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RESEARCH ARTICLE
Is crossed laterality associated with academic
achievement and intelligence? A systematic
review and meta-analysis
Marta Ferrero
1,2
*, Gillian West
3
, Miguel A. Vadillo
4,5
1Department of Experimental Psychology, University College London, London,United Kingdom, 2Facultad
de Ingenierı
´a, Universidad de Deusto, Bilbao, Spain, 3Department of Language and Cognition, University
College London, London, United Kingdom, 4Primary Care and Public Health Sciences, King’s College
London, London, United Kingdom, 5Departamento de Psicologı
´a Ba
´sica, Universidad Auto
´noma de Madrid,
Madrid, Spain
*marta.ferrero@deusto.es
Abstract
Over the last century, sporadic research has suggested that people whose hand, eye, foot,
or ear dominances are not consistently right- or left-sided are at special risk of suffering aca-
demic difficulties. This phenomenon is known as crossed laterality. Although the bulk of this
research dates from 1960’s and 1970’s, crossed laterality is becoming increasingly popular
in the area of school education, driving the creation of several interventions aimed at restor-
ing or consolidating lateral dominance. However, the available evidence is fragmentary. To
determine the impact of crossed laterality on academic achievement and intelligence, we
conducted a systematic review and meta-analysis of articles published since 1900. The
inclusion criteria for the review required that studies used one or more lateral preference
tasks for at least two specific parts of the body; they included a valid measure of crossed
laterality; they measured the impact of crossed laterality on academic achievement or intelli-
gence; and they included participants between 3 and 17 years old. The final sample included
26 articles that covered a total population of 3578 children aged 5 to 12. Taken collectively,
the results of these studies do not support the claim that there is a reliable association
between crossed laterality and either academic achievement or intelligence. Along with this,
we detected important shortcomings in the literature, such as considerable heterogeneity
among the variables used to measure laterality and among the tasks utilized to measure the
outcomes. The educational implications of these results are discussed.
Introduction
The term “crossed laterality” is employed to refer to people whose hand, eye, foot, or ear domi-
nance are not uniformly right- or left-sided. The idea that crossed laterality is linked to aca-
demic performance is becoming increasingly popular in the area of school education. A simple
search in Google of the terms "Crossed + laterality + right + hand + left + eye + exercises"
returns around 33.000 entries, most of them containing links to articles, fora and specialised
PLOS ONE | https://doi.org/10.1371/journal.pone.0183618 August 28, 2017 1 / 18
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OPEN ACCESS
Citation: Ferrero M, West G, Vadillo MA (2017) Is
crossed laterality associated with academic
achievement and intelligence? A systematic review
and meta-analysis. PLoS ONE 12(8): e0183618.
https://doi.org/10.1371/journal.pone.0183618
Editor: Jakob Pietschnig, Universitat Wien,
AUSTRIA
Received: May 23, 2017
Accepted: August 8, 2017
Published: August 28, 2017
Copyright: ©2017 Ferrero et al. This is an open
access article distributed under the terms of the
Creative Commons Attribution License, which
permits unrestricted use, distribution, and
reproduction in any medium, provided the original
author and source are credited.
Data Availability Statement: All relevant data are
within the paper and its Supporting Information
files.
Funding: MF was supported by Grant AYD-000-
235 from bizkaia:talent, Diputacio
´n Foral de Bizkaia,
and by Programa Posdoctoral de
Perfeccionamiento de Personal Investigador
Doctor, Gobierno Vasco. MAV was supported by
Grant 2016-T1/SOC-1395 from madri+d Science
Foundation. The funders had no role in study
design, data collection and analysis, decision to
publish, or preparation of the manuscript.
centres focused on this issue. The majority of these sites establish a direct association between
crossed laterality and learning difficulties and offer information about different programmes
aimed at remediating these disabilities by restoring unilateral dominance in children. For
instance, The Institute for Neuro-Physiological Psychology (INPP), founded in the UK and now
established in more than 20 countries, offers remedial training for children who show crossed
laterality, with the aim of improving their educational outcome. It also offers licenses to use the
INPP method as an independent practitioner. Similarly, The Brain Balance Achievement Centres,
with around 100 subsidiaries in United States, offer a specific training called The Brain Balance
Program, aimed at restoring natural dominance in children with a mixed-dominance profile.
Finally, The Superior Institute of Psychological Studies offers assessments and interventions for
crossed laterality in more than 15 centres in Spain. These programs may cost up to €350 [1].
