Testing Predictions from the Hunter-Gatherer Hypothesis - 2: Sex Differences in the Visual Processing of Near and Far Space

Evolutionary Psychology
www.epjournal.net – 2007. 5(3): 666-679
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Original Article
Testing Predictions from the Hunter-Gatherer Hypothesis – 2: Sex Differences
in the Visual Processing of Near and Far Space
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)

Kamila Sinclair, Department of Psychology, London Metropolitan University

Tom Walsh, Department of Psychology, London Metropolitan University

Abstract: Here, in the second of two linked reports, we focus on sex differences in visual
processing. Study 1 presented a time estimation task in virtual space and generated the
predicted Space*Sex interaction with men performing significantly better in far than in near
space. Study 2 used a laboratory-based puzzle completion task in which participants saw
their hands and the puzzle in far or near space. This time women performed significantly
better in near than far space. Study 3 simplified the puzzle completion task. Once again the
predicted Space*Sex interaction was significant but with both sexes showing significantly
different performances: women better in near, men in far space. These findings are
compatible with an evolutionary origin as predicted by the hunter-gatherer hypothesis. Far
and near space are processed in the ventral and dorsal streams, two cortical regions more
widely known as the “what” and “where” visual systems. To those traditional descriptions
we suggest adding that the two streams are sex-dimorphic, with the ventral “there” system
interacting with far space and favored in men and the dorsal “here” system interacting with
near space and favored in women. Future studies of visual systems should consider the
impact of sex differences and the spatial location of stimulus presentations.
Keywords: hunter-gatherer hypothesis; sex differences; visual processing; far and near
space; dorsal and ventral cortical streams; two visual systems.
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Introduction
Silverman and Eals (1992) proposed the hunter-gatherer hypothesis as an
evolutionary explanation for sex differences in spatial ability. They argued 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
cannot be tested directly we can test its predictions and we have adopted this approach in a

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Sex differences in visual processing
series of studies aimed to generate predictions that are novel and testable. In this, the
second of two parallel reports, we focus on predicted sex differences in the visual
processing of far and near space. In the first report we addressed sex differences in motor
control (Sanders and Walsh, 2007). In both cases we were able to demonstrate the predicted
sex differences and to identify from the literature 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 visual processing we would expect an interaction between
Space and Sex with women performing better in near and men in far space. For motor
control we would expect women to favor the hand and men the arm, a prediction that we
have confirmed (Sanders and Walsh, 2007).
Here we report three studies that focus on sex differences in the visual processing of
information from far and near space for which no published studies appear to exist. Study 1
was conducted via the Internet using a time estimation task in virtual space while Study 2
and Study 3 were laboratory-based using a puzzle completion task. In each case we
predicted a Space*Sex interaction arising from women performing better with visual input
from near space and men with visual input from far space. Data were subjected to
appropriate analyses of variance and significant interactions were explored with t-tests
using 1-tailed tests for our directional predictions and 2-tailed tests for other comparisons.
Study 1
Study 1 was conducted via the World Wide Web. Male and female participants
were required to make spatiotemporal judgments in far and near virtual space. Schiff and
Oldak (1990) used real life film of approaching vehicles projected in a laboratory and
found that women were less accurate than men at judging time to arrival because they
tended to underestimate. The effect persisted when the authors removed the element of
personal risk by using film of a tabletop toy car approaching a gateway. This task was
suitable for our study because it could be presented visually in both far and near space with
the same required response, a key press, thus ensuring that the cognitive demands and
motor movement were constant and only the source of the visual input (far and near space)
varied between the conditions. We collected our data using a two-dimensional
representation of a three-dimensional image presented in virtual space via the Internet. The
task was presented as a computer game in which potential participants were asked to judge
the time to arrival at a docking station of a toy UFO that hovered above a tabletop.

