Abstract One of the greatest challenges of developmental psychology is figuring out what children are thinking. This is particularly difficult in early childhood, for children who are prelinguistic or are just beginning to speak their first words. In this stage, children’s responses are commonly measured by presenting young children with a limited choice between one of a small number of options (e.g., “Do you want X or Y?”). A tendency to choose one response in these tasks may be taken as an indication of a child’s preference or understanding. Adults’ responses are known to exhibit order biases when they are asked questions. The current set of experiments looks into the following question: do children demonstrate response biases? Together, we show that 1) toddlers demonstrate a robust verbal recency bias when asked “or” questions in a lab-based task and a naturalistic corpus of caretaker-child speech interactions, 2) the recency bias weakens with age, and 3) the recency bias strengthens as the syllable-length of the choices gets longer. Taken together, these results indicate that children show a different type of response bias than adults, recency instead of primacy. Further, the results may suggest that this bias stems from increased constraints on children’s working memory.

Citation: Sumner E, DeAngelis E, Hyatt M, Goodman N, Kidd C (2019) Cake or broccoli? Recency biases children’s verbal responses. PLoS ONE 14(6): e0217207. https://doi.org/10.1371/journal.pone.0217207 Editor: Vera Kempe, Abertay University, UNITED KINGDOM Received: October 16, 2018; Accepted: May 7, 2019; Published: June 12, 2019 Copyright: © 2019 Sumner et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Data Availability: All of the data can be found on the Open Science Framework at this link: https://osf.io/yub7c/. Funding: This work was supported by the Jacobs Foundation Research Fellowship awarded to CK (https://jacobsfoundation.org/en/activity/jacobs-foundation-research-fellowship-program/) and the University of Rochester Bilski-Mayer Fellowship awarded to ES (http://www.sas.rochester.edu/bcs/undergraduate/independent.html). The funders did not play any role in the study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing interests: The authors have declared that no competing interests exist.

Introduction One of the greatest challenges of developmental psychology is figuring out what children are thinking. This is particularly difficult in early childhood, for children who are prelinguistic or are just beginning to speak their first words. In this stage, children’s responses are commonly measured by presenting young children with a limited choice between one of a small number of options (e.g., “Do you want X or Y?”). For example, in classical Piagetian conservation tasks [1], children are asked verbally “Does this one have more juice, does this one have more juice, or are they both the same?” A tendency to choose one response in these tasks may be taken as an indication of a child’s preference or understanding. Adults’ responses are known to exhibit order biases when they are asked questions. For example, adults who are asked to recall items from a list are more likely to remember the first and last items (primacy and recency biases, respectively)[2–4]. Adults have a tendency to choose the first of two presented options (e.g., they choose “heads” when asked, “heads or tails?”)[5,6]. These biases, however, have gone largely unstudied in children. Understanding whether young children have a bias when responding to questions is important, and not only for the applied purposes of researchers who are designing experiments that involve verbal responses from young children. Understanding whether children exhibit the same or different biases from adults has the potential to inform what we understand about the cognitive processes that underlie language production and decision-making in general. There’s an additional reason to care about the existence of response biases of young children, which is that verbal productions are typically taken as strong evidence that the child understands at least something about the word they have uttered. For example, studies of children’s language development often use children’s productions as reported by parents using the MacArthur Bates Communicative Development Inventories [7] as children’s lexical knowledge and familiarity. In fact, it is quite tempting for anyone who interacts with small children, not only developmental psychologists, to presume a child’s productions, like an adult’s productions, imply conceptual understanding. But this is not always the case. A toddler who is asked if they threw food “on accident or on purpose,” for example, may respond with one of the two choices without any actual knowledge what either difficult-to-infer abstract concept actually means. If children exhibit a stronger place-based bias than adults, this could indicate that productions of this type are only weak evidence of any underlying understanding of the word’s meaning. In other words, it may make us question whether all productions should be taken strong evidence of word comprehension. If children exhibit a different type of bias from adults, this could also be informative about what the underlying cognitive mechanisms driving these biases might be. We know that the core cognitive capabilities of young children differ substantially from those of adults. For example, children have lower levels of working memory and inhibitory control [8]. Understanding how these core abilities interact with verbal biases could inform the cause of the biases themselves. The current set of experiments looks into the following question: do children demonstrate response biases? We hypothesize that toddlers, who are just learning language, will be more likely to demonstrate a recency bias when presented with the sentence structure, “X or Y?”. Since younger children possess a limited memory capacity, children could be relying on the phonological loop to remember the option following the word “or”. The phonological loop accounts for recency biases in adults, meaning that during free recall the most recently listed options are more accessible. Together, we show that 1) toddlers demonstrate a robust verbal recency bias when asked “or” questions in a lab-based task and a naturalistic corpus of caretaker-child speech interactions, 2) the recency bias weakens with age, and 3) the recency bias strengthens as the syllable-length of the choices gets longer. Taken together, these results indicate that children show a different type of response bias than adults, recency instead of primacy. Further, the results may suggest that this bias stems from increased constraints on children’s working memory.