Since Orton [2] suggested that an ill-established cerebral dominance was the cause of several
disabilities, a number of interventions, such as the ones mentioned above, have been created to
ameliorate learning disabilities by performing specific physical exercises that are assumed to
restore or consolidate laterality. For instance, a popular method known as patterning [3] is
aimed at imposing cerebral dominance through a series of physical exercises which consist of
manipulating the child’s head and/or extremities in patterns intended to imitate prenatal and
postnatal movements. The method lacks any supporting evidence and has been the object of
severe criticism [4,5]. However, it remains popular in several countries [6]. Another interven-
tion based to some extent on Orton’s theory is Brain Gym
1
, an educational program originally
created by Paul E. Dennison and Gail Dennison in 1980. Brain Gym
1
prescribes a number of
simple movements intended to improve the integration of specific brain functions with body
movements. According to its authors, lateral dominance is essential for reading, writing, listen-
ing, speaking, and the ability to move and think at the same time [7]. In spite of its popularity in
schools all over the world, this intervention has been widely proven as ineffective [8].
Several studies have reported that crossed lateral people are at special risk of suffering learn-
ing disabilities, while others have reported the opposite result [9–18]. According to these
mixed results, crossed laterality might or might not be a risk factor for learning disabilities and
poor academic performance. In the same vein, studies addressing the relationship between
handedness (regardless of lateral dominance in other parts of the body) and intelligence have
yielded mixed and inconsistent results, although recent meta-analyses point to a slight cogni-
tive advantage of right-handers [19] and a higher prevalence of atypical-handedness among
intellectually disabled individuals [20]. In any case, to the best of our knowledge, the effective-
ness of existing interventions aimed at addressing crossed laterality has never been put to the
test in rigorous studies. The use of interventions with dubious or null evidence in the class-
room or other educational contexts can be harmful in different ways. For instance, educational
professionals and families must often devote much energy and emotion during the implemen-
tation of the interventions [21]. Many of these practices involve all the actors spending a great
deal of time and enormous sums of money in individual counselling sessions for children, for-
mative sessions for teachers and families or acquisition of reference materials by schools. This
waste of time and money involves an opportunity cost, because these resources are taken away
from effective treatments [22]. Even worse, the replacement of evidence-based practices by
non-evidence based practices can harm children [23,24].
The fact that interventions aimed at addressing crossed laterality have not been properly
tested does not necessarily mean that some of their basic tenets regarding the relationship
between crossed laterality, intelligence and academic performance have not been explored. In
fact, as previously mentioned, since the 1960’s a number of studies have tried to measure the
impact of lateral dominance on these variables with mixed results [13,25]. The goal of the pres-
ent study is to conduct a systematic review of all the available evidence to assess the impact of
Crossed laterality, academic achievement and intelligence
PLOS ONE | https://doi.org/10.1371/journal.pone.0183618 August 28, 2017 2 / 18
Competing interests: The authors have declared
that no competing interests exist.
crossed laterality on academic achievement and intelligence among students, from kindergar-
ten to high school. If a link between them exists, it would be imperative to design studies that
tackle the effectiveness of the interventions utilized in school settings. By contrast, if no link is
found, it would be reasonable to divert the available resources into testing the effectiveness of
scientifically grounded interventions.
Materials and methods
Search procedures
The present systematic review follows the recommendations of the PRISMA (see S1 Table and
[26]). On August 15th 2016 the first author (MF) conducted an electronic search on the Web
of Science entering the terms “[(preference OR dominance) AND (hand OR eye OR ear OR
foot OR manual OR ocular OR lateral OR cerebral)] OR [(cross) AND (lateralOR domi-
nance OR preference)] OR [(mix) AND (preference OR dominance)] OR [(hemispheric)
AND (indecision OR specialization)] OR asymmetry OR bilateral OR laterality OR lateraliza-
tion OR eyeness OR handedness OR hand skill” into the Title field. The search was limited
to (a) English-language articles (b) published between 1900 and 2015 (c) with categories re-
stricted to “behavioural sciences”, "neuroscience", "psychology", "psychology applied", "psy-
chology biological", "psychology developmental", "psychology experimental", "psychology
multidisciplinary" and "psychology social", and (d) research areas restricted to “behavioural
sciences”, “education/educational research”, “neurosciences/neurology”, and “psychology”.
Unpublished dissertations, reviews and meta-analysis were excluded at this stage. After remov-
ing 503 duplicates this initial search yielded a sample of 10,540 studies.
Titles and abstracts of these studies were screened by MF using the inclusion criteria C1-C4
explained below. This resulted in 36 full-text articles assessed for eligibility. Authors MF and
MAV independently read the full texts of these 36 articles to check whether they fulfilled the
inclusion criteria. Among the initial set of 36 articles assessed for eligibility, eight articles com-
plied with the inclusion criteria. Thereupon, descendancy searches of articles citing or cited by
these eight papers were conducted to identify additional studies. This resulted in 21 full-text
articles which were also read independently by MF and MAV. Among them, a total of 14 stud-
ies were selected by the inclusion criteria. A second round of descendancy searches was con-
ducted for articles citing or cited by each of these 14 papers, which led to 21 extra articles
assessed for eligibility. Among this set, 4 additional studies were selected. Therefore, the final
sample of articles reviewed for inclusion comprised 26 articles (8 + 14 + 4). These articles
[11,12,15,18,27–48] are shown in Table 1. A PRISMA flowchart summarizing the literature
search process is depicted in Fig 1. Two of the studies selected were based on data that had
already been collected and analysed in other articles, also included in our selection
[28,29,40,41]. Across all the full-text articles read for inclusion, initial inter-rater agreement
was 93.58%. Disagreements were resolved by discussion and consensus between the two
researchers until there was 100% agreement.
