Task Selection is Critical for the Demonstration of Reciprocal Patterns of Sex Differences in Hand/Arm Motor Control and Near/Far Visual Processing

Evolutionary Psychology
www.epjournal.net – 2008. 6(2): 342-364
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Original Article

Task Selection is Critical for the Demonstration of Reciprocal Patterns of Sex
Differences in Hand/Arm Motor Control and Near/Far Visual Processing
Geoff Sanders, Department of Psychology, London Metropolitan University, Calcutta House,
Old Castle Street, London, E1 7NT UK. Email address: g.sanders@londonmet.ac.uk.

Anya Madden, Department of Psychology, London Metropolitan University, London, UK.

Gemma Thorpe, Department of Psychology, London Metropolitan University, London, UK.

Abstract: Women have been reported to perform better with hand rather than arm
movements (Sanders and Walsh, 2007) and with visual stimuli in near rather than far space
(Sanders, Sinclair and Walsh, 2007). Men performed better with the arm and in far space.
These reciprocal patterns of sex differences appear as Muscle*Sex and Space*Sex
interactions. We investigated these claims using target cancellation tasks in which task
difficulty was manipulated by varying target size or the number of distracters. In Study 1
we did not find the Muscle*Sex or the Space*Sex interaction. We argue that ballistic
movement was too simple to reveal the Muscle*Sex interaction. However, a trend for the
Space*Sex interaction suggested task difficulty was set too high. Study 2 introduced easier
levels of difficulty and the overall Space*Sex interaction narrowly failed to reach
significance (p = 0.051). In Study 3 the Space*Sex interaction was significant (p = 0.001).
A review of the present, and four previously published, studies indicates that task selection
is critical if the Space*Sex interaction and its associated reciprocal within-sex differences
are to be demonstrated without the obscuring effects of Space and Difficulty. These sex
differences are compatible with predictions from the hunter-gatherer hypothesis.
Implications for two-visual-system-models are considered.

Keywords: Sex differences, hand/arm motor control, near/far visual processing, tool use,
hunter-gatherer hypothesis, two visual systems.

