Testing Predictions from the Hunter-Gatherer Hypothesis - 1: Sex Differences in the Motor Control of Hand and Arm

Evolutionary Psychology
www.epjournal.net – 2007. 5(3): 653-665
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

Original Article
Testing Predictions from the Hunter-Gatherer Hypothesis – 1: Sex Differences
in the Motor Control of Hand and Arm
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)
Tom Walsh, Department of Psychology, London Metropolitan University.
Abstract: Here, in the first of two reports that test predictions from the hunter-gatherer
hypothesis, we focus on sex differences in motor control. Published evidence confounds the
cognitive demands, the muscles used and the spatial location in which tasks are performed.
To address these issues our participants used hand or arm movements to track a moving
target within near space. Study 1 identified an optimal level of task difficulty for the
differentiation of male and female performance and showed that women tracked better
using their hands and men using their arms. Employing the optimal level of task difficulty,
Study 2 replicated the findings of Study 1 and, for men, demonstrated a significant
correlation between arm tracking and performance on the nonmotor sex-dimorphic Mental
Rotations task. This correlation suggests that the same or related events are responsible for
the development of sex differences in motor and cognitive systems. The distal (hand)
muscles are controlled by the primary motor cortex via two dorsolateral corticospinal tracts
whereas the proximal (arm) muscles are controlled via two ventromedial corticospinal
tracts. Our findings point to possible sex differences in these two neural pathways and they
are compatible with an evolutionary origin as predicted by the hunter-gatherer hypothesis.
Keywords: hunter-gatherer hypothesis; sex differences; motor control; hand and arm;
neural bases; near space.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
Introduction
The hunter-gatherer hypothesis was proposed by Silverman and Eals (1992) as an
evolutionary explanation for sex differences in spatial ability. The core idea is that sex
differences in task performance have arisen from a process of natural selection that favored
hunting-related skills in men and gathering–related skills in women. While the hypothesis

---

Sex differences in motor control
cannot be tested directly, we can test its predictions and we have adopted this approach in a
series of studies aimed to generate predictions that are novel and testable. In this, the first
of two reports, we focus on predicted sex differences in the motor control of the hand and
arm. In the second report we have addressed sex differences in visual processing (Sanders,
Sinclair and Walsh, 2007). In both cases we were able to demonstrate the predicted sex
differences and to identify from the literature potential neural bases for those differences.
We began with the premise that selection for hunting skills would favor men with
good processing of visual input from far (extrapersonal) space for detecting suitable prey
and accurately aiming a missile, together with good proximal arm muscle performance for
throwing the missile at the prey. On the contrary, selection for gathering skills would favor
women with good processing of visual input from near (peripersonal) space for the
detection of appropriate items together with good distal hand muscle performance for
grasping those items. Thus for motor performance we would expect an interaction between
Muscle and Sex with women performing better with their hand and men with their arm. Of
the motor tasks that might test these hand/arm predictions, two with good ecological
validity that show sex differences are targeted throwing at which men excel (Watson and
Kimura, 1991) and fine motor movement at which women excel (Nickolson and Kimura,
1996; Sanders and Kadam, 2001). Both tasks are dependent on aspects of hand-eye co-
ordination but they fail to pinpoint the basis of the sex difference because of confounds. In
addition to the task demands, published studies of sex differences in motor performance
(see Kimura, 1999 for a review) confound the subdivisions of the motor and visuospatial
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.
Here we report two studies that focused on fine, distal (hand) and gross, proximal
(arm) muscle use. To control for the confounds identified above we designed a
computerized tracking task that participants performed using either distal hand muscle
movements or proximal arm muscle movements. To avoid confounds between
extrapersonal/ peripersonal space, performance was restricted to near (peripersonal) space.
We predicted a Muscle*Sex interaction arising from women performing better with the
hand and men with the arm.

STUDY 1

Studies aiming to demonstrate and identify the bases of sex differences are beset
with task selection problems. Tasks must be appropriate not only in terms of the ability
they target but also in their level of difficulty because sex differences may be readily
masked by using tasks that are too easy or too difficult to differentiate between the
performances of men and women (Sanders, Sjodin and de Chastelaine, 2002). Therefore, in
Study 1, we used four levels of difficulty, and tested both preferred and non-preferred
limbs, in order to maximize the chance of finding support for our prediction that there will
be an interaction between Muscle and Sex.
Evolutionary Psychology – ISSN 1474-7049 – Volume 5(3). 2007 -654-

---

Sex differences in motor control
Materials and Methods
Participants
The study used 128 right-handed participants, 64 men (mean age 23.59 ± 2.50
years) and 64 women (mean age 24.09 ± 2.51 years). No participant had sustained an injury
to his or her upper limbs in the previous twelve months. 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.

