Sex Differences in Performance with the Hand and Arm in Near and Far Space: A Possible Effect of Tool Use

Evolutionary Psychology
www.epjournal.net – 2007. 5(4): 786-800
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Original Article
Sex Differences in Performance with the Hand and Arm in Near and Far
Space: A Possible Effect of Tool Use
Geoff Sanders, Department of Psychology, London Metropolitan University, Calcutta House, Old Castle
Street, London E1 7NT UK. Email: g.sanders@londonmet.ac.uk (Corresponding author)
Marta Perez, Department of Psychology, London Metropolitan University, Calcutta House, Old Castle Street,
London E1 7NT UK.

Abstract: Using novel tasks, we tested two predictions from the hunter-gatherer hypothesis
concerning sex differences in the motor control of hand and arm and in the visual
processing of near and far space. In Study 1 we replicated earlier findings by demonstrating
that women scored higher with the hand while men scored higher with the arm. Study 2
tested the motor and visual predictions concurrently and showed that the Muscle*Sex
interaction, seen in Study 1, occurs in far as well as near space. However, we failed to
confirm that women perform better with visual information from near and men from far
space. Instead the relative performance of women and men was the same in far as it was in
near space. Drawing on evidence from studies of selective visual neglect we suggest that
this outcome arose because tool use causes far space to be re-mapped as near space.
Finally, the selective visual neglect literature indicates that the processing of far and near
space is located in the ventral and dorsal cortical streams, previously described as two
“what”/“where” visual systems. We draw attention to their additional “there”/“here”
functions that are sex dimorphic and, as we have shown, modulated by tool use.
Keywords: hunter-gatherer hypothesis, sex differences, motor control, visual processing,
near and far space, tool use.
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Introduction
The hunter-gatherer evolutionary hypothesis suggests that sex differences in task
performance have arisen through a process of natural selection that favored the
development of brain and body structures supporting the cognitive and motor skills
required for hunting in men and for gathering in women (Silverman and Eals, 1992). We
aimed to investigate this idea by predicting the outcomes from novel tasks designed to test
different aspects of the hypothesis. For example men, as hunters, should show better

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Sex differences in hand and arm use in near and far space
performance in tasks directed at far space and using the large proximal muscles of the arm
and shoulder. These skills would facilitate the location of a target followed by accurately
aiming and throwing a weapon, at prey when hunting and at a threat when defending. On
the contrary, women, as gatherers, should show better performance in tasks directed at near
space and in using the small distal muscles of the wrist and fingers. Both abilities would
facilitate caring and also gathering behaviors by promoting the detection of suitable items
followed by precise reaching and grasping.
The potential existence of these predicted sex differences in motor control and
visual processing would be strengthened if separate neural mechanisms existed for the
constituent processes that may be sex dimorphic. Studies with monkeys (Lawrence and
Kuypers, 1968a; 1968b) have demonstrated that the neural bases for proximal and distal
muscle control are distinct. The distal (hand) muscles are controlled by the primary motor
cortex via two dorsolateral corticospinal tracts whereas the proximal (arm) muscles are