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Visual processing of near and far space
Introduction
In a recent series of studies (Sanders and Perez, 2007; Sanders, Sinclair and Walsh,
2007; Sanders and Walsh, 2007) we focused on a possible evolutionary origin for present
day sex differences in cognitive and motor performance. We started with the Hunter-
Gatherer Hypothesis in which Silverman and Eals (1992) proposed that present day sex
differences in spatial abilities arose from the division of labor associated with our ancestral
hunter-gatherer mode of life. Of course, the division of labor between men and women
extended beyond hunting and gathering per se. Skills required for hunting would also be
needed for defense and attack while the close fine motor movements required for gathering
would be needed for caring. Each of these tasks would have provided additional selection
pressures for the differentiation of sex-dimorphic skills.
Silverman and Eals noted that the spatial tasks at which males are reported to excel,
such as mental rotation, require individuals to orientate themselves with an object and to
maintain that relationship during movement by performing mental transformations, skills
that would aid hunting, especially in unknown territory. Consequently, they agued that
women should have evolved spatial skills, such as object location memory, which would
aid gathering, and they demonstrated a female advantage for such tasks (Silverman and
Eals, 1992; Eals and Silverman, 1994). Further support comes from studies of navigation
(e.g., Galea and Kimura, 1993) in which women tended to use landmarks (effective in the
known areas traversed by gatherers) whereas men used distance and cardinal directions
(effective in the unknown areas traversed by hunters).
Other evidence that was congruent with the Hunter-Gatherer Hypothesis came from
reports of sex differences in manual dexterity, favoring women (Nickolson and Kimura,
1996; Sanders and Kadam, 2001) and in targeted throwing, favoring men (Watson and
Kimura, 1991). Both tasks are ecologically valid but they did not point to a location for the
sex differences because the studies confounded three variables: the visual space in which
the tasks were performed, the muscles used to perform the tasks, and the cognitive demands
of those tasks. To overcome this problem, we derived predictions from the Hunter-Gatherer
Hypothesis by identifying motor skills (Sanders and Walsh, 2007) and visual skills
(Sanders, Sinclair and Walsh, 2007) that would differentially support hunting and gathering
and tested those predictions by devising tasks that avoided the previous confounds.
We argued that evolutionary selection for hunting would favor individuals with,
among other things, the ability to visually locate appropriate prey in far (extrapersonal)
space, then to aim and launch a projectile accurately at that distant target. Conversely,
selection for gathering would favor individuals with, among other things, the ability to
visually locate appropriate items in near (peripersonal) space, then to reach, grasp and
retrieve these items efficiently. From these observations we derived directional within-sex
predictions for men and women that were complementary and reciprocal. Men, as the
predominant hunters, should be better at processing visual information from far than from
near space and better when using the larger proximal muscles of the upper arm and
shoulder than when using the smaller distal muscles of the wrist and fingers. On the other
hand, as the predominant gatherers, women should be better at processing visual
information from near than from far space and better when using the hand than when using
arm.
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Visual processing of near and far space
It is important to note that we are not proposing a new theory as an alternative to the
Hunter-Gatherer Hypothesis. However, we are deriving novel predictions from the
ancestral division of labor between women and men which was highlighted by that
hypothesis. Traditional predictions from the Hunter-Gatherer Hypothesis point to unitary
between-sex differences, i.e. women will be better than men at some tasks and men better
than women at other tasks. In contrast, we are making within-sex predictions: a woman will
perform better with her hand than her arm and better with visual stimuli in near rather than
far space while a man will show the reverse patterns. When testing our predictions, the
traditional between-sex comparisons are not relevant because the relative superior
performance of women or men will depend on the nature of the task chosen, i.e. whether it
is female-favoring, male-favoring or sex-neutral. Our position will be supported by
significant Muscle*Sex and Space*Sex interactions together with significant within-sex
paired comparisons between hand and arm, and between near and far space that are in the
predicted direction within each sex. Our underlying thinking is that a history of
evolutionary selection for hunting in men and gathering in women has led to sex
differences in the neural mechanisms supporting motor control and visual processing. If
this thinking is correct then the reciprocal patterns of hand/arm and near/far performance
that we have predicted for women and men should appear whatever tasks are used.
The definitions of near and far space come from early studies of radial visual
neglect which pointed to a functional division of visual space into near (peripersonal) and
far (extrapersonal) domains in the sagittal plane. Near and far space, 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”. Near space is typically taken to be 500 mm or less, while far is defined as 1000 mm
or more. In our first investigation of visual processing (Sanders, Sinclair and Walsh, 2007)
we conducted three studies to test our prediction from the Hunter-Gatherer Hypothesis that
women would perform better when using visual information from near rather than far
space, whereas men would perform better with information from far rather than near space.
Sanders-Sinclair-Walsh Study 1 used a time estimation task conducted via the
Internet. Participants watched an image of a hovering toy UFO moving across a table top
towards a docking station. The UFO disappeared short of its destination and participants
indicated the moment they estimated it would have docked by pressing their space bar.
Near and far virtual space conditions were created by having the UFO move above the
front or the rear half of the table. Sanders-Sinclair-Walsh Studies 2 and 3 used puzzle
completion tasks conducted in the laboratory in which participants saw their hands and a
simple five-piece “jigsaw” puzzle as an image projected via a webcam onto a near monitor
or a far screen. All three studies generated significant Space*Sex interactions, arising
because, within-sex, women tended to perform better in the near condition and men in the
far condition, but these within-sex differences between near and far performance varied. In
Sanders-Sinclair-Walsh Study 3 women completed the puzzles significantly faster in the
near than in the far condition while men were significantly faster in the far than in the near
condition. However, this was not the case in the other two studies. In Sanders-Sinclair-
Walsh Study 2, which used a more difficult version of the puzzle task, the near/far
performance difference was significant for women but not for men. Conversely, in Sanders-
Sinclair-Walsh Study 1, which used the time estimation task, the near/far performance
difference was significant for men but not for women.
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Visual processing of near and far space
In contrast, a related report (Sanders and Perez, 2007) found no difference at all
between women and men in the patterns of their performances on a visuomotor task
conducted in near and far space. Participants were required to use either a short (near
space) or long (far space) hooked metal stylus to move colored washers from a starting
array to color-coded locations in a target array. The stylus and washers were manipulated
either by movements of the wrist and fingers (hand condition) or by movements of the
upper arm and shoulder (arm condition). These two conditions were included to test an
earlier finding (Sanders and Walsh, 2007) of a Muscle*Sex interaction that confirmed the
hand/arm prediction we had derived from the Hunter-Gatherer Hypothesis: women would
perform better with their hand than with their arm while men would perform better with
their arm than with their hand. Although the hand/arm within-sex differences were
replicated, Sanders and Perez failed to find the near/far within-sex differences; an outcome
that they attributed to the use of a tool to manipulate the washers (see General Discussion
for an account of this argument).
Given these somewhat variable outcomes from our reported studies of potential
within-sex differences in the processing of near and far space, we decided to further
investigate this issue. In selecting an appropriate set of new tasks we paid particular
attention to task difficulty as this appears to be a crucial factor in the revelation of sex
differences (Sanders, Sjodin, and de Chastelaine, 2002). Indeed, the critical nature of this
variable was seen in our study of hand and arm use (Sanders and Walsh, 2007) which used
a computer-based tracking task with four levels of difficulty determined by target speed
(slow and fast) and trajectory (simple or undulating circle). The Muscle*Sex interaction
appeared only in the slow/complex condition, with women tracking significantly better
with their hand than with their arm while men showed the reverse pattern. Consequently,
for the present studies we chose a computer-based target cancellation tasks in which task
difficulty could be varied by manipulating either target size or the number of distracters.
The targets were presented either on a computer monitor (near space condition) or
projected onto a wall-mounted screen (far space condition). Our primary interest was the
reciprocal within-sex differences in the processing of visual information from near and far
space for which we predicted a Space*Sex interaction with women performing better in
near than far space and men better in far than near space. In addition, for the first of our
three studies we also predicted a Muscle*Sex interaction with women performing better
with their hand than arm and men better with their arm than hand.

