Cognitive Development 20 (2005) 137–158
Developing domain-specific causal-explanatory frameworks: the role of insides and immanence Gail M. Gottfrieda,∗ , Susan A. Gelmanb a
Pomona College, Linguistics Cognitive Science, 550 N. Harvard Avenue, Claremont, USA b University of Michigan, USA Received 26 January 2004; received in revised form 11 June 2004; accepted 14 July 2004
Abstract Two studies investigate children’s knowledge of internal parts and their endorsement of immanent causes for the behaviors of living and non-living things. Study 1, involving 48 preschoolers, showed that domain-specific knowledge of internal parts develops between ages 3 and 4. Study 2 included 43 4-year-olds, 30 8-year-olds, and 35 adults and showed that preschoolers do not endorse these internal parts as causally responsible for familiar biological events (e.g., movement, growth). Like adults and older children, however, preschoolers endorse an abstract cause, “its own energy,” for animals but not for machines. The results suggest that children recognize domain-specific internal parts as early as age 4 but that their causal attributions are not yet anchored in a detailed biological theory. Findings are discussed in terms of theory change and an essentialist assumption. © 2004 Elsevier Inc. All rights reserved. Keywords: Preschoolers; Domain-specific; Living and non-living
1. Introduction The “theory theory” view of cognitive development (Gopnik & Wellman, 1994) specifies that the organization of particular domains of knowledge is based on underlying theories, which integrate and inform that knowledge. On this view, a basic theory for any domain ∗
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outlines the ontology of the domain and provides basic causal devices that operate within that domain. These theoretical constructs explain the evidence that individuals encounter, enabling them to invoke lawful and often abstract justifications for why objects behave as they do, and thus allow for predictions of future behaviors and judgments of category membership. Many causal mechanisms relevant in any given domain remain hidden to all but the most specialized scientists (Wilson & Keil, 2000). Regardless, individuals make assumptions about the existence of those mechanisms and then use those assumptions to guide cognition and behavior. For example, we may not understand enough about thermodynamics to explain exactly why water boils, but we understand that heat is somehow related, and we rely on that knowledge for cooking and washing. Within the biological domain, the focus is on hidden causal mechanisms that evoke particular physical characteristics or behavioral tendencies of living things. For example, what causes a tiger to have stripes or to roar? A basic folk explanation is often articulated as “there is something about tigers” that leads to those characteristics. This is a placeholder assumption: We may not know what is in the placeholder, but we believe that something is in there. This notion of a placeholder may then lead curious individuals—and, certainly, researchers in cognitive development—to search for what the mechanism is. Critically, the placeholder should contain the causal law relevant to the domain and the characteristic in question. With regard to animal behavior, a variety of mechanisms have been proposed to fill the placeholder, including a unique essence, an abstract notion of “insides,” a belief in some type of “vital energy” emanating from within the animal, and specific biological parts (e.g., certain genes, a brain). These concepts are not necessarily independent of one another (e.g., the “insides” of an animal may contain its essence, the internal parts of an animal may generate vital energy, etc.), and each may appear at different times in the development of a theory of biology. We discuss each in turn. 1.1. A unique “essence” Elsewhere we have discussed the nature of the essentialist belief: “People seem to assume that categories of things in the world have a true, underlying nature that imparts category identity. . .. The underlying nature, or category essence, is thought to be the causal mechanism that results in those properties we can see. For example, the essence of tigers causes them to grow as they do—to have stripes, large size, capacity to roar, and so forth” (Gelman, Coley, & Gottfried, 1994). Importantly, people may not know, and may in fact not necessarily care, what the essence comprises; rather, it is represented as an abstract concept. Because of its metaphysical nature, direct evidence of a belief in an essence is difficult if not impossible to obtain. Researchers thus have turned to the more concrete proposals below for empirical data. 1.2. An abstract notion of “insides” Within the domain of living kinds, children may follow an “innards” principle, namely that animals but not artifacts have “something on the inside that governs their movement and change” (Gelman, 1990; Gelman, Durgin, & Kaufman, 1995; Gelman & Williams,
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1998; Gelman et al., 1994; Massey & Gelman, 1988). Adherence to the innards principle does not require that a child know what those insides are; rather, “children may have the abstract expectation that insides are important to the distinction between animals and artifacts without having any specific knowledge of the nature of insides and how they cause phenomenal properties” (Simons & Keil, 1995, p. 139; see also Wellman & Gelman, 1998; Strevens, 2000). Evidence that children rely on an abstract notion of insides when making category judgments is often used to support the claim that children have an essentialist assumption, but the two remain distinct: “Insides” is localized and thus restricted in a way that an understanding of essence need not be. Children are clearly aware of the importance of insides for living kinds. They expect category members to share the same insides (Gelman & Wellman, 1991), and they know that animals cannot function properly without insides (Gelman & Wellman, 1991; Gottfried, Gelman, & Schultz, 1999). However, they do not regularly endorse insides as causing domain-specific behaviors. For example, when asked about whether “insides make [an animal] move,” preschool children do not consistently agree (Gelman & Gottfried, 1996). Nor do they claim that the insides of plants make them grow (Hickling & Gelman, 1995). Thus, children may have only a skeletal notion of the importance of insides rather than a more sophisticated understanding of the causal role of internal mechanisms for biological events. 1.3. Vital energy Vital energy is an example of an immanent cause, one that is somehow generated by and therefore emanates from the living thing itself (see Michotte, 1963). Vital energy itself may be seen as independent of any internal part and referring simply to a notion of life force (Morris, Taplin, & Gelman, 2000; Slaughter & Lyons, 2003), or it may be seen as “caused by activity of an internal organ, which has ‘agency’ or an activity-initiating and sustaining character” (Inagaki & Hatano, 1993, 2002). Even in the latter case, it remains distinct from a more general notion of “insides” in that the energy emanating from the insides, not the insides themselves, is causing the event to occur (i.e., insides may be necessary but are not sufficient). Although preschoolers readily endorse abstract immanent causes (e.g., “moved by itself”; see Gelman & Gottfried, 1996), research on understanding “energy” as a cause of the behaviors of natural kinds is scarce. Some indirect evidence comes from research showing that children rely on vitalistic explanations for behaviors such as eating. For example, when asked to select the best explanation for why we eat, 6-year-olds preferred “our stomach takes in vital power from the food” over “we take the food into our body after its form is changed in the stomach and bowels” (Inagaki & Hatano, 1993; see also Miller & Bartsch, 1997). Moreover, 5-year-olds generally favor vitalistic explanations referring to energy over physiological or intentional causal explanations for biological phenomena (i.e., “what happens when we eat?”; Morris et al., 2000), especially for processes such as growth and movement. On the other hand, these studies did not focus directly on vital power as an abstract causal agent—for example, they did not directly assess whether children believed that vital power (i.e., energy) caused biological processes such as growth.