controlled by the primary motor cortex via two ventromedial corticospinal tracts.
A similar neural separation exists for the visual processing of near and far space,
domains which were originally defined by Brain (1941) as “grasping distance” as opposed
to “walking distance” and later by Brouchon, Joanette and Samson (1986) as “reaching
field” and “pointing or throwing field”. Studies of selective visual neglect report a
dissociation between far and near space in monkeys (Rizzolatti, Matelli, and Pavesi, 1983)
and humans (Brouchon, Joanette, and Samson, 1986; Butler, Eskes, and Vandorpe, 2004;
Guariglia and Antonucci, 1992; Halligan and Marshall, 1991; Vuilleumier Valenza, Mayer,
Reverdin, and Landis, 1998). Three studies using normal human volunteers, one using
transcranial magnetic stimulation (Bjoertomt, Cowey, and Walsh, 2002) and two using PET
(Weiss, et al., 2000; Weiss, Marshall, Zilles, and Fink, 2003) have located the
corresponding pathways in the cerebral cortex. The performance of tasks in far space is
dependent on the ventral stream, from the primary visual cortex to the inferior temporal
cortex, while the performance of tasks in near space is dependent on the dorsal stream,
from the primary visual cortex to the posterior parietal cortex.
The predicted sex differences in these aspects of motor control and visual
processing have recently been demonstrated. In two studies using a computer controlled
tracking task performed in near space, women maintained contact with the target for longer
when using their hands to control the cursor via a short joystick than when using their arms
to control the cursor via a long joystick while men tracked better with their arms than with
their hands (Sanders and Walsh, 2007). In a time estimation task and two puzzle
completion tasks, women did better when the visual information was presented in near
rather than far space while men were better with input from far rather than near space
(Sanders, Sinclair, and Walsh, 2007). The time estimation task (Study 1) was conducted in
virtual space via the Internet with far and near space represented by the distant half and the
closer half of a table. A toy UFO hovered over the table moving towards a docking station
but disappearing before it docked. Participants had to indicate when it would have docked.
In the puzzle completion tasks (Studies 2 and 3) participants could not see their hands or
the puzzle directly but instead they saw them as an image projected either onto a monitor
(near space) or onto a wall-mounted screen (far space). Each study revealed a significant
Space*Sex interaction with men significantly better in far than near space in Study 1,
women significantly better in near than far space in Study 2, while in Study 3 women were
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Sex differences in hand and arm use in near and far space
significantly better in near than far space and men were significantly better in far than near
space.
Here we report two studies in which we attempted to replicate those predictions
from the hunter-gatherer hypothesis. In Study 1 we investigated the sex difference in motor
control of hand and arm using a novel task performed in near space. In Study 2 we
investigated the sex differences in motor control and visual processing of near and far space
concurrently. From the hunter-gatherer hypothesis we predicted that men would perform
better when using their proximal arm muscles and when performing in far (extrapersonal)
space while women would perform better when using their distal hand muscles and when
performing in near (peripersonal) space. Data were subjected to appropriate analyses of
variance and significant interactions were explored with t-tests using 1-tailed tests for
directional predictions and 2-tailed tests for other comparisons.