Selection criteria
Studies were selected for inclusion in this review if they met the following criteria: (C1) They
used one or more lateral preference tasks for at least two of the following parts of the body:
hand, eye, foot, or ear; (C2) they included a measure of crossed laterality according to the
absolute or relative operative definitions described below; (C3) they measured the impact of
crossed laterality on academic achievement or intelligence; and (C4) they included participants
between 3 and 17 years old.
Crossed laterality, academic achievement and intelligence
PLOS ONE | https://doi.org/10.1371/journal.pone.0183618 August 28, 2017 3 / 18
Table 1. Articles that met inclusion and quality criteria.
Author;
year
N Gender Age
Selection
criteria
Setting
Laterality
Academic achievement
Intelligence
Body
part
Task Reading Spelling Arithmetic
Language
Annet &
Turner, 1974
[27] [A]
224 120F
104M
60–132
months
school
population
city nursery school and
infant and junior
schools
Hand,
eye
1
,
foot
non-standardised
(
8
)
n.s. __ __ The Peabody
Picture
Vocabulary Test
The maze test of
the Wechsler
Scale
Balow, 1963
[28] [A]
302 151F,
151M
84 months school
population
n.s. hand,
eye
1
Test Harris (
8
) The Gates Reading
Readiness Tests
Gates Primary
Reading Tests
PPR-Paragraph
Reading
__ __ __ Lorge-Thorndike
Intelligence Test
Balow &
Barlow 1964
[29] [A]
250 n.s. 96 months school
population
n.s. hand,
eye
1
Test Harris (
8
) The Gates Reading
Readiness Tests
Gates Primary
Reading Tests
PPR-Paragraph
Reading
__ __ __ Lorge-Thorndike
Intelligence Test
Bishop et al.
1979 [30] [A]
147 n.s. 102 months school
population
rural general practice hand,
eye
2
non-standardised
(
8
)
Neale Reading
Ability Test
__ __ __ Wechsler Scale
for Children-R
Brod &
Hamilton, 1971
[31] [AB]
54,51,52 (A,
N,D)
n.s. 132 months school
population
parochial school hand,
eye
1
non-standardised
and the hole-in-
the- card test(
8
)
Reading of
passages
__ __ __ __
Bryden, 1970
[11] [B]
234 n.s. 84 months
(GR2)
108 months
(GR4)
132 months
(GR6)
school
population
n.s. hand,
ear
5
non-standardised
(
8
)
The Gates-
MacGinitie Reading
Test
__ __ __ The Otis Quick-
Scoring Mental
Ability Test
Clymer & Silva,
1985 [32] [A]
890 n.s. 84 months all born in the
same
hospital
n.s. hand,
eye
1
,
foot
4
Test Harris (
8
) __ __ __ The Dunedin
Articulation
Check
The Illinois Test
of Psycho-
linguistic Abilities
Wechsler Scale
for Children-R
Coleman &
Deutsch, 1964
[33] [A]
121 7F, 28M
(N)
56M (D)
26M, 4F
(D)
108.5–144.3
months (D)
120.3–144
months (N)
reading
disabilities
n.s. hand,
eye
1
,
foot
Test Harris (
8
) __ __ __ __ __
Conolly, 1983
[34] [AB]
91 29F, 62M 84–141
months
learning
disabilities
private elementary
school for learning
disabled children
hand,
eye
1
,
foot
Test Harris (
8
) n.s. __ n.s. __ __
Dunlop et al.,
1973 [12] [A]
30
(15N, 15D)
n.s. 103.3 months
(N)
118.7 months
(D)
reading
disabilities
local district primary
schools
hand,
eye
2
non-standardised
(
8
)
Neale Reading
Ability Test
__ __ __
__
Fagard et al.,
2008 [15] [A]
42
(18GR1,
24GR5)
10F, 8M
(GR1)
11F, 14M
(GR5)
72 months
(GR1)
120.4 months
(GR5)
school
population
regular public school hand,
eye
1
non-standardised
(
9
)
Alouette
standardized
reading test
__ __ __ __
(Continued)
Crossed laterality, academic achievement and intelligence
PLOS ONE | https://doi.org/10.1371/journal.pone.0183618 August 28, 2017 4 / 18
Table 1. (Continued)
Author;
year
N Gender Age
Selection
criteria
Setting
Laterality
Academic achievement
Intelligence
Body
part
Task Reading Spelling Arithmetic
Language
Gates & Bond,
1936 [35] [A]
129
(64N, 65D)
n.s. 103.32 months reading
disabilities
n.s. hand,
eye
1