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1.4. Specific internal parts Recognition that internal parts have specific functions is a key component of a fully developed, scientific theory of biology, and thus it should be late in developing, if it develops at all. Indeed, although preschoolers can identify and label at least some internal parts (see Carey, 1985; Gelman, 1990; Gelman & Gottfried, 1996; Gelman & O’Reilly, 1988), they frequently cite external parts such as eyes or feet as the most important body parts (Gellert, 1962) and have trouble matching internal parts with causally relevant behaviors. Four-year-olds, for example, recognize a link between the brain and mental activity, but only during the elementary school years do children come to understand that the brain is needed for perception and motor behaviors (Johnson & Wellman, 1982) or individualspecific thoughts and memories (Gottfried et al., 1999). Moreover, preschoolers do not accurately identify domain-appropriate insides (e.g., selecting the photograph of the insides of a fetal pig or real, preserved abdominal organs of cats) when asked to find the parts that help the animal to move or have babies (Simons & Keil, 1995). Thus, children may not only lack knowledge of what the relevant causal mechanisms may be; they may not even have expectations for the kinds of insides that may be causally relevant (cf. Au & Romo, 1999).
1.5. Two components of a theory, two studies A coherent theory requires understanding of a distinct ontology as well as domainspecific causal mechanisms, but these two components need not develop along the same trajectory or at the same time. Within the domain of biology, knowledge of causal laws may develop independent of the understanding of the relation between insides and kind, but the trajectory may also reflect an abstract-to-concrete progression. In other words, children may first discover that “something inside” causes an animal to move, without fully understanding what sorts of things are inside animals at all. Study 1 thus investigates whether children have general, somewhat abstract expectations for the types of things that are inside different kinds of objects. On the theory that children move from an abstract understanding to a concrete understanding of biological causal agents (cf. Simons & Keil, 1995), we predict a developmental trajectory in which children endorse an abstract notion of “insides” as causally relevant to the behavior of living things prior to their endorsement of specific internal parts. The relation between “insides” and “energy” in the trajectory is less clear. If “vital energy” is conceptualized by children as arising from insides, even if the specific internal parts are unknown, endorsement of it as a cause should coincide with or follow endorsement of “insides.” On the other hand, “energy” may be a more abstract concept, free from a notion of insides; endorsement of energy would therefore precede endorsement of both “insides” and internal parts. To identify the developmental pattern, Study 2 contrasts these three causal mechanisms—“its own energy,” “its insides,” and specific internal parts—in a between-subjects design. Together, the studies examine the scope of the causal-explanatory framework on which an early na¨ıve theory of biology may be based.
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2. Study 1 Study 1 addresses the nature of preschoolers’ expectations about domain-specific internal parts. We tested the hypothesis that preschoolers have abstract expectations for “insides” by asking them to identify the insides of objects for which the domain is clear but specific concrete knowledge is lacking (e.g., unknown animal, unfamiliar machine). If children have abstract expectations that do not rely on concrete knowledge, they should readily distinguish domains when presented with unfamiliar items, as long as they are able to identify the appropriate domain. In contrast, if children’s grasp of insides rests on concrete knowledge, they should perform poorly. In the experiment, preschool children were asked to select the appropriate insides for unfamiliar animals, machines, and plants. Plants were included because little research has investigated children’s knowledge of the insides of plants (but see Hickling & Gelman, 1995), perhaps because plants are importantly different from both animals and machines in that they do not have internal “parts” in the sense of clearly defined internal organs or mechanisms. Abstract notions may therefore be more readily accessible. Given the findings that children generally are quite good at providing verbal lists of internal parts (Gelman, 1990; Gelman & Gottfried, 1996) but relatively poor at visually identifying internal parts of animals and machines (Simons & Keil, 1995), we hypothesized that children’s knowledge of appropriate labels for insides may mask their erroneous conceptualizations about the referents for those labels. On the other hand, however, children may have had difficulty recognizing the specific internal parts used in Simons and Keil’s study—computer-drawn insides, photos of the “insides of a fetal pig,” and preserved feline abdominal organs suspended in a gelatin-like substance. To reduce the stimulus complexity, the visual stimuli in Study 1 thus included realistic color photographs of specific internal parts (e.g., heart) taken from an encyclopedic book for children. 