STUDY 1

Study 1 was designed to investigate the possible occurrence of sex differences in
two abilities: (a) the visual processing of near and far space; (b) the control of hand and
arm muscles. We designed a computer-based target cancellation task. Participants used
their preferred hand or arm to operate either a short (hand condition) or a long (arm
condition) joystick to position a cursor over a target which they then cancelled by pressing
the space bar with their non-preferred hand. Task difficulty was manipulated by varying
target diameter.

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Visual processing of near and far space
Materials and Methods
Participants
Forty-eight participants, 24 women (mean age 26.33, SD 5.70) and 24 men (mean
age 31.54, SD 14.04), were recruited as an opportunity sample from among our University
students and staff. All of the participants were right handed, had normal or corrected to
normal vision and all were naïve to the purpose of the study. None of the participants had
sustained an injury to the right hand or arm within the previous twelve months. The study
was approved by the Departmental 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
We used a mixed design. Sex was a between-participants factor with two
independent groups, women and men; Space, Muscle and Difficulty were within-
participants factors with repeated measures on near/far, hand/arm, and five levels of
difficulty. The computer-based study was run by a custom-written program that recorded
the time taken by participants to move a cursor from a central starting position to locate and
cancel circular targets that appeared randomly elsewhere on the screen. In the near
condition the stimuli were presented on a 430 mm monitor placed 500 mm from the
participant with the centre of the screen at eye-level. On this screen the diameter of the
targets varied across level of difficulty as follows: Level 1, 70 mm; Level 2, 35 mm; Level
3, 22 mm; Level 4, 10 mm; Level 5, 5 mm. For the far condition the stimuli were projected
2.4 times larger onto a wall-mounted screen placed 3200 mm from the participant with the
centre of the screen 600 mm above eye-level so that the display could be seen over the top
of the monitor.
In the hand condition the forearm of the participants was restrained by strapping to
the table and they moved the onscreen cursor by manipulating a short (70 mm) joystick
with wrist and finger movements. 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 move
the cursor. 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 joysticks in any direction from the central position, 42
mm for the short (hand condition) and 600 mm for the long (arm condition), produced the
same 37.5 mm onscreen movement of the cursor and was sufficient to encompass all of the
targets. Participants used the joy stick with their preferred hand or arm to move the
onscreen cursor and they pressed the space bar with their non-preferred hand to start trials
and cancel targets. The sequence of screens that constituted a trial is illustrated in Figure 1
and described below.