non-standardised
(
8
)
Gates Reading
Diagnostic Test
__ __ Tests of word
pronunciation
__
Harris, 1957
[36] [A]
561
(245D, 316N)
42F,274M
(D),
n.s (N)
120 months reading
disabilities
n.s. hand,
eye
1
Test Harris (
8
) n.s. __ __ __ __
Hillerich, 1964
[37] [A]
400 n.s. 60 months
90 months at
follow-up
assessment
school
population
kindergarten schools hand,
eye
1
non-standardised
(
8
)
California
Achievement Test
__ __ __ California Short-
Form Test of
Mental Maturity
Kirk, 1934 [38]
[B]
61 30F, 31M high school intellectual
disability
state-funded institution
for developmentally-
disabled children
hand,
eye
1
non-standardised
and Miles V-
scope (
8
)
Gray Oral Reading
Tests
__ __ __ __
Mahone et al.,
2006 [39] [AB]
99 53F, 46M 36–70 months school
population
local preschools and
day care centers
hand,
eye
1
non-standardised
(
9
)
__ __ __ Peabody Picture
Vocabulary Test
__
Muehl, 1963
[40] [B]
62
(23Y, 39O)
n.s. 48.7 months
(Y)
58.2 months
(O)
school
population
cooperative
preschools
hand,
eye
1
non-standardised
and Miles V-
scope (
8
)
non-standardised __ __ __ __
Muehl & Fry,
1966 [41] [B]
40 21F, 19M 60–72 months school
population
cooperative
preschools
hand,
eye
1
non-standardised
and Miles V-
scope (
8
)
Metropolitan,
Achievement Test
__ Metropolitan,
Achievement
Test
__ __
Roszkowski
et al., 1987 [42]
[B]
58 29F, 29M 111 months school
population
catholic elementary
school
hand,
eye
1
,
foot,
ear
6
DKSLD (
8
) Educational
Development
Series
__ Educational
Development
Series
__ Otis-Lennon
Test
Shaywitz et al.
1984 [18] [AB]
104
(32N, 37G,
35D)
104M 132 months intelligence
learning
disabilities
public school hand,
eye
7
,
foot
7
non-standardised
(
8
)
Woodcock-
Johnson
achievement
battery
Woodcock-
Johnson
achievement
battery
Woodcock-
Johnson
achievement
battery
__ Wechsler Scale
for Children-R
Smith, 1950
[43] [AB]
100 100M 120 months reading
disabilities
parochial and public
schools
hand,
eye
1
non-standardised
(
8
)
n.s. __ __ __ __
Stephens
et al., 1967 [44]
[B]
89 45F,44M first grades school
population
elementary
schools
hand,
eye
1
non-standardised
(
8
)
Metropolitan
Reading Readiness
Test
__ __ __ California Test of
Mental Maturity
Thomson,
1975 [45] [A]
120 (60N,
60D)
n.s. 84.93 months
(N)
84.82 months
(D)
reading
disabilities
primary schools hand,
eye
1
,
foot,
ear
6
non-standardised
(
8
)
Schonell Reading
Test
__ __ __ __
Trussell, 1969
[46] [A]
75 36F,39M first and
second grades
school
population
public elementary
schools
hand,
eye
1
non-standardised
(
8
)
Metropolitan
Achievement Test
__ __ __ __
Ullman, 1977
[47]
[B]
648 325F,
323M
66–149
months
school
population
public schools hand,
eye
1
,
foot
3
non-standardised
(
8
)
Wide Range
Achievement Test-
Revised
Wide Range
Achievement
Test-Revised
Wide Range
Achievement
Test-Revised
__ Lorge-Thorndike
Intelligence Test
(Continued)
Crossed laterality, academic achievement and intelligence
PLOS ONE | https://doi.org/10.1371/journal.pone.0183618 August 28, 2017 5 / 18
Table 1. (Continued)
Author;
year
N Gender Age
Selection
criteria
Setting
Laterality
Academic achievement
Intelligence
Body
part
Task Reading Spelling Arithmetic
Language
Witty & Kopel.,
1936 [48] [A]
[B]
200
(100N,100D)
34F, 66M
(N)
34F, 66M
(D)
120.4 months
(N)
108.2 (D)
reading
disabilities
public schools hand,
eye
1
non-standardised
(
8
)
Metropolitan
Achievement Test
__ __ __ __
Note: [A] Absolute crossed laterality. [B] Relative crossed laterality. [AB] Absolute or relative crossed laterality. (A) Advanced group. (N) Normal group. (D) Disabled group. (GR1)
First grade. (GR5) Fifth grade. (Y) Young group. (O) Old group. (G) Gifted group. (n.s.) Not specified. (F) Female. (M) Male.