2.1. Method 2.1.1. Participants Participants were sixteen 3-year-olds (6 boys, 10 girls; mean age 3.5; range 3.0–3.11), sixteen 4-year-olds (8 boys, 8 girls; mean age 4.5; range 4.2–4.11), and sixteen 5-yearolds (7 boys, 9 girls; mean age 5.5; range 5.0–5.10), enrolled in university preschools in a middle-class, Midwestern city. Ten additional children (mean age 5.5) and 13 college students participated in pretesting of the materials. 2.1.2. Materials Twelve target items were selected on the basis of a pilot test in which 10 children were shown photos or realistic pictures of 10 animals, 10 plants, and 10 machines taken from children’s books and adult mail-order catalogs. Eight of the objects from each domain were expected to be unfamiliar, and two were familiar (e.g., turtle, camera). The experimenter asked each child, “Is this an animal, a plant, or a machine? [Then, after they responded:] Do you know what kind of [animal/plant/machine]?” The order of the pictures was randomly determined for each child, and the choices were counterbalanced. The final target set included four animals (eland, tapir, pacarana, cavy), four plants (fern, moss, water lily,
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liverwort), and four machines (espresso maker, intercom, mini-TV, electric razor). All were accurately recognized as animals, plants, and machines (with one exception: A child called a machine a plant but then went on to label it a stove) but were incorrectly labeled at the basic level by nine or more of the children. The match-to-sample items included four color photographs of animal insides (i.e., brain, bones, muscles, heart), four of plant insides (i.e., cross-section of banana plant, cellular structure of blade of grass, cross-section of wood, plant cells), and four of machine insides (i.e., circuit board, batteries, wires, gears). The pictures were realistic in detail, taken from children’s books that discussed internal parts of objects and animals, and were mounted on 4 × 6 cards. These items were selected based on results from a pencil-and-paper pilot test including 13 college-aged volunteers asked to indicate on a 7-point scale (1 = not at all likely; 7 = very likely) whether this part would be inside a plant, animal, and a machine. Mean likelihood scores for the four selected pictures from each domain ranged from 6.1 to 6.9: the mean score for animals was 6.78 (S.D. = .50), for machines 6.70 (S.D. = .72), and for plants 6.41 (S.D. = 1.11). 2.1.3. Procedure Children were tested individually in a separate room apart from their regular classroom. Each child was randomly assigned to one of the two conditions (verbal or pictorial). In both conditions, the children were shown the pictures in a randomly determined order and were asked two questions about each. Children were first asked, “What’s inside of this animal (plant or machine)?” If they failed to respond, they were prompted: “Can you tell me what parts it has inside?” They were then introduced to a puppet and were told, “Now Sam [the puppet] has some choices. What else is inside?” In the verbal condition, the experimenter manipulated the puppet as if to speak, providing one animal part (either brain, muscles, bones, or heart), one plant part (seeds, juice, water, or wood), and one machine part (motor, wires, batteries, or gears). The choices were all internal parts provided by preschoolers in previous research (Gelman, 1990; Gelman & O’Reilly, 1988). In the pictorial condition, the experimenter showed the child three pictures of insides (one plant, one animal, one machine) mounted on cardboard and manipulated the puppet to ask, “Is it this, or this, or this [pointing to each choice in turn]?” The order of the choices was counterbalanced across items. 2.2. Results To analyze children’s answers to the open-ended question (i.e., “What’s inside of this?”), two judges coded each response as to whether it represented domain-appropriate internal parts of the object (e.g., bones, juice, batteries) or another type of response. Inter-rater reliability was 95%. Totals for the internal parts category were then compiled, ranging from 0 to 4. A 3 (age: 3, 4, 5) × 2 (condition: verbal, pictorial) × 3 (domain: animal, plant, machine) repeated-measures ANOVA on the scores for appropriate internal part answers (see Table 1) showed the predicted interaction of age and domain, F(4,84) = 5.10, p < .001. Fisher LSD tests confirm age effects for the animals and for the machines: For the animals, 4- and 5-yearolds provided appropriate internal parts significantly more often than did the 3-year-olds,
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Table 1 Mean number of domain-specific internal parts spontaneously mentioned as inside animals, plants, and machines Animals
Plants
Machines
Verbal 3-year-olds 4-year-olds 5-year-olds
2.00 (1.31) 3.75 (.46) 3.13 (1.64)
2.25 (1.39) 1.63 (1.30) 2.13 (1.36)
2.25 (1.67) 1.75 (1.16) 3.25 (1.16)
Pictorial 3-year-olds 4-year-olds 5-year-olds
0.13 (.35) 3.00 (1.85) 2.63 (1.77)
1.00 (1.41) 1.25 (1.39) 0.75 (1.39)
0.88 (1.46) 2.50 (1.20) 2.63 (1.92)
Note: Scores could range from 0 to 4. Standard deviations are in parentheses. Because children could produce any answers, no “chance” performance is computed.