Study 1

This study was designed to investigate the motor control prediction derived from
the hunter-gatherer hypothesis. Using a novel task we attempted to replicate the finding that
women perform better with the small distal muscles of their fingers and wrist while men do
better with the large proximal muscles of their upper arm and shoulder (Sanders and Walsh,
2007). Participants were required to move colored tokens from a neutral starting area to
appropriately colored target areas using the distal and proximal muscles in a
counterbalanced order.
Materials and Methods
Participants
The study involved 48 right-handed participants, 24 men (mean age 27.08 +/- 5.93)
and 24 women (mean age 27.67 +/- 5.62). None of the participants had suffered an upper
limb injury in the previous six months, all could operationally distinguish the colors of the
tokens and all had normal, or corrected to normal, vision. The study was approved by the
London Metropolitan University Psychology Department Ethics Committee. All
participants gave informed written consent and were aware that they could withdraw from
the study at any time. None withdrew.

Tasks and procedure
Hand task The participants were presented with a horizontal board (127 x 178
mm) divided into one large neutral starting zone adjacent to the closer edge and five
smaller target zones, colored yellow, brown, green, blue or red, in a left to right row
between the neutral zone and the farther edge of the board. At the start of a trial 25 tokens,
five of each color, were mixed together within the neutral starting zone. Participants were
asked to place the forearm of their preferred right limb flat on the table in front of the board
and, using short (89 mm) forceps, to move tokens as quickly as possible from the neutral
starting zone to the target zone of the same color as the token. Throughout the task
participants were required to maintain a fixed contact between the underside of their
forearm and the surface of the table so that movement was restricted to the distal muscles
of the wrist and fingers. Performance was monitored to ensure that participants complied
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Sex differences in hand and arm use in near and far space
with these instructions. The task began with a 5 second practice followed by a 20 second
test from which the total number of tokens correctly place in the color matching target
zones was recorded.
Arm task The participants were presented with a larger horizontal board (305 x
406 mm) divided as for the hand task into a neutral starting zone and five smaller target
zones with the same five colors. This time, the participants were given a short (178 mm)
hooked metal stylus, which they used to slide tokens as quickly as possible from the neutral
starting zone to the target zone of the same color as the token. Throughout the task
participants were required not to move their fingers and to keep their wrist locked so that
movement was restricted to the proximal muscles of the upper arm and shoulder.
Performance was monitored to ensure that participants complied with these instructions. In
practice, participants readily adapted to this constraint. Trials began with a 5 second
practice followed by a 20 second test from which the total number of tokens correctly place
in the color matching target zones was recorded.
The far edge of the boards used for the hand and arm tasks was placed within 500
mm of the participants so that both tasks were performed in near (peripersonal) space. The
sequence in which the tasks were completed was counterbalanced across participants with
half of the women and half of the men doing the hand task first followed by the arm task
while the other half completed the tasks in the reverse order.
Results and Discussion
The number of tokens correctly moved to the target areas were subjected to a two-
way ANOVA with Muscle as a within participants and Sex as a between participants
factor. Neither of the main effects were significant (Muscle: F1,46= 2.88, p=0.096; Sex:
F1,46= 3.51, p=0.067) but the predicted Muscle*Sex interaction (Fig. 1) was significant
(F1,46= 49.70, p<0.001) and this was explored further with appropriate t-tests.
Within-sex comparisons reveal that women scored higher with the hand than with
the arm (t23= 5.29, p<0.001, one-tailed, effect size d=0.98) while men scored higher with
the arm than with the hand (t23= 4.77, p<0.001, one-tailed, effect size d=0.89). In addition,
between-sex comparisons show that women scored higher than men with the hand (t46=
4.60, p<0.001, two-tailed, effect size d=1.11) while men scored higher than women with
the arm (t46= 2.39, p=0.021, two-tailed, effect size d=0.66). Thus the predicted sex
differences were confirmed with large effect sizes.
Apart from Sanders and Walsh (2007), other reports of sex differences in the
performance of motor tasks (for reviews see Halpern, 2000; Kimura, 1999) have used tasks
such as the male-favoring targeted throwing (Watson and Kimura, 1991) and female-
favoring tasks such as fine motor movement (Nicholson and Kimura, 1996; Sanders and
Kadam, 2001). Such tasks fail to pinpoint the basis of the sex difference because of
confounds. In addition to different task demands, those tasks confound the subdivisions of
the motor and spatial components. Targeted throwing uses the proximal muscles of the arm
and is directed into far (extrapersonal) space while fine motor movement uses the distal
muscles of the hand and is performed in near (peripersonal) space.



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Sex differences in hand and arm use in near and far space
Figure 1. Study 1: Means (+/- SEM) of the number of tokens correctly placed by women (pink/light grey
lines and squares) and men (blue/dark grey lines and diamonds) using the distal muscles of the hand or the
proximal muscles of the arm when the tasks were performed in near (peripersonal) space.
Near Space
12
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co
8
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6
Nu
4
Ha nd
Arm