(1)
Sighting task.
(2)
Sighting task and binocular task combined.
(3)
Bilateral task.
(4)
Bilateral and unilateral task combined.
(5)
Dichotic listening task.
(6)
Unilateral listening task.
(7)
Not specified.
(
8
) Behavioural tasks.
(
9
) Behavioural tasks and questionnaire.
https://doi.org/10.1371/journal.pone.0183618.t001
Crossed laterality, academic achievement and intelligence
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Definition of crossed laterality
Throughout this article, we adopted the term “crossed laterality” because it was the most fre-
quent in the literature reviewed. However, it should be noted that some researchers employ
different terms to refer to the same phenomenon, such as “confused laterality” [49], “mixed
preference” [47], “mixed hand-eye” [40] or “mixed dominance” [18]. In this regard, it is worth
mentioning that nowadays the term “mixed dominance” usually refers to the lack of preference
with respect to the same type of limb or organ, not to the combination of preferences of two
different types of limbs or organs (i.e., “mixed handedness” refers to an unclear preference of
the right or left hand).
There is little consensus on the operative definition of crossed laterality through the
reviewed literature. To address this problem, we established two operative definitions. Absolute
crossed laterality refers to always using the same opposite sides of the body while performing
different tasks with any combination of hand, eye, foot, or ear. In contrast, relative crossed
laterality refers to the marked (but not exclusive) preference for using the same opposite sides
of the body while performing different tasks. For example, a child whose dominant eye is the
Fig 1. PRISMA flowchart.
https://doi.org/10.1371/journal.pone.0183618.g001
Crossed laterality, academic achievement and intelligence
PLOS ONE | https://doi.org/10.1371/journal.pone.0183618 August 28, 2017 7 / 18
right in 80% of the observations and whose dominant hand is the left in 80% of them would
not be considered crossed-lateral according to the first operative definition, but would be con-
sidered crossed-lateral according to the second one.
Absolute and relative crossed laterality differ from mixed laterality in that the latter refers to
an undistinguishable or equivocally determined preference for using either side of the body
while performing different tasks [37]. To continue the example above, a child whose dominant
eye is the right in 50% of the observations and whose dominant hand is the left in 50% of the
observations would be considered mixed lateral according to our definitions and consequently
would not comply with the inclusion criteria of this review. Studies that only measured mixed
laterality or that conflated mixed and crossed laterality in their analyses were not included in
this review. For instance, studies that considered laterality as a continuous variable, from
totally right to totally left, were not included [10,25,50–55]. Similarly, studies where mixed and
crossed laterality were merged were also excluded [9,49,56–60]. In either case, intermediate
scores could refer either to genuine crossed laterality or to mixed laterality. In addition, studies
that failed to report any measure of crossed laterality [13,61–74] or that failed to explain how
they categorized crossed lateral participants were excluded [75–78].
Data extraction and coding
The 26 studies included were summarized in terms of characteristics of participants and set-
ting (e.g., public school, catholic school), measures of crossed laterality, academic achievement,
and intelligence (see Table 1).
Results
Participant characteristics
The number of participants included in each study ranged from 30 to 890 (mean = 201.3;
SD = 204.8). The age of participants ranged from 53 to 132 months (mean = 104.4; SD = 24.4).
Ten studies did not report the gender of participants and one study only provided the gender
of one of the groups compared (see Table 1). Among the remaining 16 studies, 984 participants
were female (38.2%) and 1590 were male (61.7%). The majority of participants (54%) were
selected solely on the basis of being enrolled in a specific school, 23.5% were selected based on
the presence or not of reading difficulties, 5.4% were selected based on the presence or not of
learning difficulties or low intelligence, and 17% were selected based on being born in a spe-
cific hospital. In addition, 37% of participants came from public schools, 6.9% from private
schools and 5% from religious schools. In the case of the remaining 49% of participants, the
type of school was not specified.
Methods used to measure laterality
The final sample of studies included in this review used a wide variety of questionnaires and
behavioural tasks to measure laterality, independently of the definition of crossed laterality
adopted (i.e., absolute or relative). It is worth mentioning that not all the tests employed to
assess laterality were equally valid and reliable. They differed substantially in terms of the
quantity of tasks included as well as in terms of the availability of normative data. The studies
included in this review also focused on different combinations of hand, eye, foot or ear to
assess crossed laterality (see Table 1). It is worth underlining that the measure of each part of
the body comprises specific peculiarities and that the type of test employed may sharply con-
strain the results. The study performed by [79] offers a comprehensive analysis of the measure-
ment of lateral preference and an extensive analysis of the prevalence and interrelationship of
Crossed laterality, academic achievement and intelligence
PLOS ONE | https://doi.org/10.1371/journal.pone.0183618 August 28, 2017 8 / 18
lateral preferences among the general population. Detailed information about each type of test
that appeared in the present study is offered in the Supporting Information.