Experiment 1 Do toddlers demonstrate recency biases during question answering? To investigate whether toddlers show any type of bias during question answering, we tested 24 one- to two-year-olds in a game that required them to choose between two (counterbalanced) choices about stickers. In this game, children had to answer several questions with two possible response options (e.g., “Should Rori bring a lunchbox or a backpack to school?”) and were much later (after enough other questions were asked to mask the repetition) again asked the same question with the choice options flipped (“Should Quinn bring a backpack or a lunchbox to school?”). Experiment 1 method Participants. Twenty-four children (mean = 2.02 years old, age range = 1.77–2.25 years old) with normal vision and hearing were recruited from the database of volunteer families from the greater Rochester, NY area. Participants were from home environments where they were exposed to at least 90% English. Families were compensated $10 and a child-sized t-shirt for their participation in the study. Six additional children were tested in the study, but were excluded from the final analysis because of failure to complete at least half of the total number of trials (mean = 2.07 years old, range = 1.80–2.17 years old). This research was approved by the University of Rochester Institutional Review Board (Protocol RSRB #43091). All families provided written consent prior to testing. Detailed versions of our protocol and our data are on the Open Science Framework (https://osf.io/yub7c/). Materials. The stimuli consisted of two sets of 20 questions, each containing two choices (see Supporting Information or our Open Science Framework page for the questions). The second set of questions was identical to the first, except that choices were presented in the opposite order. Question choices varied in terms of their commonality and frequency in child-directed speech. Some words were highly familiar to most 2-year-olds (e.g., red, cat) and some were likely to be unfamiliar (e.g., ganache, khaki). Questions also varied in terms of where the choices appeared within them. Some questions contained choices at the end (e.g., “Does Rori like to eat apples or bananas?”), some contained choices at the middle (e.g., “Should Rori bring a backpack or a lunchbox to school”), and some contained choices at the beginning (e.g., “The school has burgers and the school has pizzas. Which one should Rori eat?”). Procedure. Children were tested in a quiet testing room without their parents present to prevent parental influence on children’s responses. Testing sessions were videotaped so the data could be coded and entered at a later time. The testing room contained a small, child-sized table with a 17'' x 14'' double-sided whiteboard easel. Participants sat across the table from the researcher who explained that they were going to play a game together. The researcher introduced the child to a bear character (named either Rori, a polar bear, or Quinn, a grizzly bear) whose image was printed on a static-cling sticker and placed on the easel in front of the child. The researcher asked the child if they could help Rori/Quinn make some choices and that for each answer, the child would get a sticker to place on the board (see Fig 1). PPT PowerPoint slide