Task
Participants performed a computerized tracking task in which they attempted to
keep a cursor in contact with a moving target (diameter 5 mm). Task difficulty was
manipulated by using combinations of two circular target paths (simple or complex) and
two target speeds (slow or fast). The simple path was a circle with a radius of 30 mm. The
complex path was an undulating circle with six peaks and troughs that deviated 5 mm
without and within an imaginary circle with a radius of 30 mm. At the slow speed the target
completed one revolution in 12 seconds. At the fast speed the target completed one
revolution in 6 seconds. Each participant was assigned to one of four levels of difficulty:
Level 1 used the slow speed and simple trajectory; Level 2 used the slow speed and
complex trajectory; Level 3 used the fast speed and simple trajectory; Level 4 used the fast
speed and complex trajectory.
The tracking task was performed under two conditions, Hand and Arm. For each
condition the computer screen was placed at a distance of 600 mm, i.e. in peripersonal
(near) space, in order to avoid near/ far space confounds. In the Hand condition participants
operated a joystick with their distal, hand and wrist, muscles while in the Arm condition
they used the proximal, upper arm and shoulder, muscles. For the Hand condition the
forearm of the participants was retrained by strapping it to the table to prevent arm
movements and they were instructed to track the moving target by manipulating a short (70
mm) joystick with wrist and finger movements. The maximal movement of the top of the
short joystick was 42 mm in any direction from its central position. For the Arm condition
the same joystick was moved from the table to the floor and its length extended to 1200
mm by attaching a rod. Participants were instructed to hold a 49 mm diameter ball at the
top of the rod in the palm of their hand, to keep their wrist locked and to use their upper
arm and shoulder muscles to perform the tracking task. The size of the ball and length of
the rod encouraged, and the instructions ensured, that finger and wrist movements were
effectively eliminated and that the extended joystick was manipulated by the proximal
muscles of the upper arm and shoulder only. The maximal movement of the top of the long
joystick was 600 mm in any direction from its central position. Maximal movement of both
the short and long joystick produced the same 37.5 mm on screen movement of the cursor.




Evolutionary Psychology – ISSN 1474-7049 – Volume 5(3). 2007 -655-

---

Sex differences in motor control
Procedure
We used a mixed design with independent groups of 16 men and 16 women
randomly allotted to each of the four levels of task difficulty (Level 1 slow/ simple, Level 2
slow/ complex, Level 3 fast/ simple or Level 4 fast/ complex) and repeated measures on the
Hand/ Arm and Non-preferred/ Preferred Limb factors. Participants were tested first using
their non-preferred left limb and then their preferred right limb with the order of the Hand
and Arm conditions counterbalanced. Both limbs were tested in order to manipulate task
difficulty and to include the possibility of replicating any finding. Each test began with a 5
second practice followed by a 30 second test. Performance was recorded as the percentage
of the 30 second period that the participant succeeded in keeping the cursor in contact with
the target. 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.
Results and discussion
Figure 1 shows that the patterns of performance obtain with the preferred right and
non-preferred left limbs were remarkably similar although the time on target was higher
with the preferred limb (grand mean 17.32% right, 12.54% left). Men tended to perform
better than women when the target followed a simple circular trajectory (Level 1 and 3) but
the predicted Muscle*Sex interaction appeared with the complex undulating trajectory
(Levels 2 and 4). Data from the preferred and non-preferred limbs were analyzed
separately.

Preferred right limb
The three-way interaction between the Sex, Muscle and Level was significant (F3,120
= 8.16, p<0.001) so separate two-way ANOVAs were conducted for Levels 1 to 4. At
Level 1 the Muscle*Sex interaction was significant (F1,30 = 4.53, p = 0.042) because
women (t30 = 7.32, p = 0.001, two-tailed) but not men (t30 = 2.02, p = 0.061, two-tailed)
scored significantly lower with the arm. The predicted Muscle*Sex interaction appeared at
Level 2 (F1,30 = 84.23, p<0.001). In the Hand condition, women achieved time on target
scores that were higher than those of men (t30 = 4.21, p<0.001, two-tailed) and higher than
their own scores in the Arm condition (t15 = 10.07, p<0.001, one-tailed). Conversely, in the
Arm condition, men achieved time on target scores that were higher than those of women
(t30 = 8.25, p<0.001, two-tailed) and higher than their own scores in the Hand condition (t15
= 4.49, p<0.001, one-tailed). At Level 3 the Muscle*Sex interaction was not significant but
the predicted interaction appeared again at Level 4 (F1,30 = 16.71, p<0.001) because women
scored significantly lower with the arm than with the hand (t15 = 4.28, p = 0.001, one-
tailed) and significantly lower with the arm than men (t30 = 4.65, p<0.001, two-tailed).
However, for men, the hand and arm scores did not differ significantly in this condition. A
similar picture emerged from performance with the non-preferred left limb.