In Study 1 participants performed similar tasks, the sorting of colored tokens by
moving them from a mixed pile to appropriately colored target zones, either by using the
muscles of the wrist and fingers (hand condition) or by using the muscles of the upper arm
and shoulder (arm condition) with both tasks performed in near space. Hence, the previous
confounds of task demands and spatial location were controlled. The outcome from the
within-sex comparisons are consistent with our motor control prediction based on the
hunter-gatherer hypothesis and they confirm the findings reported by Sanders and Walsh
(2007); women perform a motor task better when using their hand rather than their arm
while men perform better with their arm rather than with their hand. Between-sex
comparisons are not strictly relevant to our hypothesis because their nature will always
depend on the relative difficulty that a task presents to women and men. If the task used is
easy for women but difficult for men then it may happen that women out-perform men
using both the hand and the arm. Therefore we made no predictions regarding these
comparisons. As it happened, women scored only slightly higher than men and the
difference was not significant. As a consequence, both of the between-sex comparisons
were significant: women scored higher than men with the hand while men scored higher
than women with the arm.
Given that common confounds were controlled these findings point to a sex
difference in the motor control of the distal and proximal muscles and are consistent with
the view that these differences have been inherited from our evolutionary ancestors who
acquired them as a result of selection for hunting and gathering. While early childhood
experiences may serve to enhance any inherited predisposition it also is possible that such
sex differences are entirely the product of experience. However, explanations that exclude a
biological input are unlikely to be correct (see Halpern, 2000, pp. 253-254; Kimura, 1999,
pp. 57-63). In fact, studies have found that partialing out sports history has little effect on
the size of sex (Watson and Kimura, 1991) and sexual orientation (Hall and Kimura, 1995)
differences in targeted throwing accuracy. Consequently our findings support the view that
such present day sex differences are inherited.
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Sex differences in hand and arm use in near and far space
Study 2
In a second test of predictions from the hunter-gatherer hypothesis, Sanders,
Sinclair and Walsh (2007) demonstrated that women perform tasks better when the visual
information is presented in near rather than far space whereas men show the reverse
pattern. We designed Study 2 as a concurrent investigation of the visual processing and the
motor control predictions. Using a similar token sorting task we attempted to replicate two
findings: [1] that women perform better with the distal muscles of their fingers and wrist
while men do better with the proximal muscles of their upper arm and shoulder and [2] that
women perform better with visual information from near space while men do better with
visual information from far space. Participants were required to move colored tokens from
a neutral starting area to appropriately colored target areas using the distal and proximal
muscles and a short or long stylus that allowed manipulation of the tokens to occur in near
or far space.
Materials and Methods
Participants
The study involved a new sample of 48 right-handed participants, 24 men (mean
age 26.17 +/- 4.56) and 24 women (mean age 27.09 +/- 5.12). None of the participants had
suffered an upper limb injury in the previous six months, all could operationally distinguish
the colors of the tokens and all had normal, or corrected to normal, vision. The study was
approved by the London Metropolitan University Psychology Department Ethics
Committee. All participants gave informed written consent and were aware that they could
withdraw from the study at any time. None withdrew.
Tasks and procedure
Participants were required to move six colored washers from their positions in a
neutral starting array to color-matching squares in a 2x3 target array (Fig. 2). The six colors
used were blue, purple, orange, red, black and yellow. Each of the six locations within the
two arrays had a central peg on which a washer could be suspended. Participants used a
slightly hooked wire stylus to lift and move the washers, one at a time, from their position
in the starting array to their appropriate position in the target array. They performed this
task in four ways characterized by the muscle (hand or arm) used and the spatial location
(near or far) in which the task was performed, i.e. hand/near, hand/far, arm/near and
arm/far.
Hand condition Participants used a slightly hooked wire stylus, held between their
thumb and first finger, to move the washers using the muscles of the fingers and wrist only.
Throughout the task their arm was strapped to a board in order to prevent movement of the
upper arm and shoulder muscles (Fig. 2a). The restraint was used because without it
participants were inclined to move their arm as well as their hand.
Arm condition Participants used a slightly hooked wire stylus, held between their
thumb and first finger, to move the washers using the muscles of the upper arm and
shoulder without moving their fingers or wrist (Fig. 2b). No restraint was used because,
unlike the requirement of the hand condition, the arm condition appeared to be a natural
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Sex differences in hand and arm use in near and far space
action and participants were readily able to move their arm while keeping the wrist and
fingers locked.

Figure 2. Schematic drawings, not to scale, showing the experimental set-up for two of the four condition
used in Study 2. Figure 2a shows the far/hand condition in which the array is 1000 mm from the eyes of the
participant whose arm is restrained. Figure 2b shows the near/arm condition in which the stimulus array is
300 mm from the eyes of the participant whose arm is unrestrained. See text for further explanation of the
procedures.







Figure 2a

Figure 2b

Near condition The stylus for this condition was 203 mm long, the circular
washers had an external diameter of 12 mm and a central circular hole with a diameter of 6
mm, and the vertical arrays were placed in near (peripersonal) space at a distance of 300
mm from the participant. Each of the cells in the arrays measured 38 x 38 mm.
Far condition The stylus for this condition was 734 mm long, the circular washers
had an external diameter of 38 mm and a central circular hole with a diameter of 9 mm, and
each of the cells in the arrays measured 85 x 85 mm. The vertical arrays were placed in far
(extrapersonal) space at a distance of 1000 mm from the participant. The suitability of this
distance as an operational definition of far space is addressed in the General Discussion.
The four tasks, arm/near, arm/far, hand/near and hand/far, were completed in a
counter-balanced order. Each task began with a 10 second practice during which the
experimenter ensured that the participant performed the task correctly using the appropriate
set of muscles. Following this practice the participant had one attempt to move all of the
washers from the neutral starting array to the color-matching positions in the target array. If
a washer fell it was immediately retrieved by the experimenter and placed back in its
original position on the starting array. The total time taken to complete the task was
recorded in seconds.