ps < .001. For the machines, 5-year-olds provided appropriate internal parts significantly more than the 3-year-olds did (p < .02). No age differences were found in children’s responses when asked about plant insides. Main effects of each variable also emerged. First, the 3-year-olds provided significantly fewer appropriate internal parts than did the 4- and 5-year-olds, who did not differ from one another, F(2,42) = 5.67, p < .007. Second, children were less likely to provide appropriate internal parts for plants than they were to provide appropriate internal parts for animals or machines (F(2,84) = 6.87, p < .002; Fisher LSD, ps < .01). Finally, children in the verbal condition produced more appropriate internal parts than children in the pictorial condition, F(1,42) = 9.45, p < .004. Post-hoc analysis suggests that the effect of condition was due to the children in the verbal condition repeating choices they had heard during the forced-choice questioning for previous items (e.g., heart, bones, seeds, juice, wires, batteries). These children fairly frequently named internal parts that Sam had previously provided for that domain, and they named those specific parts more often in the verbal condition (for the animals 55% of responses in the verbal condition versus 37% in the pictorial condition; for plants 49% versus 24%; for machines, 63% versus 45%). A second ANOVA on the data with these answers removed eliminated any effect of condition. Furthermore, children’s error patterns show that children rarely provided one of Sam’s choices for the wrong domain; there were only six such errors in the verbal condition (one for animals, five for machines) and two in the pictorial condition. In other words, children in the verbal condition were appropriately selective, not arbitrary, when they provided an internal part previously suggested by Sam. Table 2 shows children’s correct selection of insides on the forced-choice question. A 2 (condition: verbal, pictorial) × 3 (age: 3, 4, 5) × 3 (domain: animal, plant, machine) repeated-measures ANOVA on these data also showed effects of age, F(2,42) = 15.38, p < .001, and domain, F(2,84) = 7.08, p < .002. The 3-year-olds were significantly less likely to select the correct insides than were the 4- and 5-year-old children (ps < .001), and across age, children were less likely to select the correct plant insides than they were to select correct animal (p < .001) or machine (p < .05) insides. There were no systematic patterns to children’s errors.
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Table 2 Number of correct selections of insides in the forced-choice questions Animals
Plants
Machines
Verbal 3-year-olds 4-year-olds 5-year-olds
2.13 (1.46) 3.50 (.53) 3.75 (.46)
1.75 (1.04) 2.25 (.89) 2.88 (1.13)
2.50 (1.20) 3.13 (.83) 3.00 (.93)
Pictorial 3-year-olds 4-year-olds 5-year-olds
1.75 (.89) 3.50 (.53) 3.50 (.53)
1.38 (1.06) 3.00 (.93) 3.13 (.83)
1.88 (1.36) 3.13 (.83) 3.00 (.93)
Overall 3-year-olds 4-year-olds 5-year-olds
1.94 (1.18)+ 3.50 (.52)** 3.63 (.62)**
1.56 (1.03) 2.63 (.96)** 3.00 (1.03)**
2.19 (1.28)* 3.13 (.81)** 3.00 (1.16)**
Note: Scores could range from 0 to 4. Standard deviations are in parentheses. Because no condition difference was found, t tests versus chance (score of 1.33) were computed on the overall scores. + Significance level, p < .10. ∗ Significance level, p < .05. ∗∗ Significance level, p < .001.
Because we found no significant effects involving condition in this analysis, we collapsed the data across this variable and conducted single-mean t tests comparing the children’s scores to chance performance, a score of 1.33. Four- and 5-year-olds responded at abovechance levels for all domains, p < .001. Three-year-olds were above chance only in selecting the machine insides, p < .02, but showed a trend toward selecting the appropriate animal insides as well, p < .06. 2.3. Discussion The goal of Study 1 was to test whether preschoolers display an abstract understanding of the insides of objects, and furthermore whether such an understanding develops in a domainspecific way. The results suggest that at least by age 4, children had general expectations of the sorts of “insides” that animals, plants, and machines may have, with 3-year-olds not far behind. How do 3- and 4-year-olds know what is inside of animals and machines? To succeed on the verbal task requires children to have specific knowledge: they cannot simply infer that certain words go with animals whereas others go with machines. Acquiring this knowledge so early in development shows attentiveness to environmental evidence about insides from an early age (i.e., even 3-year-olds have enough interest in insides to note and remember this information). Furthermore, these particular animals, plants, and machines were unfamiliar, and therefore some degree of conceptual abstraction or generality was needed to perform as well as they did. Even stronger evidence of children’s early understanding is the accuracy with which children selected pictures of domain-specific insides in the forced-choice task. Without any prompting or information regarding the appearance of things that might be “inside”
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animals, plants, or machines, children’s choices reflected their expectations about the nature of those insides. Compared to the verbal task, where children must have learned specific words in the past, it seems less likely that children had familiarity with these particular images. Rather, children may have been using low-level perceptual cues that differentiate domains at an abstract level, such as sharpness of contour, presence of right angles, or texture cues. For example, just as animals and artifacts can be distinguished on this basis (Smith & Heise, 1992), children may assume that the insides of animals and artifacts can as well. Some of these cues may be innate or early-emerging to enable a rough parsing of environment into animate versus inanimate or natural versus human-made. What is of interest is that these cues then get extended to novel exemplars of the categories in question. Critically, children at each age performed equally well (or equally poorly) on the visual and the verbal forced-choice tasks. This finding is particularly noteworthy because the input to each should vary. For example, children hear the word “heart” presumably much earlier and more frequently than they see an image of an actual heart, and although they may have seen inside machines, which can easily be disassembled, they are less likely to have direct experience with the insides of animals. Yes these very different media get mapped appropriately onto ontological domain at roughly the same age. This finding suggests a broad capacity to appreciate that insides are domain-specific: Children have at least a rudimentary expectation regarding the appearance of the “insides” of animals and machines from an early age. Our findings on this point differ from those of Simons and Keil (1995), who argued that expectations about the appearance or substance of internal parts of animals generally develop between ages 5 and 8. We suggest that, in their work, children’s emerging knowledge may have been masked slightly by the unfamiliar presentation of the “insides” choices in that study (e.g., preserved animal parts suspended in a gelatinous substance) as well as by salient distractors that may have been hard for preschoolers to identify (e.g., rocks suspended in gelatin). Even so, regardless of the age of onset, the developmental pattern is replicated, with a general understanding of the nature of insides in place despite a lack of concrete knowledge about the items in question. Finally, understanding of internal parts seems to develop in a domain-specific way, with expectations about plants developing later. That children had fewer expectations about plant insides than about machine or animal insides may reflect the fact that plants have less distinct internal parts; it may also reflect that preschool children lack knowledge about plants in general. For example, they frequently deny that plants are alive (Carey, 1985; Piaget, 1929; Richards & Siegler, 1986; Stavy & Wax, 1989) and do not understand the cyclical nature of plant growth and reproduction (Hickling & Gelman, 1995). Although it is possible that the 4- and 5-year-old children’s accuracy on this task resulted from their rejecting the animal and machine parts rather than having any knowledge about plant insides, even an exclusionary selection technique minimally suggests an expectation that the animal or machine parts were not appropriately found inside plants. Overall, the results of Study 1 demonstrate that preschoolers can successfully infer the insides of a set of items they have never seen before, knowing simply that each is an animal, plant, or machine. This result suggests that the expectations children hold about the substance of internal parts are abstractly tethered to their knowledge of the domain.