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larger image TIFF original image Download: Fig 1. Set up for Experiment 1. Static-cling stickers used in the experiment, along with verbally presented questions. Children were asked a series of questions about each bear–first in one order (e.g., choices in blue on the left) and later in the opposite order (e.g., choices in red on the right). Children were only shown the stickers after they responded to the question verbally. https://doi.org/10.1371/journal.pone.0217207.g001 The researcher asked the child a set of questions about items for the bear character (e.g., “Is Rori’s shirt red or yellow?”). The stickers corresponding to the options were kept out of view of the participant while the questions were asked verbally. Once participants gave an answer, they were given the static-cling sticker that depicted their choice to stick on the board. The question was repeated one more time if the child did not make a choice after the question was asked the first time. If the child still did not respond, the researcher held up the two corresponding stickers and repeated the question up to two more times. The two stickers were presented in a randomized left right configuration, each equidistant from the child. Children either responded to this final question verbally or by pointing, and were given the appropriate sticker to place on the board. Once the child ran through the first set of questions, they were asked the same set of questions for the new bear character with the choices presented in the opposite order. For example, if they were asked if Rori’s shirt was “red or yellow” during the first part of the experiment, they were asked if Quinn’s shirt was “yellow or red” during the second part. The order of the question sets was randomized across participants. Coding. Two researchers coded each child’s responses from the video recording of the testing session. Additionally, the child’s verbal skills were ranked on a scale of 1 to 4 independently by two researchers who observed the participant in the waiting room in advance of the study, according to a predetermined set of rating criteria relating to mean utterance length (see S1 File). The two coders assigned the same verbal-skill rating to children 70.83% of the time and differed by one 29.17% of the time. When coders differed, the two verbal-skill rating values for that child were averaged. Child participants had a median verbal-skill rating of 2 (mean = 2.1, range = 1–4). Analysis. Our primary analysis examined the proportion of second-choice responses children made when verbally responding to the questions. This analysis allowed us to determine if children have a bias to respond with the most recently mentioned option. We used a Wilcoxon signed-rank test in order to compare this proportion to the proportion we would expect by chance or preference—mu = 0.5. We also compared the proportion of second-choice responses that children made verbally to those that they made non-verbally. If, in fact, children exhibit a recency bias because recent linguistic material is more readily available in their phonological loop, we would expect to see an effect only for verbally answered questions. We used a Wilcoxon signed-rank test because we cannot assume that our data is normally distributed, we are comparing proportions, and it is a more conservative measure than a t-test. Additionally, we used R and lme4 to conduct a generalized linear mixed effect analysis in order to evaluate the influence of whether the response was verbal, along with other factors (e.g., age, verbal skills, choice familiarity/frequency, whether or not the choices occurred at the end of the question, and which bear they were answering questions about) on children’s likelihood of responding with the second choice. We used the following as fixed effects: verbal response, age, bear, trial, and frequency of choice words (using the Google Books NGram corpus for English [9]). We used random slopes and intercepts for each of these factors by individual. We included crossed random effects of participant and item pair. The analysis script can be found on our Open Science Framework page (https://osf.io/yub7c/). Experiment 1 results Recency biases in verbal responses. Overall, when participants responded verbally, they chose the second choice 85.2% of the time—a value that a Wilcoxon signed rank test confirmed to be significantly above chance (V = 231, p < .0001). Further, 22 of the 24 subjects showed significantly more second-choice responses than chance would have predicted in an analysis of individual subjects (Fig 2). PPT PowerPoint slide

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larger image TIFF original image Download: Fig 2. Robust recency bias across ages in Experiment 1. Each dot represents an individual’s proportion of second choices during the trials in which they responded verbally. The error bars depict 95% confidence intervals. The dotted line represents chance. Twenty-two of the twenty-four participants exhibited significantly more second-choice responses than would be predicted by chance. https://doi.org/10.1371/journal.pone.0217207.g002 Participants responded verbally 78.7% of the time. When looking at trials where children responded by pointing, participants chose the second choice only 51.6% of the time, which was not significantly different from chance (V = 102, p = .79). The proportion of second choices was significantly higher for verbal than non-verbal responses (W = 384.5, p < .0001). This comparison of verbal to nonverbal responses suggests that this bias is limited to the verbal domain. In the generalized linear mixed effect model, we found that when the participant responded verbally, the chance of choosing the second option increases (β = 2.13, p = 9.6e-12). We also found a small effect of age, suggesting that the younger participants are more likely to respond with the second option (β = -0.29, p = .02). Verbal skills, choice familiarity/frequency, sentence structure, and the bear they were answering questions about did not reach significance (see Table 1 for more detail). PPT PowerPoint slide