Qualitative analysis
Crossed laterality and reading skill. As can be seen in Table 1, 23 studies measured
the association between crossed laterality and reading performance. Only four of them
[11,12,31,41] detected a significant positive association between crossed laterality and reading
performance (i.e., children with crossed hand-eye laterality performing significantly worse
than children without crossed laterality). Furthermore, two studies obtained significant results
in the opposite direction: Shaywitz et al. [18] found that crossed lateral children showed fewer
serious problems in reading than uncrossed lateral children, while Stephens et al. [44] found
that children with crossed hand-eye laterality performed better in reading than children with-
out crossed laterality.
Crossed laterality and spelling. Two studies measured the association between crossed
laterality and spelling performance (see Table 1). None of them found a statistically significant
association.
Crossed laterality and arithmetic skills. Four studies analyzed the effects of crossed later-
ality in arithmetic performance (see Table 1). Only Muehl and Fry [41] reported a significant
contribution of crossed dominance to arithmetic achievement. Specifically, children with con-
sistent right preference in hand and eye obtained better results than children with right hand
and left eye preference.
Crossed laterality and language. Four studies measured the association between crossed
laterality and language in terms of vocabulary or articulation (see Table 1). None of them
found a significant relationship between crossed laterality and language performance.
Crossed laterality and intelligence. Eleven studies analyzed the relationship between
laterality and intelligence (see Table 1). Only one of these studies found a relationship between
crossed laterality and intelligence. Specifically, Bishop et al. [30] found a significant association
between crossed laterality and intelligence in a small subgroup of children classified as pre-
dominantly uncrossed or predominantly crossed according to reference eye.
Other results. Together with the outcomes described above, one of the studies included
in this review [39] explored the association between crossed laterality and visual attention. The
results showed no relationship between these variables. In addition, Conolly [34] explored the
association between crossed laterality and learning disabilities. The results showed that crossed
laterality was more prevalent in learning disabled children than in normal children.
Quantitative meta-analysis
The 26 studies selected for the present systematic review rely on a wide range of methods to
assess crossed laterality, academic achievement, and intelligence. Consequently, any quantita-
tive synthesis of these studies must be taken with caution. However, to avoid relying exclu-
sively on a vote-counting approach we conducted a meta-analysis of the effect sizes reported in
these studies. Unfortunately, about half of them failed to report sufficient information to com-
pute a valid effect size estimate. The remaining studies provided enough information to com-
pute an effect size in the Cohen’s dscale or reported contingency tables that could be used to
compute an odds ratio, which we converted to a Cohen’s deffect size using the equations pro-
vided by [80]. All the effect sizes we computed focused on the comparison of crossed lateral
and uncrossed lateral participants, which means that some of the comparisons covered in the
previous qualitative review, which focused in specific sub-groups of crossed-lateral or prefer-
entially crossed lateral children, were not included in the meta-analysis.
Crossed laterality, academic achievement and intelligence
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Although we could only extract a single effect size from most studies, some of them con-
tained sufficient information to compute several effect sizes. For instance, Bishop et al. [30],
measured crossed laterality using two different measures of eyeness (sighting eye and reference
eye) and they also measured two outcome measures, reading and intelligence. Furthermore,
they reported results in a way that allowed crossed laterality to be conceptualized according to
both our absolute and relative definitions. Accordingly, we were able to extract six equally
valid effect sizes from this study. Six other studies also contributed with several effect sizes to
our meta-analysis. More information about the procedure followed to compute each effect size
can be found at the Open Science Framework, https://osf.io/akg3h/.
Given that some studies contributed several effect sizes, while others were represented by a
single effect size, these studies cannot be integrated using a simple univariate meta-analysis,
because this would give extra weight to studies reporting several effect sizes. To overcome this
problem, we fitted a three-level random effects models, clustering effect sizes at the study level.
All the statistical analyses were conducted with the metafor R package [81].
All the effect sizes included in the general meta-analysis, with their corresponding 95% con-
fidence interval, are depicted in Fig 2. Negative effect sizes denote a relative disadvantage of
crossed lateral children over children without crossed laterality. As can be seen, only a handful
of studies yield effect sizes that depart noticeably from zero, in either direction. The meta-ana-
lytic effect size for all studies, shown at the bottom, is d= -0.03, 95% CI [-0.10, 0.04], which is
not significantly different from zero, z= -0.73, p= .46. Furthermore, the amount of heteroge-
neity across studies failed to reach statistical significance, Q(26) = 31.50, p= .21, suggesting
that in principle the variation observed across studies can be attributed to chance alone. Fig 2
also presents the meta-analytic average for two subsets of studies that focused on the impact of
crossed laterality on reading and intelligence. The meta-analytic estimates were d= -0.03
[-0.13, 0.06] and -0.04 [-0.17, 0.08], respectively, and in both cases failed to reach statistical sig-
nificance, z= -0.67 and z= -0.69.