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3. Study 2 To credit children with a distinct theory of biology requires that they can accurately link biological causal agents with biological processes while also denying that biological agents cause similar behaviors in non-living things. Study 2 thus focuses specifically on children’s emerging understanding of the role of insides for the behavior of animals, as contrasted with machines. The study provides a systematic test of children’s understanding of the causal role of “insides.” Three types of internal causes, representing three levels of abstraction, were evaluated. The most concrete level specified individual parts (e.g., brain and muscles; motor and batteries) as causal agents (Simons & Keil, 1995, Study 4). The middle level of abstraction specified “insides” abstractly as causal agents (Gelman & Gottfried, 1996; Gelman & Kremer, 1991; Hickling & Gelman, 1995). The most abstract level involved asking whether the object used “its own energy” to engage the behavior (Inagaki & Hatano, 1993; Morris et al., 2000). For all three levels of abstraction, we used the wording, “Does it use its own X [e.g., insides] to Y [e.g., move]?” rather than “Do its X make it Y?”, to allow the possibility that the animal or machine is the agent of its own behavior. In contrast, “Do its X make it Y?” seems to suggest that the animal or machine is a passive recipient, which could bias participants against endorsing animate choices. Thus, the wording we used was intended to be neutral with regard to domain (plausible for either animals or machines). In the Discussion, we return to the issue of how the wording may have influenced the patterns of responses we obtained. 3.1. Method 3.1.1. Participants Participants in the experiment included 43 preschoolers (range 4.0–5.4, mean age 4.9), 30 second graders (mean age 7.11; range 7.4–8.11), and 35 college student volunteers. The youngest children were enrolled in racially mixed preschools serving a professional, uppermiddle-class population in a large urban environment; the older children were enrolled in public or private elementary schools in the same areas. Two additional adults were dismissed for not completing the experiment or for answering both Yes and No to every question, and one adult and two preschoolers were dismissed due to experimenter error in recording the data. 3.1.2. Materials The materials included a subset of the color photographs from Study 1. One animal, one machine, and one plant picture were used in a warm-up task, described below. Test items included the remaining three animal pictures (tapir, cavy, and eland) and three machine pictures (intercom, mini-TV, and electric razor). The set of three plants were included but were not analyzed further, as children’s limited knowledge about the insides of plants (shown in Experiment 1) should be a counfounding factor in their endorsement of cause. Each domain of items was associated with three properties. Properties of animals included move, sit still, and grow. Movement and growth were selected because they are
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salient and characteristic behaviors of animals (Inagaki & Hatano, 2002; Richards & Siegler, 1986), Rosengren, Gelman, Kalish, & McCormick, 1991). We predicted that adults would endorse internal parts, “insides,” and energy when explaining both movement and growth. We included sitting still as the contrast to movement—the animal is not doing anything—and predicted that adults would say that animals do not use their parts, insides, or energy to sit still. For the machines, the properties were move, sit still, and make little beads. Machine movement is highly similar to animal movement in being potentially self-sustaining (Gelman & Gottfried, 1996; Gelman & Kremer, 1991) and is often linked to complex internal parts that contribute to the movement; we thus predicted adults would endorse all three internal causes for machine movement. Further, we predicted that adults would not endorse any internal cause for sitting still. We included “making beads” as an approximate parallel for growth (hereafter “production”). Nothing in the artifact domain fully corresponds to growth in the sense of regular transformations that are internally driven, and unlike growth, artifact production is not an inherent or internally mediated process (cf. Backscheider, Shatz, & Gelman, 1993). In fact, this is one of the distinctive aspects of living things. However, “making beads” involves a creation process in which something new is generated, somewhat akin to growth. Although none of the machines in actuality produces beads, they were all unfamiliar to children and thus we were able to control for children’s reporting previous knowledge. 3.1.3. Procedure All children first participated in the warm-up task designed to encourage each child to say both yes and no, to help ensure against a response bias. They were shown the picture of a pacarana (i.e., a mammal) and were asked (one question at a time), “Is this a plant? What is it? Can this animal run fast? Is this animal made out of metal?” They then saw the picture of a liverwort and were asked, “Is this a machine? What is it? Can this plant run fast? Is this plant made out of metal?” Finally, they saw a picture of an unusual espresso maker and were asked, “Is this an animal? What is it? Can this machine run fast? Is this machine made out of metal?” Children were corrected if they gave an incorrect answer. Children were then randomly assigned to one of three conditions: the Parts condition, the Insides condition, or the Energy condition. In each condition, the children saw the six target pictures, presented in random order for each participant. Each picture was paired with two of the three properties, to reduce the length of the procedure. For example, one animal was associated with movement and growth, one was associated with movement and sitting still, and one was associated with growth and sitting still. Thus, each child saw a total of six pictures, three per domain, and was asked a total of 12 target questions (i.e., six for each domain—two questions about sitting still for animals and two for machines, two questions about moving for animals and two for machines, two questions about growth for animals, and two questions about production for machines). Following presentation of each property, the participants answered a question about the cause of the characteristic described. In the Parts condition, the question for animals was, “Does it use its own brain and muscles and stuff like that to X [e.g., move]?” and for machines, “Does it use its own motor and batteries to X?” In the Insides condition,
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the question was always, “Does it use its own insides to X?” In the Energy condition, the question was always, “Does it use its own energy to X?” For example, on one trial participants saw a picture of a cavy and heard, “This animal can move. Does it use its own insides to move?” For each yes or no answer, children were asked to explain why or why not. Adults participated in a paper-and-pencil task including a larger set of stimuli and questions. Only the conditions that correspond to those that the children received are reported here. 3.2. Results The mean percentages of yes responses for each of the question types were compiled and analyzed with a 3 (age: 4, 8, adult) × 3 (condition: parts, insides, energy) × 2 (domain: animals, machines) × 3 (question movement, sitting still, growth/production) repeatedmeasures ANOVA involving the complete design of the study, from which the following findings were generated. As predicted, a domain difference appeared, with participants endorsing the internal causes more for animals than for machines, F(1,99) = 100.64, p < .001. Furthermore, this difference increased linearly with age, F(2,99) = 20.37, p < .001, with the 4-year-olds not showing a significant difference (See Fig. 1). Additionally, t tests comparing these scores to a conservative estimate of chance (3, or 50% yes response) show that participants at all ages endorsed the internal causes when asked about the behavior of animals (t(42) = 3.68 for the 4-year-olds, t(29) = 7.15 for the 8-year-olds, and t(34) = 9.61 for the adults, ps < .001).
