SPATIAL COGNITION

return to a home base and select the Arabic numeral that corresponded to the sum of the orange slices. So, for example, if one site had two orange slices, a second had three, and the third had none, Sheba would have to select the Arabic numeral

5

to be correct. This she did on the very first trial, requiring virtually no training.

In

a second experiment, the arrays of oranges were replaced by Arabic numerals. So instead of finding two and three orange slices at two sites as in the example above, she would find only the numerals

2

and

3.

Sheba performed significantly above chance on this task beginning with the first experimental session. Sheba’s simple arithmetic performance is comparable to the simple counting strategies observed for

3-

and 4-year-old children (Starkey & Gelman,

1982)

and suggests that simple, humanlike addition abilities can be used

by

at least some chimpanzees.

Basic quantitative abilities are important not only for humans but for many species. Humans, however, have made especially great use of mathematics, and Geary’s claim that there are a handful of biologically primary quantitative abilities finds support in the infancy and child develop- ment literatures. We are also intrigued by the evidence that some monkeys and chimpanzees can develop counting and simple arithmetic skills, suggest- ing that the origins of humanlike mathematics may extend back to our simian ancestors. We should emphasize that despite the primacy of these abilities, they do not appear,

de

^nooo,

^fully

formed when first seen. Rather, they develop over infancy and childhood and, interestingly, there may be a discontinuity in their development, with early expression of these skills being implicit in nature, whereas the more mature forms are expressions of explicit cognition. Despite the unresolved issues, we believe that the evi- dence is strong that humans are prepared to deal with quantitative relations, that such preparation exceeds that shown

by

other species, and that it is displayed early in ontogeny.

that they can store up to

33,000

seeds in more than

6,600

locations and recover most of them months later (Hauser,

2000).

When looking at the variety of impressive spatial abilities displayed

by

a broad range of species, it seems wholly reasonable to assume that natural selection has worked to prepare animals with relatively specific mechanisms to find their way through space and to remember spatial locations, specific to the ecological needs of each species. Homo supiens should be no exception.

Development of Spatial Abilities in Children

Spatial abilities in humans clearly develop with age. But infants seem to represent spatial location early in life and to use the location of an object, rather than other characteristics such as shape or color, to define that object. In one set of experiments (Quinn, 1994), 3-month-old infants were habituated to visual displays in which a dot was presented in one of several locations above a solid bar. (Other infants were habituated to dots consis- tently presented below a solid bar.) After habituating, infants were presented with two new stimuli: One was a dot presented in a new location but still above the bar, and the other was a dot presented below the bar. Both stimuli were “new” to the extent that neither had been shown to the infant previously. However, the stimulus with the dot above the bar was similar to the other above-the-bar stimuli the infants had seen during the habitua- tion trials.

If

infants categorize spatial location, they should look longer at the stimulus with the dot below the bar than the one with the dot above the bar. This is what the infants

did,

indicating that they were able to abstract a spatial (geometric) relation.

In other research, 5-month-old infants watched as an object was hidden in a specific location in a sandbox. After a 10-second delay, the object was dug out.

This

was repeated four times. O n the

fifth

occasion, instead of digging up the object at the location where it had been hidden, the experi- menter dug out the object from a different location (as near as

6

inches from where the first had been hidden). Infants looked significantly longer when the object was retrieved from a location at which it had not been hidden, indicating that these infants had coded the specific location of the objects (Newcombe, Huttenlocher, & Learmonth,

1999).

In a final experiment in this report, infants were not surprised (i.e.,

did

not look longer) when a different object was retrieved from the sand.

This

finding suggests that spatiotemporal characteristics may play a central role in defining objects to young infants, with shape and color being unimportant.