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larger image TIFF original image Download: Table 1. Experiment 1 generalized linear mixed effect model results. https://doi.org/10.1371/journal.pone.0217207.t001 Experiment 1 discussion In Experiment 1, we show that toddlers exhibit a robust recency bias when responding verbally to “or” questions. Since this bias appears to be limited to the verbal domain, it suggests that the cause of this bias may be working memory constraints.

Corpus analysis Are recency biases present in natural parent child speech? Experiment 1 found that one- to- two-year-olds are more likely to choose the second option when presented with an “or” question. Do these types of sentences exist in naturalistic speech? Or is our finding limited to the laboratory setting? To determine if the same bias applies in real-world child-parent interactions, we further explore Experiment 1’s findings through analyzing naturalistic parent-child interactions. Corpus analysis method The Child Language Data Exchange System or CHILDES [10] is an online databank of transcribed conversations between parents and their children. We limited our analysis to CHILDES transcript files that involved two participants—one adult and one child—and that included the age of the child. This way we could ensure that questions we extracted from adult speech were most likely directed to a child of a known age (as opposed to a sibling or other adult). From this predefined subset, we extracted 534 two-alternative choice questions. The questions were posed to children ranging in age from 0.74 to 4.54 years of age (mean = 2.51). We then coded children’s responses to the question for whether they chose the first option, the second option, or neither option (e.g., by failing to respond, or responding with an irrelevant verbal response). Of all of the two-choice questions adults asked children in CHILDES, children responded with the first or second option 58.17% of the time (as opposed to not responding or providing an irrelevant response). We then further limited our analysis to only the questions where the child responded with an appropriate responses option, bringing the age range to 1.59–4.39 years of age (mean = 2.68). The final data set included data from the following corpora: Bates [11], Bernstien [12], Demetras [13], Higginson [14], Kuczaj [15], MacWhinney [16], New England [17], Providence [18], Rollins [19], Sachs [20], Snow [21], Suppes [22], Tardif [23], and Van Houten [24]. The dataset used, as well as the script we used to extract the data from CHILDES is openly available on the Open Science Framework (https://osf.io/yub7c/). Corpus analysis results Recency bias present natural parent-child setting until around age three. A linear regression suggests that as children get older, the likelihood of choosing the second option decreases (t = -2.65, p = .009). The proportion of first and second choice responses children gave binned by age appears in Fig 3A. As evident from the plot, binned responses from 2-year-olds are significantly more biased towards the second-choice option, as compared to chance. This bias is no longer significant in the 3- and 4-year-olds, who are equally likely to select the first and second choices. Fig 3B shows the raw data (displayed as circles at the top and bottom of the plot to represent the 1’s and 0’s encoding whether each response was for the second-choice option), as well as a LOESS curve fitted to the data. The smoothed fit to the data suggests that young children exhibit a strong second-choice bias, but that choices are at chance starting at between 2.5 and 3 years of age. The data suggests that this recency bias persists until around the third year of life. PPT PowerPoint slide

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larger image TIFF original image Download: Fig 3. Recency bias present in natural parent-child setting. (A) Proportion of second-choice responses in CHILDES binned by age of the children in years. (B) A smoothed LOESS curve fitted to the raw data from CHILDES (empty dots). https://doi.org/10.1371/journal.pone.0217207.g003 Corpus analysis discussion Here, we found further evidence that children under the age of 3 are biased towards choosing the second response option when faced with the sentence structure, “X or Y?”. Interestingly, this bias seems to disappear with age. Unlike Experiment 1, where parents are not present in order to minimize parental bias, the Corpus Analysis only looks at conversations between parents and their children. Thus, these results may be confounded by nonverbal cues which older children may be better at picking up than younger children. Regardless, these results can be considered a conceptual replication of the results we found in Experiment 1 because the recency bias is still present in children under the age of 3.

