al., 2000) change, when the
structure of the brain shifts?
4.2. Longitudinal studies
Iris data may be able to map out how impulsive and
approach-related behaviors of genetic origin, in interaction
with environmental factors, influence dyadic relationships
between family members. For example, it would be interesting
to find out if scores on child disclosure, parental solicitation,
parental control, parental support and parents’ bad reactions to
disclosure, differ from what is normal, in parental–child dyads
where there is a low concordance between the child and the
parent iris configurations. The fact that the expression pattern
for Pax6 in the brain overlaps with the cortical foundation for
maternal behaviors (mainly BA 24/32 and 47) and hurtful
rejections (BA 24/32), in conjunction with our results with
regard to approach-related and impulsive behaviors, suggests
that the iris can be useful marker for biological mechanisms that
influence the interaction between such child and parent based
behaviors (Eisenberger and Lieberman, 2004; Kerr and Stattin,
2003; Lorberbaum et al., 2002; Neiderhiser et al., 2004).
4.3. Twin-studies
The monozygotic (MZ) intra-pair twin correlation for
crypts, pigment dots and furrows are .66, .58 and .79,
respectively (Larsson and Pedersen, 2004). Thus, given that
it actually is quite common, at least in some MZ pairs, to
observe intra-pair differences that are relatively large,
especially with regard to crypts and pigment dots, and smaller
iris differences than this were associated with personality
differences in our sample, it is quite possible that small MZ
intra-pair iris differences could be associated with non-
systematic non-shared environmental effects for personality,
that is, random embryological events that make MZ twins less
similar. In any event, since there is a quest for more theory and
research in this area (Plomin et al., 2001; Plomin and Daniels,
1987; Turkheimer and Waldron, 2000). It is worth mentioning
that by collecting iris data from MZ twins it may be possible to
learn more about how non-systematic non-shared environ-
mental effects, influence personality as well as other behaviors.
In conclusion, our hypothesis was largely confirmed. Crypt
frequency is likely, via Pax6, to be associated with tissue
M. Larsson et al. / Biological Psychology 75 (2007) 165–175 173
deficiencies in the left anterior cingulate cortex, which in turn
influences how much individuals engage in approach-related
behaviors. Furthermore, the self-organizing property of the
developmental system that is reflected in the iris appears to be
quite efficient in pinpointing individuals who differ in their
personalities. When both crypts and furrows were taken into
consideration in the person-oriented analyses, mean differences
in all main domains of personality emerged. Furthermore, the
effect sizes in the person-oriented analyses were about twice as
large as in the variable-oriented analyses. Overall, these
findings support the notion that people with different iris
configurations tend to develop along different trajectories in
regards to their personalities. Thus, it may be fruitful to
investigate further the usefulness of the iris as a biomarker
within a holistic interactionistic framework, using both
variable- and person-oriented approaches. Future genetic
association studies as well as MIR-, longitudinal- and twin-
studies may benefit from collecting iris data and testing
candidate genes for crypts and contraction furrows.
Acknowledgements
This study was supported by funds from the Department of
Social Sciences at O
¨rebro University in Sweden and The
Swedish Foundation for International Cooperation in Research
and Higher Education. We are grateful to the participants for
their cooperation.
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