Evolutionary Psychology – ISSN 1474-7049 – Volume 5(3). 2007 -656-

---

Sex differences in motor control
Figure 1. Study 1. Performance measured as percentage time on target (Mean ± SEM)
using distal muscles (Hand condition) and proximal muscles (Arm condition). Eight
independent groups (n = 16) completed the task at one of four levels of difficulty: Level 1 –
slow simple; Level 2 – slow complex; Level 3 – fast simple and Level 4 – fast complex.
Upper row – preferred right limb; lower row – non-preferred left limb; pink/ lighter grey
lines and squares – women; blue/ darker grey lines and diamonds – men.
Level 1
Le v e l 2
Le v e l 3
Le v e l 4
45
45
45
45
40
t
40
40
40
e
35
35
35
35
r
g
30
30
30
a
30
t
25
25
25
25
n
20
20
20

o
20
15
15
15
15
i
me 10
10
10
10
5
5
5
% t
5
0
0

0

0


45
45
45
45
40
40
40
40
et 35
35
35
35
g 30
30
30
30
tar 25
25
25
25
n 20

o
20
20
20
e 15
15
15
15
m 10
10
ti
10
10
5
%
5
5
5
0
0
0
0
Hand Arm
Hand
Arm
Hand Arm
Hand Arm



Non-preferred left limb
The three-way interaction between the Sex, Muscle and Level was a significant
(F3,120 = 11.33, p<0.001) so separate two-way ANOVAs were conducted for Levels 1 to 4.
The two-way interaction was not significant at Levels 1 and 3 but at Level 2 the predicted
Muscle*Sex interaction was again significant (F1,30 = 86.76, p<0.001). In the Hand
condition, women achieved time on target scores that were higher than those of men (t30 =
3.36, p = 0.002, two-tailed) and higher than their own scores in the Arm condition (t15 =
5.01, p<0.001, one-tailed). Conversely, in the Arm condition, men achieved time on target
scores that were higher than those of women (t30 = 7.65, p<0.001, two-tailed) and higher
than their own scores in the Hand condition (t15 = 9.57, p<0.001, one-tailed). Finally, the
predicted interaction appeared again at Level 4 (F1,30 = 8.83, p = 0.006) because women
scored significantly lower with the arm than with the hand (t15 = 4.17, p = 0.001, one-
tailed) and significantly lower with the arm than men (t30 = 2.50, p = 0.018, two-tailed).
However, for men, the hand and arm scores did not differ significantly in this condition.
Hence, with the previous confounding variables of task demands and spatial
location controlled, Study 1 confirmed our prediction from the hunter/gatherer hypothesis
Evolutionary Psychology – ISSN 1474-7049 – Volume 5(3). 2007 -657-

---

Sex differences in motor control
that women would perform better with their hand and men with their arm. It is notable that
the pattern of performance obtained with the preferred right limb was replicated with the
non-preferred left limb (Fig. 1). The data also show the importance of level of difficulty
with the complex undulating circular pathway (Levels 2 and 4) revealing the predicted
Muscle*Sex interaction and only Level 2 (slow/complex) optimally differentiating the
opposite patterns of male and female performance. Consequently, the Level 2 tracking task
was selected for use in Study 2.

STUDY 2
Here we aimed both to replicate the interaction between Sex and Muscle that we
found in Study 1 and also to demonstrate correlations between performance on the tracking
task and performance on five other sex-dimorphic motor and nonmotor cognitive tasks. We
predicted that tracking with the hand would correlate positively with female favoring tasks
and negatively with male favoring tasks whereas tracking with the arm would correlate
negatively with female favoring tasks and positively with male favoring tasks.