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Figure 1. An example of the sequence of screen presentations (not drawn to scale) that were used for the
target cancellation tasks in Studies 1 and 2 in which task difficulty was manipulated by varying target
diameter. The cross represents the cursor, the square the starting position and the circle the target which was
light grey in Study 1 but in Study 2 is was orange as shown in Figure 1c. Further explanation is presented in
the text.




Figure 1a

Figure 1b




Figure 1c

Figure 1d
Trials began with the presentation of a starting position, a light grey square (12 x 17
mm in the near condition) at the centre of the screen (Figure 1a). Participants moved the
cursor (a cross) onto the light grey square which turned dark grey to show it was activated
(Figure 1b). Pressing the space bar at this point caused the dark grey square to disappear,
timing to begin, and an appropriately sized target to appear elsewhere on the screen (Figure
1c). Participants were required to move the cursor as quickly as possible from its central
starting position to a point over the target which turned dark grey (Figure 1d). Pressing the
space bar at this point caused the target to disappear, timing to stop, and, following a 100
ms clear screen, the sequence returned to the starting position screen (Figure 1a). At the
end of each block of five trials a clear screen was displayed for 5000ms to mark the change
to the next level of difficulty.
The order of presentation of the four conditions, hand/near, hand/far, arm/near and
arm/far, was counterbalanced across participants. Within each condition, each target size
was presented as a block of five trials starting with the easier Level 1 and progressing
sequentially to the more difficult Level 5. Targets could appear in any one of six different
screen positions so that at each level of difficulty participants experienced targets presented
at 5 of the 6 screen positions randomly selected by the computer. Response times, i.e. the
time between cancelling the central square and cancelling the circular target, were recorded
for each trial in ms. The median response times for the five trials in each condition were
used for statistical analyses. Before starting the experiment, participants were given verbal
instructions and, to ensure familiarity with the procedure, they completed 10 practice trials
at Level 1, 5 in the near and 5 in the far space condition.

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Results and Discussion
Initial analysis
Target cancellation times were submitted to a 4-way mixed ANOVA with Sex
(women/men) as a between participants factor and Space (near/far), Muscle (hand/arm) and
Difficulty (Levels 1 to 5) as within participants factors. Three of the four main effects were
significant, Space, Muscle and Difficulty but not Sex (Table 1). Participants were faster in
near than far space (F1,46 = 4.781, p = 0.034), faster with the hand than the arm (F1,46 =
16.302, p < 0.001), and faster with larger than smaller targets (F1,46 = 323.094, p < 0.001).

Further analysis
Apart from the interaction between Muscle and Difficulty (F1,46 = 7.083, p = 0.001),
which arose because responses with the hand were faster at all levels of target size except
for the smallest, none of the other interactions was significant. However, we had argued
that the predicted Muscle*Sex and Space*Sex interactions may appear only at optimal
levels of task difficulty because hand/arm and near/far differences may not be seen if the
task is either too easy or too difficult. Differences in target size significantly affected task
difficulty which varied markedly from Level 1 (mean response time = 1086 ms) to Level 5
(mean response time = 2649 ms). Hence we conducted separate 3-way ANOVAs at each
level of difficulty. As seen in Table 2, the predicted interactions were not significant at any
level of difficulty. However, while there is no pattern across levels of difficulty for the
muscle data, the space data show larger effects at the easier Levels 1 and 2 than at the more
difficult Levels 3-5.

Sex differences in control of hand and arm
The absence of a pattern in the muscle data (Table 2) suggests the present failure to
replicate the Muscle*Sex interaction is not a question of task difficulty but rather the result
of differences between the tasks used. The present target cancellation task required a
relatively simple ballistic movement from the starting position to the target. In contrast, the
tracking tasks used by Sanders and Walsh (2007), which showed women were better with
the hand than the arm and men better with the arm than the hand, were more complex,
requiring a constant speed and continuous changes of direction. Perhaps a ballistic
movement from a start point to an end point is too simple to reveal differences in hand and
arm use.