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Sex differences in visual processing
Materials and Methods
Participants
The target participants were men and women with a minimum age of 18 years. The
Web address for the study was spread through a network of informal contacts, principally
via email. Potential participants logged onto the site and decided whether or not to
participate and, if they completed the task, whether or not to submit their data. Submissions
were received from 166 participants. Nine were discarded because the data were
incomplete, or because the arrival time estimates fell outside reasonable limits (i.e., those
corresponding to times when the UFO was visibly distant from the docking station), or
because the submitted data came from the same computer and/or email address as a
previous submission. There remained submissions from 93 men and 64 women. We
analyzed the data from all the women and from the first 64 male respondents. The study
was approved by the London Metropolitan University Psychology Department Ethics
Committee. On-screen instructions indicated that by submitting their data participants were
giving consent for it to be used in the study.
Task and procedure
Via the website, potential participants could access a program, written by KS in
Java Script, which appeared as two html pages: the Instructions and the Task. On the Task
page, participants were presented with a 2-D representation of a 3-D scene in virtual space.
A hovering toy UFO projected a spot of light onto a tabletop. The UFO then moved
obliquely towards the participant as it traveled to a docking station. Before reaching the
docking station the UFO and its spotlight disappeared and participants were asked to press
their space bar at the precise moment they thought the UFO’s spotlight would have
coincided with the centre of the, still visible, docking station.
Three aspects of the UFO’s trajectory, spatial location (far/near), starting side
(left/right) and disappearance time (early/late), were manipulated as independent variables.
Space was represented by a receding table with one chair on its near left side and another in
the centre of the far end. The orientation of the scene indicated that the participant was
sitting at the near end of the table. From this position, given the size of the chairs in relation
to the table, it was apparent that a participant would be able to reach the full width and at
least across the nearest third, but not quite the nearest half of the table. In the far condition
the UFO appeared at the distant end of the table in one corner and traveled towards a
docking station in the middle of the opposite side. In the near condition the UFO appeared
at the edge of the table in the middle of one side and traveled towards a docking station in
the near corner of the opposite side. Thus, in the far condition, the entire trajectory of the
UFO was beyond reaching distance in far (extrapersonal) space, whereas, in the near
condition, all but the starting point was in near (peripersonal) space. Side was represented
by the starting point, left or right, of the journey made by the UFO. Disappearance was
determined by the point in the journey when the UFO disappeared. The total journey time
was fixed at five seconds. In the early (more difficult) condition the UFO disappeared 2.53
seconds before docking while in the late (easier) condition it disappeared 1.40 seconds
before docking. This “level of difficulty” factor was introduced because demonstrations of
sex differences may be masked by using tasks that are too easy or too difficult to
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Sex differences in visual processing
differentiate the performances of men and women (Sanders, Sjodin, and de Chastelaine,
2002).
We used a mixed design: Sex was a between participants factor with two
independent groups, women and men; while Space, Side and Disappearance were within
participants factors with repeated measures on near/far, left/right and early/late
respectively. These three factors were tested over eight repeated measures trials: near-left-
early, near-left-late, near-right-early, near-right-late, far-left-early, far-left-late, far-right-
early and far-right-late that were presented in a random sequence to the participants.
Results and Discussion
The time estimations were subjected to a 4-way ANOVA with Sex as a between
participants factor and Space, Side and Disappearance as within participants factors. With
performance collapsed across all conditions the male grand mean score (5.127 seconds)
was closer to the correct value of 5 seconds than the female grand mean score (4.640
seconds) and this main effect of Sex was significant (F1, 126 = 34.433, p < 0.001). This
finding confirms the report by Schiff and Oldak (1990) that men are more accurate than
women who tend to underestimate time of arrival. However, our major interest here is the
accuracy of male and female arrival time estimates in far and near space so for all
subsequent analyses we have used absolute error (time estimate minus 5 with the sign
ignored) as the dependent variable. A 4-way ANOVA based on the absolute error scores
revealed main effects of Sex (F1, 126 = 6.652, p = 0.011) with men more accurate than
women, Space (F1, 126 = 4.642, p = 0.033) with participants more accurate in the near
condition and Disappearance (F1, 126 = 123.372, p<0.001) which arose because participants
were more accurate in the easier late than the more difficult early condition. There were
also significant Space*Sex (F1,126 = 6.125, p = 0.015) and Space*Disappearance*Sex
(F1,126 = 6.038, p = 0.015) interactions. The Disappearance factor effectively manipulates
level of difficulty. Comparing the early with the late condition shows that participants see
the UFO for less time (2.47 compared with 3.60 seconds) and have to remember its speed
for a longer time (2.53 compared with 1.40 seconds). It is known that level of difficulty can
be a critical factor in the demonstration of sex differences (Sanders, Sjodin, and de
Chastelaine, 2002) so we analyzed the early (difficult) and late (easy) disappearance data
separately.