Materials and Methods
Participants
The study used 100 right-handed participants, 50 men (mean age 22.74 ± 2.68
years) and 50 women (mean age 21.48 ± 2.35 years). The participants all had English as
their first language and none had sustained an injury to his or her upper limbs in the
previous 12 months. 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
We used the Level 2 (slow/complex) tracking task from Study 1 and five
established sex-dimorphic motor and nonmotor cognitive tasks. The three motor tasks were
the male-favoring Targeted Throwing (Watson and Kimura, 1991) which emphasizes gross,
proximal muscle activity and the female-favoring Purdue Pegboard Single Peg Condition
with the left and right hands (Tiffin, 1987), which emphasizes fine, distal muscle activity.
We used a modified version of the throwing task in which participants directed six Velcro
covered table tennis balls one at a time, using an underarm throw, to towards the centre of a
1450 mm square cloth target placed 1500 mm away and we recorded performance as mean
radial error in cm. We used the standard Purdue pegboard single peg condition in which
participants fit single pegs vertically into a column of holes on a horizontal board and
performance is recorded separately for the left and right hands as the total number of pegs
correctly placed in 30 seconds. The two cognitive tasks were the female-favoring
Controlled Associations (Ekstrom, French, Harman, and Dermen, 1976) and the male-
favoring Mental Rotations (Vandenberg and Kuse, 1978). We used a reduced version of the
Controlled Associations task in which participants had 4 minutes to write down as many
words as they could think of with the same or similar meaning to the target word “weak”
Evolutionary Psychology – ISSN 1474-7049 – Volume 5(3). 2007 -658-

---

Sex differences in motor control
and performance was recorded as the total number of correct words. In the Vandenberg and
Kuse mental rotations test participants are required to select, from four figures, the two that
match a given example but are rotated. Six minutes were allowed for the completion of the
20 items and performance was the recommended measure, correct responses adjusted for
guessing.

Procedure
We used a mixed design with independent groups of 50 men and 50 women and
repeated measures on the tasks. The tracking task (Level 2 slow/complex) was
administered as in Study 1. On completion of the tracking task the five sex-dimorphic
motor and nonmotor cognitive tasks were completed in the following order: Purdue
Pegboard Single Peg Condition first with the left non-preferred and then with the right
preferred hand; the Controlled Associations; Mental Rotations; and Targeted Throwing.
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 and correlations.

Results and discussion
First, as seen in Figure 2, Study 2 confirmed the findings of Study 1. Once again the
predicted Muscle*Sex interaction was significant for both the preferred (F1,98 = 174.27,
p<0.001) and the non-preferred (F1,98 = 28.85, p<0.001) limbs. Preferred limb performance
showed that women scored higher than men in the Hand condition (t98 = 8.39, p<0.001,
two-tailed) and higher than their own scores in the Arm condition (t49 = 10.17, p<0.001,
one-tailed). Conversely, men scored higher than women in the Arm condition (t98 = 10.19,
p<0.001, two-tailed) and higher than their own scores in the Hand condition (t49 = 8.49,
p<0.001, one-tailed). A similar picture emerged from performance with the non-preferred
limb.















Evolutionary Psychology – ISSN 1474-7049 – Volume 5(3). 2007 -659-

---

Sex differences in motor control

Figure 2. Study 2. Percentage time on target (Mean ± SEM) recorded by 50 men (blue/
darker grey lines and diamonds) and 50 women (pink/ lighter grey lines and squares) using
distal muscles with a short joystick (Hand condition) and proximal muscles with an
extended joystick (Arm condition). All completed the Level 2 (slow complex circle)
tracking task from Study 1. Left panel – preferred right limb; right panel – non-preferred
left limb.

18
18
16
16
14
14
t
e
r
g 12
12

ta
n 10
o
10
e
m
8
8
ti
%
6
6
4
4
2
2
Hand
Arm
Hand
Ar m


Second, as illustrated in Figure 3, the widely reported sex differences for each of the
five sex-dimorphic tasks were confirmed at p<0.001 and with substantial effect sizes (d).
Women scored higher than men on each of the three female-favoring tasks: Controlled
Associations (t98 = 8.54, p<0.001); Purdue Pegboard Single Peg Condition, non-preferred
left hand (t98 = 4.00, p<0.001) and preferred right hand (t98 = 9.41, p <0.001). Conversely,
men scored higher than women on the two male-favoring tasks: Mental Rotations Task (t98
= 10.94, p<0.001) and Targeted Throwing (t98 = 7.07, p<0.001).