Results and Discussion

The task completion time data were submitted to a 3-way ANOVA with sex
(male/female) as a between participants factor plus muscle (hand/arm) and space (near/far)
as within participants factors. On the basis of the hunter-gatherer hypothesis and results
from previous studies we had predicted that women should perform better with the hand
than arm and better in near than far space while men should be better with arm than hand
and better in far than near space. Thus we expected a Muscle*Space*Sex interaction and
were surprised to find that it was not significant (F1,46=2.11, p=0.15). However, following
further investigation of our findings, the absence of that 3-way interaction has presented us
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Sex differences in hand and arm use in near and far space
with an interesting serendipitous finding concerning the impact of tool use on the
perceptual division between near and far space.
Of the two predicted 2-way interactions, Muscle*Sex was significant (F1,46=15.61,
p=0.001) but Space*Sex (F1,46=0.34, p=0.711) was not. We shall explore these central
findings below. The remaining 2-way interaction, Space*Muscle, approached significance
(F1,46=4.03, p=0.051) because the difference between near and far space performance was
greater for the hand than for the arm. Of the main effects, sex was not significant
(F1,46=0.83, p=0.366), but space (F1,46=424.40, p<0.001), and muscle (F1,46=15.82,
p<0.001) were significant. Overall performance was better in near than far space, and better
with the arm than with the hand. However, these space and muscle differences merely
reflect the relative difficulty of the near/far and hand/arm tasks.

Figure 3. Study 2: Means (+/- SEM) of the time taken by participants to place the tokens correctly using the
distal muscles of the hand and the proximal muscles of the arm when the task was performed in near
(peripersonal) and far (extrapersonal) space. (Women – pink /lighter grey lines and squares; Men – blue
/darker grey lines and diamonds.)

Near Space
Far Space
7 0
7 0
6 0
6 0
secs) 50
5 0
(
4 0
aken
4 0
t
e
i
m 3 0
T
3 0
2 0
2 0
Hand
Arm
Hand
Ar m


Hand “v” arm
As seen in Figure 3, the significant 2-way interaction between Muscle and Sex,
noted above, arose because men completed the task significantly faster with their arm than
with their hand and this was true for both near (t23 = 5.76, p<0.001, one-tailed, effect size
d=1.18) and far (t23 = 4.47, p<0.001, one-tailed, effect size d=0.91) space. For women the
differences between performance with the hand and arm were not significant (near space:
t23 = 0.16, p = 0.44, one-tailed, effect size d=0.03; far space: t23 = 0.12, p = 0.45, effect size
d=0.02). Thus the findings confirmed the predicted interaction and our specific predictions
for men but not for women. Clearly, the relative ease of the arm task compared with the
hand task had an impact here, increasing the hand/arm difference for men and reducing it
for women. Consequently, it is important that the Muscle*Sex interaction was significant
and in the predicted direction. The outcome of between-sex comparisons depends on the
relative difficulty of the tasks for men and women. In this case, the overall performances
were similar. Women were faster than men with the hand in both near space (t46= 3.03,
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Sex differences in hand and arm use in near and far space
p=0.004, two-tailed, effect size d=0.81) and in far space (t46= 2.23, p=0.031, two-tailed,
effect size d=0.62). Men were faster than women with the arm in near space (t46= 2.85,
p=0.007, two-tailed, effect size d=0.77) but not in far space (t46= 1.90, p=0.063, two-tailed,
effect size d=0.53) space. As shown, these sex differences displayed moderate to large
effect sizes.

Near “v” far
Participants in both hand and arm conditions took longer to complete the tasks in far than in
near space (Fig. 3) but there is no evidence to support our prediction that women would
perform better in near than in far space and men better in far than in near space. Indeed, the
relative pattern of male and female performance in near and far space is remarkably
consistent. Combining the hand and arm data quantifies this consistency and reveals almost
identical female to male ratios for near and far space (Table 1).