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Sex differences in visual processing
Figure 1. Study 1: Mean (+/- SEM) of the absolute error (time to arrival estimate minus 5
with sign ignored) in seconds recorded with (a) late disappearance (easy) and (b) early
disappearance (difficult) conditions for men (blue/ darker grey line and diamonds) and
women (pink/ lighter grey line and squares) with the visual information presented via the
Internet in near (peripersonal) and far (extrapersonal) virtual space.
(a) Late Disappearance
(b) Early Disappearance
(Easy)
(Difficult)
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A 3-way ANOVA based on the absolute error data from the easy late disappearance
condition reveal a significant main effect of Sex (F1,126 = 13.777, p<0.001) with men more
accurate than women (Fig. 1a). The only other significant outcome was the main effect of
Side (F1,126 = 3.963, p = 0.049) with overall accuracy better when the UFO moved from
right to left, but the effect of Side was not significant for either sex alone (women t63 =
1.903, p = 0.062; men t63 = 0.902, p = 0.370, both two-tailed). The 3-way ANOVA on the
absolute error data from the difficult early disappearance condition reveal only one
significant outcome, the Space*Sex interaction (F1,126 = 7.635, p = 0.007). As seen in
Figure 1b, this interaction arose because men were more accurate when the UFO was
moving in virtual far space (t63 = 3.762, p = 0.001, one-tailed) while women were
marginally more accurate in near space although the difference was not significant (t63 =
0.721, p = 0.237, one-tailed).
In summary, data from the more difficult early disappearance condition confirmed
two of our three predictions; the Space*Sex interaction and the greater accuracy of men in
far than in near space, but not that women were more accurate in near than in far space. In
addition, the combined data confirmed the Schiff and Oldak (1990) report of a sex
difference in the estimation of time to arrival with men more accurate than women and
extended this finding from a real world laboratory-based study to the virtual world of the
Internet. One other finding emerged from the complex interactions that occurred between
Sex, Space, Side and Disappearance; performance was better when the UFO moved from
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Sex differences in visual processing
right to left rather than from left to right. At present we have no explanation for this
right/left difference other than the possibility that the predominant use of the right hand to
press the space bar produced a priming effect similar to that reported by Frassinetti, Rossi
and Ladavas (2001). However, as noted above, the effect of Side disappeared when the
male and female data were analyzed separately.
Study 2
In Study 2 we investigated sex differences in the visual processing of information
from far and near space in the real world space of a laboratory. Participants were required
to arrange pieces of foam-board to form a square while viewing an image of their hands
and the pieces of board projected onto a closely placed monitor for the near space condition
or onto a more distant wall-mounted screen for the far space condition. Once again we
predicted a Space*Sex interaction arising from women performing better with visual input
from near space and men with visual input from far space.
Materials and Methods
Participants
Forty participants, 20 women and 20 men, were recruited as an opportunity sample
from among our University students. 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 and procedure
Participants were required to correctly assemble simple puzzles constructed from 3
mm thick white foam-board. Each puzzle was made by cutting a 100 mm square of board
into five pieces using combinations of curved and straight lines. The pieces were displayed
and assembled on a large sheet of matt black card that covered the table closest to the
seated participant. At the start of each trial the five pieces were arranged in the same
randomly determined positions in front of the participant. A curtain, suspended from a
wooden frame, prevented participants from looking directly at their hands or the puzzle.
Instead a webcam was used to display an image of the puzzle and the participant’s hands.
In the near space condition the image was displayed on a 380 mm monitor placed 500 mm
from the participant. By looking straight ahead, across the top of the framed curtain,
participants could see the entire monitor screen but not their hands or the puzzle. In the far
space condition, the image was projected onto a wall-mounted screen placed 2000 mm
from the participant. By looking straight ahead, across the top of the framed curtain and the
monitor, participants could see the entire wall-mounted screen but not their hands or the
puzzle. The size of the image on the wall-mounted screen was adjusted so that it subtended
the same visual angle as the image on the monitor. We used a mixed design with Sex as a
between participants factor and Space as a within participants factor with repeated
measures in the far and near conditions. The order of the far and near conditions was
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Sex differences in visual processing
counterbalanced across participants. The time taken to complete the puzzle was recorded
with a stopwatch to the nearest 0.01 seconds.