Fig. 1. Mean number of causal attributions for animals and machines. Significant domain × age interaction.
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Fig. 2. Mean number of causal attributions for animals and machines. Significant domain × condition interaction.
However, the 4-year-olds also showed a trend to endorse the internal causes when asked about the behaviors of machines, t(42) = 1.82, p < .08, whereas the adults consistently denied that the machines used internal causes to engage in the behaviors, t(34) = −3.94, p < .001. Critical to the primary focus of the study, the domain difference also increased as the level of abstraction increased, F(2,99) = 7.80, p = .001 (see Fig. 2) and was particularly clear in the energy condition. Participants endorsed energy at above-chance levels only for animals, t(37) = 10.4, p < .001. Furthermore, this effect was superseded by a 3-way interaction involving domain, age, and condition, F(4,99) = 2.54, p < .05. Planned comparisons revealed that the adults showed marked domain differences in each condition (ps < .03), endorsing internal parts, insides, and energy more for animal behaviors than for machine behaviors. The 8-year-olds endorsed insides and energy significantly more for animals than machines (ps < .03) and showed a trend in the same direction when asked about specific parts (p = .07). The 4-year-olds showed a significant domain difference only when asked if the object used its own energy, endorsing energy as an internal cause only when asked about animals (p < .01, see Fig. 3). Finally, a domain × age × question interaction also emerged, F(4,198) = 2.8, p < .03 (see Fig. 4). In particular, adults endorsed internal causes more for animals than machines for movement, sitting still, and growth/production (ps < .03), whereas 8-year-olds showed similar differences only when asked how the objects move or sit still (ps < .001), and the 4year-olds showed domain differences only when asked about internal causes for movement (p < .001).
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Fig. 3. Mean number of causal attributions for animals and machines. Significant domain × age × condition interaction.
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Fig. 4. Mean number of causal attributions for animals and machines. Significant domain × age × question interaction.
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3.3. Discussion The data from Study 2 suggest a pattern of developmental change in children’s understanding of the role of internal causes for the observable behaviors of animals and machines. Most notably, preschoolers do not consistently differentiate among domains with regard to internal causes, whereas grade-school children (i.e., by age 8) attribute internal causes to animals but not machines. On first glance, it appears that the 4-year-olds in this study make no distinction between internal causes relevant for animals and those relevant for machines: When the data are analyzed across the three conditions (i.e., parts, “insides,” energy), the children seem to have attributed the internal causes equally to each domain. However, the level of specificity of the question appears critical. In particular, the preschoolers showed distinct patterns when asked about whether animals and machines used their “own energy” to move, regenerate, and sit still. This finding is consistent with those of Gelman and Gottfried (1996) as well as Massey and Gelman (1988), who found that preschoolers readily claimed that animals, but not machines, can move “by themselves.” Furthermore, the results support those of Morris et al. (2000), who found that more than 80% of 5-year-old children endorsed vitalistic explanations referring to energy when asked to explain growth and movement. Together, these studies suggest that children have an abstract understanding that animals have an immanent or inherent causal mechanism necessary for external behaviors like movement and growth, one which may be characterized generally as “its own energy.” The data do not, however, clarify what “its own energy” actually is—a distinct mechanism (e.g., possibly an animate or a psychological force, or something akin to electricity) or simply a placeholder term for an as-yet-undiscovered mechanism (see also Morris et al., 2000). In contrast to children’s endorsement of “energy” selectively for animals, no domain distinction appeared in preschoolers’ responses to questions about whether animals and machines used their “own insides” for the given behaviors. This finding suggests that the preschool children do not equate the inherent causal mechanism with the term “insides” and perhaps may not localize the force as necessarily internal to the animal. That children fail to treat “insides” as causal in this condition is consistent with previous findings that preschoolers report that machines use their “insides” to move more often than they report that animals use their “insides” to move (Gelman & Gottfried, 1996; Gelman & Kremer, 1991). The data on the movement question of the present study are particularly relevant to this issue. On this question, the 4-year-olds were significantly more likely to endorse internal causes for animals than for machines. For this most typical animate property (Richards & Siegler, 1986), children differentiate causes relevant for animals from those relevant for artifacts by age 4 years. This finding should not be surprising in light of the task question: One can argue more readily that animals “use” their insides (in terms of bones, muscles, etc.) to move more than to grow, for example. The link between the behavior and the cause is more direct and likely more familiar. Again, this finding raises questions about the ways in which children operationalize “insides” when asked to endorse causal explanations of this sort. On the other hand, children may not consistently be interpreting “insides” as referring to particular internal parts, or at least not the internal parts selected in this task. Because