Sex differences in processing near and far space
In contrast to the muscle data (Table 2), in the space data the Partial Eta Squared
values, although small, indicate that the Space*Sex interaction accounted for more of the
variance at the easier levels (1 and 2) than at the more difficult levels (3-5). There is a
related tendency for the Space*Sex interaction to approach significance at the easier levels
of difficulty. Indeed, at Level 1, the easiest task, women were faster in near than in far
space as predicted (t23 = 1.762, p = 0.046, one-tailed) while men were nominally faster in
far space than near space but not significantly so (t23 = 0.595, p = 0.279, one-tailed). These
observations suggest that the use of easier target cancellation tasks might reveal the
predicted Space*Sex interaction.
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Table 1. Study 1: Mean (SEM) response times for men and women when using their hand or arm muscles to
cancel targets presented in near or far space. Task difficulty was manipulated by varying the target size from
large (Level 1) to small (Level 5). Significant main effects are high-lighted in bold color (Space; Muscle;
Difficulty).
Task
Near space
Far space
Muscle
difficulty
Difficulty
(target size)
Men Women Men Women

Level 1
940.46
913.25
993.96
1001.25
962.23
(70 mm)
(47.05)
(47.05)
(73.40)
(73.40)
(38.53)

Level 2
1226.08
1236.08
1269.67
1357.83
1272.42
(35 mm)
(74.19)
(74.19)
(87.45)
(87.45)
(53.88)
Hand
Level 3
1277.00
1478.63
1393.17
1572.38
1430.29
(22 mm)
(64.25)
(64.25)
(87.44)
(87.44)
(45.03)

Level 4
1609.71
1900.581
1869.29
2072.67
1863.06
(10 mm)
(89.83)
(89.83)
(111.12)
(111.12)
(63.21)

Level 5
2457.75
2960.67
2541.29
2855.96
2703.92
(5 mm)
(183.71)
(183.71)
(210.41)
(210.41)
(116.75)
1502.20
1697.84
1613.48
1772.02
1646.38
Hand overall
(68.30)
(68.30)
(97.83)
(97.83)
(55.44)

Level 1
1258.25
1215.38
1145.71
1221.92
1210.31
(70 mm)
(56.78)
(56.78)
(61.11)
(61.11)
(37.46)

Level 2
1428.54
1442.46
1401.92
1511.88
1446.20
(35 mm)
(55.19)
(55.19)
(81.27)
(81.27)
(44.36)
Arm
Level 3
1556.17
1634.17
1569.42
1704.96
1616.18
(22 mm)
(78.94)
(78.94)
(104.34)
(104.34)
(55.88)

Level 4
1846.38
2166.17
1985.29
2206.08
2050.98
(10 mm)
(88.45)
(88.45)
(119.09)
(119.09)
(64.89)

Level 5
2377.75
2596.13
2636.04
2768.33
2594.56
(5 mm)
(131.96)
(131.96)
(213.43)
(213.43)
(106.39)
1693.42
1810.86
1747.68
1882.63
1783.65
Arm overall
(67.48)
(67.48)
(104.49)
(104.49)
(56.34)
Space
Near: 1676.08 (44.87)
Far: 1753.95 (65.50)
1715.02
(53.24)
Sex
Men: 1639.19 (75.30)
Women: 1790.84 (75.30)
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Table 2. Study 1: F-values, probabilities and effect sizes (? 2
p ) for the predicted Muscle*Sex and Space*Sex
interactions at each level of difficulty from Level 1 (easy) to Level 5 (difficult).
Muscle*Sex interaction
Space*Sex interaction
Task
Women faster with hand than arm
Women are faster with near than far
difficulty
Men faster with arm than hand
Men are faster with far than near
(target size)
Level 1
(F
2
2
1,46 = 0.264, p = 0.610, ?p = 0.006)
(F1,46 = 1.854, p = 0.180, ?p = 0.039)
(70 mm)
Level 2
(F
2
2
1,46 = 0.036, p = 0.850, ?p = 0.001)
(F1,46 = 2.137, p = 0.151, ?p = 0.044)
(35 mm)
Level 3
(F
2
2
1,46 = 0.985, p = 0.326, ?p = 0.021)
(F1,46 = 0.030, p = 0.864, ?p = 0.001)
(22 mm)
Level 4
(F
2
2
1,46 = 0.044, p = 0.835, ?p = 0.001)
(F1,46 = 0.717, p = 0.402, ?p = 0.015)
(10 mm)
Level 5 (F
2
2
1,46 = 1.319, p = 0.257, ?p = 0.028)
(F1,46 = 0.445, p = 0.508, ?p = 0.010)
(5 mm)