Evolutionary Psychology – ISSN 1474-7049 – Volume 5(3). 2007 -660-

---

Sex differences in motor control

Figure 3. Scores (Mean +/- SEM) from men (blue/ lighter grey) and women (pink/ darker
grey) on five sex-dimorphic tasks. Female-favoring tasks: Controlled Associations (CA)
number of words generated; Purdue Pegboard Right Hand (PPB-R) and Left Hand (PPB-L)
number of correctly peg fitted. Male-favoring tasks: Mental Rotation Task (MRT) number
of items correct adjusted for guessing; Target Throwing (TT) an accuracy score calculated
by subtracting the male and female mean error scores in cm from their sum.

8
25
20
15
40
7
35
20
6
15
30
10
5
15
25
r
e
o
4
10
20
Sc
10
3
15
5
2
5
10
5
1
5
0
0
0
0
0


CA

PPB-R

PPB-L

MRT

TT
Female-favoring
Male-favoring
Effect Sizes
d = 1.30
d = 1.37
d = 0.75
d = 1.47
d = 1.16


Finally, using the preferred limb data from the total sample we looked at
correlations between performance on the tracking task and performance on the five sex-
dimorphic motor and nonmotor cognitive tasks. Using the whole group data we found, as
predicted, that tracking scores from the Hand condition correlated positively with female-
favoring task performance and negatively with male-favoring task performance while the
reverse was true for tracking scores from the Arm condition, all with Pearson correlations
at p<0.022 or beyond (Table 1). However, given the marked sex differences that were
recorded for these tasks (Figures 2 and 3) significant correlations with the combine male
and female data are to be expected as for each task female scores will tend to be high and
male scores low or vice versa. Consequently, real interest centers on the within-sex
correlations of which only one was significant. Men showed a positive correlation between
their scores for tracking in the Arm condition with the preferred right limb and the Mental
Rotation Task (r = 0.287, p = 0.043). It is notable that this correlation is in the predicted
direction and that it occurred between tracking and a nonmotor cognitive task suggesting
the same factor(s) predispose the development of sex dimorphic patterns of performance
for both types of task, at least in males. Why tracking with the arm and mental rotation
should provide the only significant within-sex correlation is not clear especially as the other
Evolutionary Psychology – ISSN 1474-7049 – Volume 5(3). 2007 -661-

---

Sex differences in motor control
within-sex correlations were all small (the majority were less than r = 0.1 and p>0.500)
although the effect size of the sex difference for mental rotation was 7% to 21% greater
than for any of the other tasks.


Table 1: Pearson correlations Between Tracking and Sex-dimorphic Tasks Using
Performance with the Preferred Right Limb (probabilities are two-tailed)
Female-favoring
Male-favoring
Motor Non-Motor
Motor Non-Motor

Purdue
Controlled
Target
Mental
Pegboard
Association
Throwing
Rotations
Hand
r = +0.451 r = +0.491 r = -0.228 r = -0.540
tracking
p < 0.001
p < 0.001
p = 0.022
p < 0.001
Arm
r = -0.549 r = -0.416 r = +0.392 r = +0.404
tracking
p < 0.001
p < 0.001
p < 0.001
p < 0.001

General discussion
Behavioral sex differences
As noted in the Introduction, published studies of sex differences in motor
performance confound the motor and visuospatial subdivisions as well as task demands.
The present investigation was designed to avoid these confounds. Male and female
performance with hand and arm were compared using the same tracking task performed in
near space so that both task demands and spatial location remained constant. The data from
Study 1 confirmed our prediction from the hunter-gatherer hypothesis. With the previous
confounds avoided there was an interaction between Muscle and Sex that arose because
women performed better with their hands and men better with their arms (Fig. 1). We used
four levels of difficulty in Study 1 in an attempt to avoid tasks that were too easy or too
difficult to differentiate the performances of men and women. As seen in Figure 1, the
interaction that we predicted appeared only with an appropriate level of tracking difficulty
(Levels 2 and 4, slow and fast complex) in which frequent changes of direction were
required. However, only Level 2 fully differentiated male and female performance.
Consequently in Study 2 we used the same level of difficulty and replicated the finding of
opposite patterns of performance in women and men with both the preferred right and non-
preferred left limbs (Fig. 2).

Neural sex differences
The reality of such sex differences as those we have demonstrated for the control of
distal hand and proximal arm muscles would be reinforced by the presence of separate
underlying neural mechanisms for each of the behaviors. Separate mechanisms would
Evolutionary Psychology – ISSN 1474-7049 – Volume 5(3). 2007 -662-

---

Document Outline

• ??
• ??
• ??
• ??
  • ??
    • ??
    • ??
    • ??
    • ??
    • ??
    • ??
    • ??
    • ??
    • ??
    • ??
    • ??
    • ??
    • ??
    • ??
• ??

---