Results and Discussion
The puzzle completion time data were subjected to a 2-way ANOVA with Sex
(male/ female) as a between participants factor and Space (far/ near) as a within
participants factor. The main effects of Sex and Space were not significant (sex F1,38 =
0.002, p = 0.999; space F1,38 = 0.000, p = 0.999). However, as predicted, there was a
significant Space*Sex interaction (F1,38 = 4.979, p = 0.032) that arose because women
completed the puzzles faster in near space whereas men were faster in far space (Fig. 2).
The far/near space effect was significant for women (t19 = 1.936, p = 0.034, one-tailed) but
not for men (t19 = 1.365, p = 0.094, one-tailed).

Figure 2. Study 2: Mean (+/- SEM) time to complete puzzle in seconds for men (blue/
darker grey line and diamonds) and women (pink/ lighter grey line and squares) in near
(peripersonal) and far (extrapersonal) space with free movement of the hand and arm.

100
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Near
Far


Thus the finding from Study 2 replicates in the laboratory the predicted Space*Sex
interaction that we found for virtual space in Study 1. However, whereas in Study 1 the
interaction arose because men performed significantly better in far than near space, this
time, women performed significantly better in near space while the far/near difference was
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Sex differences in visual processing
not significant for men. In Study 3 we hoped to demonstrate significant far/near differences
concurrently for men and women by repeating Study 2 with some modifications, including
procedures to control muscle use.
Study 3
In Study 2 participants used their hands and arms freely to complete the puzzles.
For Study 3 we decided to control muscle usage because sex differences in the use of hand
and arm have been reported (Sanders and Walsh, 2007): women are better with their hands
but men with their arms. Consequently participants were required to complete the puzzles
separately in the far and near conditions using either hand (wrist and finger) or arm (upper
arm and shoulder) movements. Hence, in addition to the predicted Space*Sex interaction,
with women performing better in near and men in far space, we also predicted a
Muscle*Sex interaction with women performing better with their hands and men with their
arms.
Materials and Methods
Participants
Sixty participants, 30 women and 30 men, were recruited as an opportunity sample
from among our University students. 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 and procedure
The basic task and procedure were similar to those used in Study 2 but, anticipating
that restricting movement to either hand or arm use would increase task difficulty,
completion of the puzzles was made easier. As before, participants were required to
correctly assemble five-piece foam-board puzzles to form a 100 mm square but, this time, a
puzzle was judged to be completed when all five pieces were correctly assembled within a
105 mm square outline, i.e. perfect contact between the pieces was not required. As a
second aid, a drawing of the completed puzzle, which participants were instructed to use as
a guide, was displayed just beyond the five pieces in their starting positions.
We used a mixed design. Sex was a between participants factor with two
independent groups, women and men; Space and Muscle were within participants factors
with repeated measures hand/ arm and far/ near respectively. The far and near conditions
were established as in Study 2. For the hand condition participants were instructed to rest
their forearms on the edge of the table and to complete the puzzle using movements of their
fingers and wrists. In the arm condition participants were instructed to hold the pieces
between their first finger and thumb but to restrict all other movements to the muscles of
the upper arm and shoulder. As a reminder not to move wrists or fingers, the participants in
the arm condition wore skating wrist protectors. Participants completed one of the four
puzzles in each of four conditions: far-hand, near-hand, far-arm and near-arm. The
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Sex differences in visual processing
sequence in which the puzzles were presented was randomized across participants and the
time taken to complete them was recorded with a stopwatch to the nearest 0.01 seconds.