Table 1. Study 2: Mean (SEM) of the combined hand and arm scores (i.e. time to complete the token
re-location task in seconds) recorded for women and men when operating in near (peripersonal) and
far (extrapersonal) space.

Near Space
Far Space



32.835
51.44
Women (n = 24)
(1.93)
(3.48)




35.46
54.75
Men (n = 24)
(2.03)
(3.49)
Female/Male Performance Ratio
0.93
0.94

In Study 2 we attempted to test the motor control and visual processing predictions
from the hunter-gatherer hypothesis concurrently. As in Study 1, for motor control we
obtained the predicted Muscle*Sex interaction in both near and far space (Fig. 3) but, this
time, although men performed significantly better with their arm than their hand, women
did not show a significant difference between their hand and arm performances. However,
the performance of women with hand and arm relative to men was in the predicted
direction. On the contrary, we found no evidence to support the visual processing
prediction from the hunter-gatherer hypothesis. In fact, the relative performance of women
and men was the same in far space as in near space (Fig. 3 and Table 1). This failure to find
the predicted interaction between space and sex is explored further in the general
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Sex differences in hand and arm use in near and far space
discussion together with a consideration of whether or not the definition of near and far
space as 300 mm and 1000 mm is sufficient to generate known near/far effects.
General Discussion
Studies 1 and 2 are extensions of two recent investigations that tested predictions
from the hunter-gatherer hypothesis (Sanders and Walsh, 2007; Sanders, Sinclair and
Walsh, 2007). The authors argued that selection for hunting skills would favor, among
other things, two abilities: [1] visual processing of far space that would be essential for
detecting and aiming at prey and [2] motor control employing the upper arm and shoulder
muscles that would be used to direct missiles at the prey. Conversely, selection for
gathering skills would favor [1] visual processing of near space that would be essential for
detecting suitable items and for guiding appropriate reaching and grasping movements and
[2] motor control employing the wrist and finger muscles that would be used to collect the
chosen items. These predictions would apply to tasks that directly mirrored aspects of
hunting and gathering behavior such as targeted throwing and manual dexterity tasks.
However, as the authors noted, tasks of that kind usually confound task demands, the
muscles used and the spatial location in which the task is performed. Hence, tasks chosen
to test the hunter-gatherer predictions should avoided such confounds. Suitable tasks may
be identified by deconstructing the activities of hunting and gathering to find abilities that
should be differentially present in men as hunters and in women as gatherers. Whether or
not the chosen tasks mirror an aspect of hunting or gathering behavior is then not
important. As indicated above, this process led to the selection of [1] visual processing of
near and far space and [2] motor control by hand and arm. As predicted women were better
with near and men with far space (Sanders, Sinclair and Walsh, 2007) and women were
better with the hand and men with the arm (Sanders and Walsh, 2007). The present Studies
1 and 2 attempted to confirm those findings using novel tasks and, in Study 2, to test the
motor control and visual processing predictions concurrently.

The findings and potential limitations of Study 1: Hand “v” Arm
In Study 1, we used a novel token re-location task performed in near space and
successfully replicated the findings reported by Sanders and Walsh (2007). As seen in
Figure 1, women performed better when movement was restricted to the distal muscles of
the wrist and fingers (hand task) while men were better when using the proximal muscles
of the upper arm and shoulder (arm task). It is true that our hand and arm tasks differed in
one aspect in addition to the muscle used; in the hand task the tokens were moved with
forceps whereas in the arm task they were moved with a hooked stylus. However, it seems
unlikely that the difference between the tools would of itself have generated the sex
difference. While it has been suggested that the widely reported, female-favoring, sex
difference in fine motor skill is the result of the different average finger size in men and
women (Peters and Campagnaro, 1996) it seems unlikely that it could be a causal factor
here. Our forceps and stylus were of a size and construction that could be equally well
operated by both large and small fingers. In fact, in the light of our findings it is interesting
to look more closely at the evidence presented by Peters and Campagnaro.
In what became an influential paper, Peters and Campagnaro (1996) challenged the
general claim that women have greater manual dexterity than men. They pointed out that
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