A funnel plot, with all the effect sizes plotted against their standard errors, is offered in Fig
3. All the effect sizes falling within the grey contour would be statistically non-significant in a
two-tailed test. As can be seen, only studies with low precision (high standard errors) yield
effect sizes that depart noticeably from zero, while studies comprising more participants tend
to yield effect sizes consistently close to zero. Furthermore, the funnel plot shows that the dis-
tribution of effect sizes is clearly symmetrical, suggesting that the results of the meta-analysis
are unlikely to be affected by publication bias.
Discussion
The majority of the studies included in this review failed to find any relationship between the
preference for different sides of the body when performing tasks that involved any combina-
tion of hand, eye, foot, or ear and performance in reading, spelling, arithmetic, language or
intelligence tests. As explained in the qualitative analysis, some studies showed that crossed
laterality was associated with lower scores in reading [11,12,31,41], arithmetic [41,42,47], intel-
ligence [30] or other variables [34]. However, these results were not consistently replicated in
other studies and sometimes they even failed to be replicated within the same study with differ-
ent selection criteria or different dependent variables. Furthermore, the quantitative meta-
analysis shows that among those studies reporting sufficient information to compute an effect
size the average effect size was not significantly different from zero.
For instance, in the study of Dunlop et al. [12] the lower performance of children with
crossed laterality was only observed when measuring eye dominance with the criterion of the
controlling eye but not with the criterion of the sighting eye, which is the most widely used in
Crossed laterality, academic achievement and intelligence
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the literature (see the S1 File and [82]). In addition, they measured the preferred hand, using a
unique writing task, which has questionable validity for being frequently culturally biased [83].
It is also worth noting that the sample size of the study conducted by Dunlop et al. [12] is only
30 participants (the smallest sample in Table 1) and that the two groups of children (dyslexics
Fig 2. Forest plot of the effect sizes included in the meta-analysis.
https://doi.org/10.1371/journal.pone.0183618.g002
Crossed laterality, academic achievement and intelligence
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and controls) came from very different educational and social backgrounds, without any
attempt to match them in terms of age or other variables. In a study specifically designed as a
replication of Dunlop et al., [12] Bishop et al. [30] failed to replicate these results with a much
larger sample (N= 147), although as discussed below she did detect lower intelligence in a
small subset of crossed lateral children.
Similar problems apply to the other three studies reporting a positive relationship between
crossed laterality and reading problems. In the study by Bryden [11] the relationship between
crossed laterality and lower reading performance was confined to boys with an intermediate
or low reading achievement in relation to their IQ. In the case of girls, no relationship between
crossed laterality and reading disabilities was observed under any assumption. In a study with
only 40 participants (the second smallest study in Table 1, after [12]), Muehl and Fry [41]
Fig 3. Funnel plot of the effect sizes included in the meta-analysis.
https://doi.org/10.1371/journal.pone.0183618.g003
Crossed laterality, academic achievement and intelligence
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found that uncrossed lateral children (right-hand and right-eye) performed better in a reading
task than crossed lateral children (right-hand and left-eye) and left-eyed children. However,
due to the reduced number of children with left-hand and right-eye laterality and left-hand
and left-eye laterality, the authors could not determine whether crossed laterality or eyeness
was the outcome associated to lower reading achievement. The same problem applies to the
study conducted by Brod and Hamilton [31], who actually concluded that the poor perfor-
mance of crossed lateral children was not due to crossed laterality itself but to the fact that
most children in this sample were left-eye dominant.
Furthermore, two studies reported better reading skills in crossed-lateral children than in
control participants. Specifically, Shaywitz et al. [18] found that children with crossed laterality
showed fewer problems of a severe nature in reading. Similarly, Stephens et al. [44] found that
children with crossed laterality had a tendency to perform better in reading than those with an
uncrossed laterality pattern, although this trend failed to reach full statistical significance. As
in other studies finding a significant relationship between crossed laterality and reading per-
formance, these two studies relied on relatively small samples of only 104 and 89 participants,
respectively.
Overall, the collective weight of this evidence does not support the conclusion that there is a
reliable association between crossed laterality and reading performance. Significant (either
positive or negative) associations between both variables were observed predominantly among
the smallest studies and only in some of the dependent variables or in small subgroups of par-
ticipants. The majority of studies, including those with the largest sample sizes, found no rela-
tionship between crossed laterality and reading skills. This conclusion is further supported by
the results of the meta-analytic synthesis, as can be seen in Fig 3.