Results and Discussion
The puzzle completion time data were submitted to a 3-way ANOVA with Space
(far/ near) and Muscle (hand/ arm) as within participant factors and Sex as a between
participants factor.

Figure 3. Study 3: Mean (+/- SEM) time to complete puzzle in seconds for men (blue/
darker grey line and diamonds) and women (pink/ lighter grey line and squares) in near
(peripersonal) and far (extrapersonal) space using (a) combined hand and arm data, (b)
hand data and (c) arm data.

(a) Combined
(b) Hand
(c) Arm
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As seen in Figure 3a, there was a significant Space*Sex interaction (F1,58 = 11.838,
p = 0.001) that arose because men, as predicted, completed puzzles faster in the far
condition (t29 = 1.949, p = 0.031, one-tailed) whereas women performed better in the near
condition (t29 = 3.013, p = 0.003, one-tailed). In addition there were significant main effects
of Sex (F1,58 = 6.302, p = 0.015) and Muscle (F1,58 = 16.352, p = 0.001) that arose because
overall completion times were faster for women than men and faster for the hand than the
arm condition (compare Fig. 3b and c). Contrary to expectation, the Muscle*Sex and
Muscle*Space*Sex interactions were not significant; however, observation during the
study indicated that our control for muscle use was not effective. While the instructions to
restrict movement to hand or arm were followed in preliminary tests, in the main study
participants used combinations of hand and arm muscle movements.
On finding that the Muscle (hand/ arm) condition effectively manipulated difficulty
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Sex differences in visual processing
we used 2-way ANOVA to analyze the hand and arm data separately. Both analyses
generated similar outcomes (Fig. 3b and c), a significant main effect of Sex (hand: F1,58 =
6.50, p = 0.013; arm: F1,58 = 4.47, p = 0.039) with women faster overall than men and a
significant Space*Sex interaction (hand: F1,58 = 5.75, p = 0.020; arm: F1,58 = 4.23, p =
0.044) with women faster in near space and men faster in far space. On these reduced data
sets the far/near difference was significant for women with the hand data (t29 = 3.00, p =
0.006) but not for the arm data (t29 = 1.53, p = 0.138) while the faster performance of men
in the far condition failed to reach significance (hand: t29 = 1.01, p = 0.319; arm: t29 = 1.40,
p = 0.172).
We had expected the muscles used to influence performance because women have
been reported to perform better with their hands and men with their arms (Sanders and
Walsh, 2007). However, in the present study neither the Muscle*Sex or the
Muscle*Space*Sex interactions were significant. Observation of participant behavior
during data collection revealed the reason. Although the minimal constraints that we used
to promote use of either the hand or the arm were sufficient during preliminary tests, in the
main study participants either forgot or ignored those instructions. Hence the absence of a
muscle effect is not surprising because the hand and arm conditions in Study 2 did not
represent robust distinctions between separate hand and arm muscle use. Nevertheless, the
use of combined hand and arm data for the space analysis remains appropriate because the
distinction between far and near was sound. Therefore the findings from Study 3, have
reconfirmed the Space*Sex interaction with women performing better in near and men in
far space.
General Discussion
Behavioral sex differences and implications for the hunter-gatherer hypothesis
Sex-dimorphic motor tasks that are typically cited as support for the hunter-gatherer
hypothesis, e.g. 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) confound task demands, motor responses and spatial location. Here we have
reported three studies in which spatial location was systematically varied between near
(peripersonal) and far (extrapersonal) while task demands and motor responses were held
constant. We argued that selection for hunting skills should favor men with good
processing of visual input from far (extrapersonal) space to facilitate the detection of
suitable prey and for accurately aiming a missile. However, selection for gathering skills
would favor women with good processing of visual input from near (peripersonal) space to
facilitate the detection and grasping of appropriate items. Thus we predicted that women
would perform better when stimuli were presented in near space and men when the stimuli
were in far space.
With the previous confounds removed, each of our studies confirmed the predicted
Space*Sex interaction, arising because men performed significantly better with visual
information from far space in Studies 1 and 3 while women performed significantly better
with information from near space in Studies 2 and 3. It is notable that these sex differences
were detected not only in the real space of laboratory-based tasks (Studies 2 and 3) but also
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