STUDY 2
Here we continued to investigate sex differences in the visual processing of
information from far and near space using a target cancellation task but, in the light of the
findings from Study 1, we introduced two changes. First we adjusted the range of target
sizes to include some levels of difficulty that were easier than those used in Study 1.
Second, given the absence of a hand/arm effect from Study 1, we dropped the Muscle
(hand/arm) factor by requiring participants to use a mouse to move the cursor. The same
participants completed both Study 2 and 3 (see below), one after the other in a
counterbalanced order within the same test session.
Materials and Methods
Participants
Forty-eight participants, 24 women (mean age 27.28, range 20 to 46 years) and 24
men (mean age 27.38, range 19 to 44 years), were recruited as an opportunity sample from
among our undergraduate and postgraduate students, and graduates from other universities.
All were right handed and had normal or corrected to normal vision. None was color-blind
and none had sustained a right hand injury during the previous six months. All were naïve
to the specific aims and predicted outcomes of the study which was approved by the
Departmental Ethics Committee. Each participant gave informed written consent and was
aware that they could withdraw from the study at any time. None withdrew.
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Task and procedure
Here we describe those aspects of the task and procedure that differed from Study 1.
As before we used a mixed design but without the Muscle (hand/arm) factor. In Study 2,
Sex was a between-participants factor with two independent groups, women and men;
Space and Difficulty were within-participants factors with repeated measures on near/far
and level of difficulty from the easy, largest diameter, target (Level 1) to the difficult,
smallest diameter, target (Level 6). In the near condition the diameter of the targets at each
level of difficulty was: Level 1, 109 mm; Level 2, 94 mm; Level 3, 72 mm; Level 4, 55
mm; Level 5, 36 mm; Level 6, 26 mm. Hence, Levels 1 and 2 were easier than Level 1 in
Study 1 while Levels 3 to 6 spanned a similar range to Levels 1 to 3 in Study 1. Image size
in the far condition was adjusted so that both near and far stimuli subtended the same visual
angle at the retina. The joysticks used in Study 1 were replaced by a standard ball-roll
mouse that was used by participants to move the cursor and to click on and cancel targets.
As before, within each condition participants completed one block of 5 trials at each level
of difficulty which started with Level 1 and continued sequentially through the levels, this
time to Level 6. Other aspects of the task and procedure were as shown in Figure 1 and
described for Study 1.
Results and Discussion
Initial analysis
Table 3 summarizes the target cancellation times from Study 2 that were submitted
to a 3-way mixed ANOVA with Sex (women/men) as a between-participants factor and
with Space (near/far) and Difficulty (Levels 1 to 6) as within-participants factors. There
were significant main effects of Space, Sex and Difficulty. Overall, participants were faster
in near than in far space (F1,46=59.90, p<0.001), faster with larger than with smaller targets
(F5, 230=317.00, p<0.001), and men were faster than women (F1, 46=4.18, p=0.047). There
was a significant two-way interaction between Space and Difficulty (F5, 230=5.03, p=0.001)
that arose because the increase in response times from Level 1 to 6 was greater in far than
in near space. It is likely that men were faster than women overall because male
participants frequently reveal greater competitiveness when completing such tasks.

Further analysis
The predicted Space*Sex interaction (Table 3) narrowly failed to reach significance
(F1,46=4.00, p=0.051). However, we had argued a priori that the Space*Sex interaction
might appear at some but not all of the Levels 1 to 6 because sex differences are sensitive
to task difficulty. Consequently, even though the three-way interaction between Space,
Difficulty and Sex was a not significant (F1,46=0.10, p=0.991), we conducted two-way
ANOVAs at each level of difficulty. From Study 1 (Table 2) we would expect the
Space*Sex interaction to appear at target sizes greater than 70mm, essentially Levels 1 or 2
in Study 2. In fact, we found that Level 2, which had a target size of 94 mm, showed a
significant Space*Sex interaction (F1,46=4.560, p=0.038) arising because, compared with
men, women were relatively faster in near than in far space (Figure 3).

Evolutionary Psychology – ISSN 1474-7049 – Volume 6(2). 2008. -351-

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