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the results of Study 1 suggest general awareness of the domain-specific nature of internal parts, the results of Study 2 suggest that although preschool children have a knowledge base from which to work, they do not consistently rely on this knowledge of internal parts when explaining cross-domain behaviors such as production or movement. By age 8, in contrast, children show a tendency toward endorsing specific internal parts as causally relevant for these behaviors. As noted previously, children may have a broad capacity to appreciate that internal parts are domain-specific but not yet have a more concrete understanding of the ways in which internal parts serve mechanistically to cause observable behaviors. The findings suggest a developmental trajectory from an abstract understanding that some cause must exist to a more concrete understanding of how particular internal parts of animals or machines affect behaviors, but they leave open a critical question: How do children come to learn, by age 8, that brain/muscles, insides, and energy are causal for animals, but motor/batteries, insides, energy are less causal for machines? For animals, additional biological knowledge may be necessary. Certainly, the history of science shows developmental change in our understanding of the biological role of the heart, brain, and other parts—we no longer believe, for example, that the heart is a direct cause of emotions. Furthermore, Slaughter and colleagues have found that as body-part knowledge increases, children’s vitalistic explanations that these parts are needed “for life” also increase (Jaakkola & Slaughter, 2002; Slaughter, Jaakkola, & Carey, 1999; Slaughter & Lyons, 2003). However, cognitive development also clearly plays a role: Johnson and Wellman (1982) found that grade-school children claimed the brain was unnecessary for certain sensory processes despite having recently completed a series of lessons focusing specifically on that topic. Thus, accommodation and theory change come into play. For machines, the relation is less direct. Whether or not adults endorse causes for machines is determined by a complex interaction between the kind of insides and the property under consideration. For example, adults are most likely to endorse internal cause for machines when particular internal parts (e.g., motor, batteries) are specified and when the property entails activity (e.g., moving or manufacturing). When the property is “sitting still” or when the cause is abstract (i.e., “energy”), adults are least likely to endorse internal cause for machines. This finding suggests two things. First, machines are viewed as less likely than animals to have their own self-contained energy, and second, animals, but not machines, require internal cause simply for continued existence (i.e., without activity). Thus, we do not see an absolute domain difference in the role of internal causes, but rather a modulation of the relation between internal parts and external behaviors, based on the ontological differences between items that contain their own energy sources and require energy continuously (i.e., animals) and those that do not (i.e., machines, for the most part). Responses of the 8-year-olds followed the same pattern as those of the adults with somewhat less differentiation between domains, but the 4-year-olds generally responded at chance when asked about internal causes of machine movement, production, and sitting still. Given the likelihood of children’s greater familiarity with specific machine insides as compared to internal animal parts, this finding suggests again that knowing what is inside a particular kind of machine or animal is separate from or perhaps a first step toward understanding the causal roles of those internal parts. One caveat is in order, regarding how the wording may have influenced the results that were obtained. The wording was intended to convey that the insides are part of the object
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(“Does it use its own [insides] to. . .?”), and that the insides do not have their own agency (in contrast to, e.g., “Do its [insides] make it. . .?”). Although the wording was intended to provide a neutral test of the causal force of insides, it may have favored an animate interpretation. Specifically, the wording may have implied that the object in question was intentional: A “yes” response may have implied that the object chose to use the insides in order to achieve a goal. If this is the case, then a domain effect would likely result—not necessarily because insides are seen as more causally efficacious for animals than machines but rather because animals are seen as more intentional than machines. However, this interpretation is unlikely to account fully for the results, because for both children and adults, we did not find an overall animacy effect. Instead, the results varied systematically as a function of the property and condition. Nonetheless, it will be important in future work to examine potential subtle implications of wording that might influence the patterns of endorsement. To conclude, the findings of Study 2 suggest that referring to children’s early understanding of the causes of biological phenomena as a domain-specific “innards” principle (Gelman, 1990) may be inaccurate. The preschoolers’ failure to consistently endorse “insides” or specific parts (despite being able to accurately label and match to category) for animals over machines but endorsement of “energy” suggests an early causal understanding best conceptualized as “immanent cause,” as we have argued elsewhere (Gelman & Gottfried, 1996).