The finding of infants being able to “solve” this problem is particularly interesting, given that a similar task, in which children must retrieve an object hidden in a sandbox after they moved to the other end of the box, is typically not solved above chance levels until

21

months of age

(Newcombe, Huttenlocher, Drummey, & Wiley, 1998). Perhaps the act of moving around the sandbox makes the task too demanding for children’s limited mental resources or disrupts their memory, accounting for the dis- crepancy. Another possibility refers to the difference between the implicit and explicit natures of the two tasks. Similar to the argument we made with respect to judgments of numerosity, children may understand spatial relations at first only implicitly, with explicit understanding, as reflected by an overt search task, being displayed only later in ontogeny.

Despite its early appearance and its saliency in identifying objects, spatial cognition in infancy does not simply “mature” but develops as a result of experience. For example, depth perception can be assessed

by

how infants behave on the visual cliff, an apparatus that consists of a glass- topped table with a board across its center. O n one side of the board the infant sees a checkerboard pattern directly under the glass. This is referred to as the “shallow” side. O n the other side the checkerboard pattern is several feet below the glass; this side is referred to as the “deep side.’’ In the original research using this apparatus, infants rarely crawled over the deep side to their calling mothers, suggesting that once infants can crawl, they display fear, indicating that little learning about depth is necessary (Walk & Gibson,

1961).

However, more recent research has found that the tendency to show fear on the visual cliff is related to the extent of previous locomotor experience (Bertenthal, Campos, & Barrett, 1984; Bertenthal, Campos, & Kermoian,

1994).

Infants with more locomotor experience are more likely to show fear than their less experienced peers, suggesting that such experience is related to depth perception. In explaining these results Bertenthal and

his

colleagues

(1994)

suggest that the

active control of locomotion, unlike passive locomotion, demands con- tinuous updating of one’s orientation relative to the spatial layout. This information is provided through multimodal sources, such as visual and vestibular coding of angular acceleration. With locomotor experience, changes in angular acceleration detected by the visual system are mapped onto analogous changes detected by the vestibular system. Fear or avoid- ance ensues when the expected mapping between visual and vestibular information is violated. (p. 142)

Classic research with kittens (Held & Hein, 1963) demonstrated that self-propelled movement is critical in the development of depth perception.

Kittens were raised in a darkened room and had visual experience only during training and testing. Pairs of kittens of the same age were used, one given normal visual-motor experience and the other given visual experience without associated motor experience.

To

do this, a special training apparatus was developed. The active kitten was harnessed to walk around the brightly decorated track while the passive kitten was placed in a gondola to have

I70

THE ORIGINS OF HUMAN NATURE

identical visual experience without the associated motor feedback. That is, although both kittens had the same visual experience, and both had motor experience in their darkened room, only the active kitten had visual experi- ence concomitant with movement. After training, both kittens were tested on the visual cliff to determine if they showed a preference for sides. The passive kittens showed no preference for sides, indicating a lack of depth perception. By contrast, the active kittens consistently chose the “shallow”

side, indicating that they could perceive depth. In research with humans, 8.5-month-old infants were divided into three groups: (a) prelocomotive (i.e., not yet crawling);

(b)

prelocomotive but with experience using a walker; and (c) locomotive (i.e., crawling; Kermoian & Campos, 1988).

The infants were then tested with a series of tasks in which they had to retrieve an object hidden under a cloth. Infants with locomotive experience, either

by

crawling or in a walker, displayed more advanced performance on the object-retrieval task than

did

the noncrawlers. Also, there was no difference in performance between the crawlers and the walkers, suggesting that it was the locomotor experience and not maturation that was responsible for the advanced spatial memory. Other research has similarly shown advan- tages on some visual-spatial tasks associated with self-produced locomotion, although these effects tend to be limited to specific abilities (Arterberry, Yonas, & Bensen, 1989; Bai & Bertenthal, 1992).

The infant literature is consistent with the position that humans are born with an ability to code spatial relations and that they may use spatial location as a primary cue in defining objects. That is, where an object is may be the defining feature of that object. However, spatial abilities improve with age and develop in relation to infants’ self-propelled movements. Spatial skills can get very complex, and substantial development occurs in these abilities during childhood. One interesting finding in spatial abilities con- cerns sex differences that have been reported consistently beginning during the preschool years (Halpern,

1996;

Maccoby & Jacklin, 1974). From an evolutionary psychological perspective, males and females have somewhat different self-interests and have evolved different strategies to best deal with the problems they have faced. Evolutionary psychologists Irwin Silverman and Marion Eals (1992) have interpreted the sex differences observed in males and females from an evolutionary perspective, proposing that the pattern of differences makes sense given the division of labor that has been assumed for ancient men and women.