A small number of studies reported positive correlations between crossed laterality and
other variables related to academic achievement or intelligence. Muehl and Fry [41] detected
that uncrossed lateral children (right hand and right eye) obtained better results in arithmetic
tasks than crossed lateral children (right hand and left eye). However, as explained above, the
authors could not establish whether crossed laterality or eyeness was the outcome related to
poorer performance. Additionally, the small sample size of this study raises suspicions about
the reliability of this finding, bearing in mind that the other three studies exploring this associ-
ation failed to detect a significant result.
One study detected a significant association between crossed laterality and intelligence [30].
Interestingly, in this study there were no significant differences in intelligence between
uncrossed- and crossed-lateral children. Rather, the significant differences found by Bishop
et al. [30] were entirely due to the poor performance of a small group of children (N= 31) with
‘predominantly uncrossed reference’ or ‘predominantly crossed reference’, that is to say, chil-
dren meeting our relative definition of crossed laterality. Far from concluding that there is a
reliable association between crossed laterality and intelligence, the author herself concluded
that ‘this study finds no evidence that “crossed reference” [i.e., laterality] has any particular sig-
nificance.’ (p. 665). Furthermore, this trend fails to reach statistical significance when all par-
ticipants are considered, as confirmed by the confidence intervals reported in Fig 2.
Likewise, one study found that the proportion of children with crossed laterality was higher
among learning disabled than among normal subjects [34]. However, the author did not spec-
ify the tools employed to classify children as learning disabled. In addition, she did not use a
control group, but simply compared her results with the pattern observed in two earlier studies
with completely different and unmatched samples [84].
Finally, no study detected an association between crossed laterality and spelling or lan-
guage. Overall, the fact that the majority of studies failed to observe significant differences and
that the few studies with significant findings yielded inconsistent results suggests that the
Crossed laterality, academic achievement and intelligence
PLOS ONE | https://doi.org/10.1371/journal.pone.0183618 August 28, 2017 13 / 18
significant effects obtained in children with crossed laterality are likely to be unreliable. This
conclusion is consistent with the results of similar studies that were excluded from this review
because they failed to provide an operative definition of crossed laterality or because they
treated crossed and mixed laterality indistinctly (e.g., [16,56,57,59,60,85], but see [13]). Simi-
larly, these results are also consistent with previous non-systematic reviews done in the field of
laterality [86,87].
This review also highlights the substantial degree of heterogeneity that characterises studies
of crossed laterality. In particular, there is little or no consensus on the operative definition of
crossed laterality. While some studies establish strict criteria to define this phenomenon, other
studies are quite flexible and vague or do not provide any operative definition of the concept at
all (e.g., [60]). Furthermore, the tools employed to measure laterality are quite miscellaneous
(see the Supporting Information) and, in many cases, the authors do not report indices of reli-
ability and validity for their measures [12,15,39,45]. Finally, there is great diversity among the
characteristics of the samples and the outcomes measured. Taken together, these limitations
hinder any progress in this area of research.
The results of this study have important implications for education. The lack of strong evi-
dence in favour of a relationship between crossed laterality and academic achievement calls
into question the fact that educational psychologists spend time measuring crossed laterality
and, even more, that crossed laterality should be the object of direct intervention to ameliorate
learning disabilities. These results, in turn, echo the voices that have been raised previously
against evaluations and interventions of this kind [49,86,88]. At present, there is no solid evi-
dence that justifies the adoption of interventions addressed at treating laterality by education
practitioners, let alone the use of them as a replacement for evidence-based interventions
directly aimed at the target difficulties.
Supporting information
S1 Table. PRISMA checklist.
(DOC)
S1 File.
(DOCX)
Acknowledgments
MF was supported by Grant AYD-000-235 from bizkaia:talent, Diputacio
´n Foral de Bizkaia, and
by Programa Posdoctoral de Perfeccionamiento de Personal Investigador Doctor, Gobierno
Vasco. MAV was supported by Grant 2016-T1/SOC-1395 from madri+d Science Foundation.
Author Contributions
Conceptualization: Marta Ferrero, Miguel A. Vadillo.
Formal analysis: Marta Ferrero, Miguel A. Vadillo.
Funding acquisition: Miguel A. Vadillo.
Investigation: Marta Ferrero, Gillian West, Miguel A. Vadillo.
Methodology: Marta Ferrero, Gillian West, Miguel A. Vadillo.
Writing – original draft: Marta Ferrero, Gillian West, Miguel A. Vadillo.
Writing – review & editing: Marta Ferrero, Gillian West, Miguel A. Vadillo.
Crossed laterality, academic achievement and intelligence
PLOS ONE | https://doi.org/10.1371/journal.pone.0183618 August 28, 2017 14 / 18
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