4. General discussion Natural kind concepts, and animals in particular, are interesting to researchers because they appear to comprise a structured domain that has been argued to be universally available, consistently organized, theory laden, ontologically distinct, and foundational (Inagaki & Hatano, 2002; Wellman & Gelman, 1998). They clearly have rich inductive potential; children and adults alike behave as if category membership confers important, immutable properties: Tigers have stripes, ferocity, etc., because they are tigers. Elsewhere, we have argued that this behavior appears consistent with a belief in an essence as a causal mechanism: There is something essential about tigers that leads to the outward behavioral and physical properties (Gelman et al., 1994). The question, then, is what this assumption entails. We suggest that early in development, children hold an abstract understanding that “something essential” is intrinsic to animals themselves. This is effectively a placeholder understanding (see Strevens, 2000): Children recognize the need for an inherent causal mechanism for behaviors of living things but lack specificity as to what that mechanism is. Theory change occurs as individuals gain a concrete knowledge base with which to fill the placeholder. It is clear that young children already have impressive knowledge about the categories of plants, animals, and machines. Not only can preschoolers distinguish between animals, plants, and machines, but by age 4, they display clear expectations for the kinds of things that are inside each kind of object (Study 1). They accurately report that things like gears and batteries are inside machines whereas hearts and bones are inside animals; to a lesser extent they report that seeds and juices are inside plants. Their correct identification of pictures of those insides suggests that they are not simply repeating labels associated with
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specific concepts. Increased knowledge may account for the rapid change between ages 3 and 4, but as discussed previously, changes in underlying cognitive abilities may occur at this time as well. Although preschool children know what the internal parts of animals are, they do not seem to understand what they do: Study 2 shows they do not consistently endorse internal parts such as bones and muscles when explaining behaviors such as growth and movement. Rather, they are most likely to endorse some type of vital energy as causally relevant for animals but not for machines and even claim that animals use “their own energy” when sitting still (cf. Gelman & Gottfried, 1996). This finding suggests a rather abstract, placeholder explanation, as children cannot directly perceive energy, which is not a concrete entity. Our results do not rule out the possibility that children construe “energy” as either a physical or a psychological, intentional force (Inagaki & Hatano, 1993; Morris et al., 2000; Slaughter & Lyons, 2003). However, they do not appear to construe it as an internal physical quality, as they reject “its own insides” as a causal explanation. This finding suggests that an animal’s “energy” is seen as independent of any internal part. We suggest children’s theory of living kinds, and animals in particular, develops along a specific developmental pattern—children’s early expectation that animals behave in accordance with their category identity combines with a belief that events are caused (causal determinism; Brown, 1990; Bullock, Gelman, & Baillargeon, 1982; Macnamara, 1986). With experience, this expectation becomes more refined: Something inherent causes that behavior. A placeholder is thus represented as children recognize that some mechanism is necessary (i.e., essential) to evoke the specific behavior and also that it is inherent in the organism itself (i.e., immanent; cf. Gelman, 1990; Gelman & Gottfried, 1996). Without concrete knowledge of causal links, something abstract goes inside the placeholder—energy, a kind of vital force—as children reject “its insides” or specific internal parts as causal mechanisms. As more concrete understanding of internal parts develops, perhaps through schooling, a truly biological understanding of cause should arise: An animal’s “insides,” such as its bones, brain, and muscles, cause it to behave in particular ways. One issue raised by these findings is how to characterize the relationship between na¨ıve biology and essentialism. Children’s placeholder understanding suggests that essentialism does not require or build on scientific biological knowledge. Can we therefore assume that essentializing of living kinds is not biological? The answer to this question depends in part on what one means by “biological.” If “biological” means that one understands the correct mechanisms underlying biological processes such as growth, illness, or contagion, then preschool children fall far short (Au & Romo, 1999; Carey, 1985; Solomon et al., 1996). However, if an initial biological theory is construed to mean one in which there is a realm that is distinct from human activities or mental states and that is specific to living things, then the data largely support this understanding. Furthermore, na¨ıve biology possesses at least three essentialist elements: (a) biological processes are natural (outside the realm of human control); (b) biological properties are inherent (inborn, and located within the individual); and (c) pursuit of biology entails a search for unobservables, including causally powerful unobservables. In all three of these respects, there is a dovetailing of na¨ıve folk essentialist beliefs and biology as a science (see also Gelman, 2003; Inagaki & Hatano, 2002). At the same time, a true biological account of species differs from an essentialist account of species in being population based rather than individual based. For example, species
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differences are determined by interbreeding populations, not by inherent properties of individual members of the species (Sober, 1994). Furthermore, biological categories evolve and change over time, in contrast to the essentialist assumption that categories are fixed and unchanging (Mayr, 1991). This analysis has implications for the ongoing debate regarding whether children undergo theory enrichment or theory change in the domain of biology (e.g., Inagaki & Hatano, 2002; Wellman & Gelman, 1998). The present studies suggest that several core tenets of a folkbiological theory are firmly in place by the preschool years, including that animal processes are natural, inherent, and non-obvious. In this sense, folk biology appears to be a domain distinct from both folk psychology and folk physics, by an early age. Over developmental time, folk biology undergoes enrichment in the fleshing out this framework theory, but the fundamental structure does not change in childhood. Where we sometimes see genuine theory change is in the adoption of Darwinian evolutionary theory, though even here many adults may never fully drop their essentialist assumptions that contradict evolution. To summarize, although children may not have an adult-like theory of biology, they appropriately distinguish between animals and other sorts of objects and have a general understanding that animals differ from inanimate things in terms of the causes of their behaviors. It may be said, then, that children’s early theory of biology is organized around the animate-inanimate distinction, with an abstract causal understanding (i.e., energy) endorsed appropriately only for animals. A more specific, biological-mechanistic link between insides and behavioral outcomes (e.g., how insides cause an animal to grow) may develop as concrete knowledge of the specific parts increases.
Acknowledgments The project was supported by NSF Grant 910034, NIH Grant HD36043, and a J.S. Guggenheim Fellowship to Susan Gelman, as well as a University of Michigan Summer Research Opportunity Program fellowship to Sonia Anglade. We thank Sonia Anglade, Rebecca Caldwell, Lori Akers, Carson McCann, Amy Field, Gabe Niles, and Kristen Pezone for research assistance at various stages of the project. Special thanks are also due to the parents, children, and staff at the Child Education Center at the Jet Propulsion Lab, Eagle Rock Elementary School, Mayfield Junior School, and the University of Michigan Children’s Center. Address written correspondence to Susan Gelman at Department of Psychology, 525 E. University Ave., University of Michigan, Ann Arbor, MI 48109-1109 (email
[email protected] or
[email protected]).
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