Sex Differences in Spatial Abilities

There is a division of labor among males and females in traditional cultures that likely reflects the way our ancestors lived in the Pleistocene.

Although men engage in some gathering, the tasks of finding and gathering

edible fruits and vegetables, typically found close to the home base, is primarily the job of women. In contrast, hunting is nearly exclusively in the male domain, with men often traveling far from the home base over a series of days in pursuit of game. Given this division of labor, spatial skills that permitted men to mentally manipulate space (cognitive maps of geographic locales) would be to their benefit because of the advantage this would provide in navigating over large areas: Relatedly, males’ greater use of projectile weapons, in the service of male-male competition, may also have contributed to a male advantage in some aspects of spatial cognition (e.g., the ability to track objects moving in space; Geary,

1998).

In contrast, women may be most advantaged with spatial skills that permit them to recall the location of specific caches of fruit or edible plants and for making fine perceptual discriminations, such as between ripe and unripe berries. In general, sex differences, beginning in childhood, have been found in the predicted direction in these areas.

For example, males, beginning in the preschool years (Levine, Hut- tenlocher, Taylor, & Langrock, 1999) tend to perform better than females on tasks that involve manipulating spatial relations (Baenninger & Newcombe,

1995;

Casey,

1996),

finding their way through physical (or virtual) environ- ments (Moffat, Hampson, & Hatzipantelis,

1998;

Silverman et al.,

2000),

making maps (Matthews,

1987),

and using maps (Dabbs, Chang, Strong,

& Milun, 1998; Gibbs

6r

Wilson, 1999; Money, Alexander,

6r

Walker,

1965). In

one study, kindergarten, second-grade, and fifth-grade children walked repeatedly through a large model town consisting of buildings, streets, railroad tracks, and trees. Following the walks, the children were asked to re-create the layout from memory. Performance increased with age, with boys performing better than girls (Herman & Siegel,

1978).

This pattern is what one would expect if males were prepared by natural selection for spatial skills related to navigating large spaces.

In comparison to these spatial-analytic skills, there is a consistent female advantage for tasks that assess object-location memory. For example, females are better than males at remembering the locations of each item in an array of objects, with this ability increasing with age for both sexes (Eals & Silverman, 1994; Silverman & Eals,

1992).