Discussion We have found that young children demonstrate a robust recency bias when answering two-alternative forced-choice questions verbally. This recency bias can be seen both in the laboratory and within naturalistic contexts. The bias decreases with age, but returns when response options are novel and have more syllables. The response bias we have identified has many implications, both theoretically and practically, in how we evaluate young children’s lexical knowledge during early stages of speech production. Additionally, our findings shed light on how we view the connection between decision-making biases and working memory. Theoretical implications Although it is known that young language learners utilize repetition during the early stages of language acquisition (e.g., echolalia), what is novel about our findings is that imitation alone cannot describe our results. In fact, it has been posited that the purpose of the phonological loop is to help us learn new words, particularly in its ability to enable imitation, which is a common phenomenon during the early stages of language learning. Indeed, this simple strategy could be very useful in enabling toddlers to engage in verbal exchanges before they possess fully developed semantic representations for the words that they are using. It’s important to note here, however, that the recency bias was present throughout all of Experiment 1, even when the response options were embedded in the middle of the question (e.g., “Should Rori bring a lunchbox or a backpack to school”). This rules out the possibility that children are simply repeating back the last option that they hear. The phonological loop could still be involved, but the mechanism must be a bit more sophisticated to account for the data from non-utterance-final choices across our experiments. Children must recognize the word “or” separates choices, and be able to identify and hold on to the second item until the moment of their response. We note that previous work has questioned whether children understand the meaning of the word “or” on the basis of children sometimes mistaking the meaning of disjunctives (e.g., “or”) to be a conjunctive (e.g., “and”) [27]. For example, preschoolers may interpret the sentence, “Every boy is holding an apple or a banana” as “every boy is holding an apple and a banana.” While we saw no evidence that children struggled with the meaning of the word “or” in our study, this difference could be due to the visual pragmatics of the task, which may have more readily suggested to children that the choice was one or the other, not both. In any case, our results, particularly the word-length effect we found in Experiment 2, suggests that this bias may be related to children’s limited working memory capacity. However, we did not find an effect of age and working memory in Experiment 2. Perhaps if our age range was larger and if we didn’t do the syllable repetition task directly after the toy naming task, we would have found an effect of working memory or age. While we focused our studies on children, who are known to have limited working memory capacities, the same effects would be expected to apply to anyone with sufficiently taxed working memory. In adults, conditions that impair working memory (e.g., sleep deprivation, inebriation, multitasking, disorders associated with working memory deficits) would all be expected to result in similar recency biases. In other words, these results shed light not only on children’s verbal behavior, but more generally on memory and response biases. Practical implications Modern studies of language development commonly assume that children’s lexical productions are indicators of their lexical knowledge. The production norms in the MacArthur-Bates Communicative Development Inventories [7] are commonly used to estimate children’s comprehension abilities throughout development. However, our findings suggest that some of children’s earliest of utterances may be the result of a recency bias, and do not necessarily indicate that the child comprehends the words they speak. Thus, data collected via vocabulary surveys like the MacArthur-Bates Communicative Development Inventories may be an overestimation of children’s true word knowledge. Additionally, researchers working with this age group should take special care with any experimental methodologies that rely on children’s verbal responses. Methods that require verbal responses should ensure question choices are carefully counterbalanced. Parents, however, may wish to use such a biased design: “Would you like cake or broccoli?”

Acknowledgments We thank R. Malachai Goodman for the inspiration and unsolicited pilot data for these experiments; Chelsea Parlett-Pelleriti, Steve Piantadosi, and Shirlene Wade for assistance with analyses; Susanne Jaeggi for useful discussion; Holly Palmeri, Claire Hart, Anetaijia Porter-Monroe, Ashley Rizzieri, Emily Roemer, and Kelsey Spear for recruiting and scheduling participants; Carla Macias, Eric Partridge, and Madeline Pelz for coding; the Jacob’s Foundation & the University of Rochester Bilski-Mayer Fellowship Program for funding; members of the Kidd Lab for helpful feedback and discussions; and the anonymous reviewers for their useful feedback.