In several of the studies

by

Silverman and Eals, participants ranging in age from

8.5

years to college adults were shown an array consisting of a variety of objects. Sometime later, they were shown a larger array and were to cross out all items that

~~~~

+Sex differences in spatial ability favoring males have been found in nonhuman mammalian species in which males have a larger home range during mating season than do females; the reverse pattern is observed in species in which females are more active in spatial search (see Sherry, 2000). Spatial ability in these species is also correlated with the size of the hippocampus. Thus, the pattern of sex differences observed in humans is seemingly tied to ancestral lifestyle differences and is analogous to that of other species.

172

THE ORIGINS OF HUMAN NATURE

were not in the original array.

At

all ages tested, females performed better than males. When asked about the correct location of items (e.g., where in an array a particular item was originally located), females again performed better than males, although differences were not significant until adolescence (see also Kail & Siegel,

1977).

This pattern is consistent with Silverman and Eals’ theory that natural selection has favored such detailed spatial- perceptual skills in females. (These results are also reminiscent of the

“Where’s the mayonnaise?” phenomenon, frequently observed in some homes, in which men have a difficult time finding a specific item, such as the mayonnaise jar, when it is among a large collection of somewhat similar objects in the refrigerator.)

Although our interpretation of these results is similar to that of Silver- man and Eals (1992)-that there have been differential selection pressures on hominid males and females for the development of spatial skills and that this is reflected in the abilities of modem men and women-it does not address the proximal mechanisms responsible for these differences. One possibility is that differences in brain organization, mediated relatively di- rectly

by

the action of different genes, or indirectly through differences in hormones, are responsible for these effects. An alternative possibility is that sex differences in locomotor play between boys and girls may be responsible, in part, for some of the sex differences observed in spatial cognition (Bjor- klund & Brown,

1998;

see also chapter

10

for a discussion of sex differences in play style).

In

other research, Newcombe and her colleagues have found a significant relation between adult females’ spatial abilities and the extent to which they engage in everyday tasks involving high-spatial content (Newcombe, Bandura, & Taylor,

1983).

Similarly, sex differences in chil- dren’s abilities to form cognitive maps of large-scale environments is associ- ated with sex differences in experience with the environment. For example, across a wide range of cultures, boys tend to have a larger exploration range than girls and, as a result, may acquire more information about their physical surroundings than girls (Matthews, 1992). Support for this comes from a study that equalized the experience preschool girls and boys had for their physical environment and found no sex difference in spatial-orientation task performance (Hazen,

1982).

In one study with preschool children, researchers found sex differences in visual-spatial play activities, with boys engaging in more spatial activities than girls (Connor & Serbin, 1977). The amount of boys’ visual-spatial play correlated significantly with their performance on the Block Design subtest of the Wechsler Intelligence Scale for Children (Wechsler, 1974) and the Preschool Embedded Figures Test (Karp & Konstadt, 1971), suggest- ing that sex differences in play activity are responsible in part for boys’

generally greater spatial skills. However, other studies have found smaller differences in spatial play between preschool boys and girls and no significant

relation between spatial play and spatial cognition (Caldera, O’Brien, Truglio, Alvarez, & Huston,

1999).

Thus, the relation between physical play, spatial cognition, and sex differences is not a simple one. It would be interesting to examine more specific relations between locomotor activity and spatial cognition to see how these effects, if they exist, vary with age.

For example, are there sex differences in the specific types of physical play that boys and girls engage in, and do some of these types (e.g., play involving eye-hand coordination) predict spatial abilities better than others?

Clearly spatial abilities improve with age and are largely a result of the specific experiences children have, both in infancy and in later child- hood. The results of a meta-analysis support this contention, finding signifi- cant relations between children’s spatial cognition and locomotor experi- ences, with these effects being greater during early childhood than during the infancy and toddler periods (Yan, Thomas, & Downing,

1998).

That sex differences in spatial abilities may also be partially attributed to differences in locomotor experience is provocative, but obviously not the entire answer.

Psychologist

M.

Beth Casey (1996; Casey, Nuttall, & Peraris,

1999)

has suggested that a combination of genetic and experiential factors likely con- tribute to the typically observed sex differences, with most males being biased from an early age toward activities involving spatial cognition, such as block building, carpentry, and throwing and catching objects; this experience eventually leads to enhanced spatial

skills.

Geary (1998) has made a similar proposal, stating that sex hormones influence boys’ and girls’ tendencies to explore environments, which in turn affects the development of the brain.

Hormones, however, also directly affect the organization of the brain, which in turn influences spatial cognition. The bulk of the evidence, we believe, argues strongly that humans are prepared to process spatial information and that males and females, because of selection pressures experienced

by

their ancestors, develop, on average, different strengths in spatial skills. Although we are confident that both the universal and sex-differentiated skills are mediated

by

inherited (genetic) differences, proximal factors, such as loco- motor and play experiences, contribute significantly to these patterns, consis- tent with the argument that development proceeds as the result of the continuous and bidirectional relationship between multiple levels of organi- zation over time, and is not the simple product of either “genes” or “envi- ronment

.”

LANGUAGE ACQUISITION

The area in which the most ink has been spilled concerning Homo sapiens’ special preparation for a cognitive ability is that of language. In fact, the modem neonativist perspective can trace its roots to the pioneering

1

74 THE ORIGINS OF HUMAN NATURE