Faculty Bibliography

Developmental trajectories provide the empirical foundation for theories about change processes during development. However, the ability to distinguish among alternative trajectories depends on how frequently observations are sampled. This study used real behavioral data, with real patterns of variability, to examine the effects of sampling at different intervals on characterization of the underlying trajectory. Data were derived from a set of 32 infant motor skills indexed daily during the first 18 months. Larger sampling intervals (2-31 days) were simulated by systematically removing observations from the daily data and interpolating over the gaps. Infrequent sampling caused decreasing sensitivity to fluctuations in the daily data: Variable trajectories erroneously appeared as step functions, and estimates of onset ages were increasingly off target. Sensitivity to variation decreased as an inverse power function of sampling interval, resulting in severe degradation of the trajectory with intervals longer than 7 days. These findings suggest that sampling rates typically used by developmental researchers may be inadequate to accurately depict patterns of variability and the shape of developmental change. Inadequate sampling regimes therefore may seriously compromise theories of development.

Locomotion-moving the body from place to place-is one of infants' greatest achievements. In addition to conquering gravity, infants must cope with variable and novel constraints on balance and propulsion. At the same time that they are learning to move, changes in infants' bodies, skills, and environments change the biomechanical constraints on movement. Recent work highlights both flexibility and specificity in infants' responses to novel and variable situations, demonstrating that infants are learning to learn as they master locomotion. Within sitting, crawling, cruising, and walking postures, experienced infants adapt their locomotor responses to the current biomechanical constraints on movement. However, what infants have learned about coping with variability and novelty in earlier-developing postures does not transfer to later-developing postures.

The social cognition and perception-action literatures are largely separate, both conceptually and empirically. However, both areas of research emphasize infants' emerging abilities to use available information--social and perceptual information, respectively--for making decisions about action. Borrowing methods from both research traditions, this study examined whether 18-month-old infants incorporate both social and perceptual information in their motor decisions. The infants' task was to determine whether to walk down slopes of varying risk levels as their mothers encouraged or discouraged walking. First, a psychophysical procedure was used to determine slopes that were safe, borderline, and risky for individual infants. Next, during a series of test trials, infants received mothers' advice about whether to walk. Infants used social information selectively: They ignored encouraging advice to walk down risky slopes and discouraging advice to avoid safe slopes, but they deferred to mothers' advice at borderline slopes. Findings indicate that 18-month-old infants correctly weigh competing sources of information when making decisions about motor action and that they rely on social information only when perceptual information is inadequate or uncertain.

The content of mothers' emotional, verbal, and gestural communication to their infants was examined under conditions of potential physical risk in a laboratory motor task. Mothers encouraged and discouraged their 12- and 18-month-old infants to crawl or walk down a sloping walkway. Mothers expressed positive affect on nearly every trial. They rarely expressed purely negative affect in their faces and voices, even when discouraging. Instead, they discouraged infants with a mixture of positive and negative expressions. In both encourage and discourage conditions, mothers coupled their emotional messages with rich verbal and gestural information to elicit infants' attention, regulate their location, guide their actions, and describe the situation and potential consequences of their actions. The content of mothers' communication was attuned to infants' age and locomotor experience.

Standing and walking generate information about friction underfoot. Five experiments examined whether walkers use such perceptual information for prospective control of locomotion. In particular, do walkers integrate information about friction underfoot with visual cues for sloping ground ahead to make adaptive locomotor decisions? Participants stood on low-, medium-, and high-friction surfaces on a flat platform and made perceptual judgments for possibilities for locomotion over upcoming slopes. Perceptual judgments did not match locomotor abilities: Participants tended to overestimate their abilities on low-friction slopes and underestimate on high-friction slopes (Experiments 1-4). Accuracy improved only for judgments made while participants were in direct contact with the slope (Experiment 5), highlighting the difficulty of incorporating information about friction underfoot into a plan for future actions.

Maintaining balance is a central problem for new walkers. To examine how infants cope with the additional balance control problems induced by load carriage, 14-month-olds were loaded with 15% of their body weight in shoulder-packs. Both symmetrical and asymmetrical loads disrupted alternating gait patterns and caused less mature footfall patterns. Walking was most severely compromised by back loads. Infants with less walking experience, lower levels of walking proficiency, and chubbier body proportions were more adversely affected. In addition, infants displayed a unique postural response to asymmetrical loads. In contrast to older children and adults, infants leaned with loads rather than in the opposite direction to the loads. Findings are discussed in terms of development from accommodation to compensatory strategies.

Seven hundred and thirty-two parents reported when and how their infants learned to climb stairs. Children typically mastered stair ascent (mean age=10.97 months) several months after crawling onset and several weeks prior to descent (mean age=12.53 months). Most infants (94%) crawled upstairs the first time they ascended independently. Most infants (76%) turned around and backed at initial descent. Other descent strategies included scooting down sitting, walking, and sliding down face first. Children with stairs in their home were more likely to learn to ascend stairs at a younger age, devise backing as a descent strategy, and be explicitly taught to descend by their parents than children without stairs in their home. However, all infants learned to descend stairs at the same age, regardless of the presence of stairs in their home. Parents' teaching strategies and infants' access to stairs worked together to constrain development and to influence the acquisition of stair climbing milestones.

Objectives. In 2 studies, we aimed to describe the content of mothers' verbal warnings to their young children and to investigate whether mothers modify their warnings based on the type of dangerous situation and children's age. Study 1. Mothers of 12-, 18-, and 24-month-olds reported in a telephone interview the words and phrases they would use to prevent their children from falling, touching dangerous objects, ingesting poisonous substances, and running away. The words "no," "don't," and "stop" were the most frequent warnings across ages. Mothers also used warnings to elicit their children's attention, regulate children's location, modify children's actions, and to highlight the properties and consequences of specific dangers. The content, diversity and complexity of mothers' warnings varied with children's age and the type of dangerous situation. Study 2. We observed mothers in the laboratory as they warned their 12- and 18-month-old children not to walk down 50 degrees slopes. As in Study 1, mothers primarily relied on the words "no," "don't," and "stop," but again used warnings to elicit attention, regulate location, modify actions, and describe the danger. Mothers used more complex and diverse warnings with older versus younger children. Conclusions. Although simple warnings, such as 'no," "don't," and "stop" hold privileged status at all ages, mothers express a rich array of warnings that are attuned to children's age and the dangers of the situation.

Objective. We examined how parents' expectations about their infants' crawling ability and crawling attempts in a locomotor task affect parenting choices about ensuring infants' safety and providing appropriate challenges. Design. Mothers and fathers of 34 11-month-old infants adjusted a ramp to the steepest slopes they thought their infants could safely crawl down, would attempt to crawl down, and they would allow their infants to crawl down independently. Results. Most parents expected their infants to attempt slopes that were steeper than their ability and generally emphasized safety only by permitting infants to crawl down slopes that were within infants' expected ability. More fathers than mothers displayed parenting choices emphasizing challenge by allowing their infants to attempt slopes beyond their ability. Conclusions. Both mothers and fathers expected infants to attempt impossibly steep slopes, but mothers were more likely to adopt safety-oriented parenting choices. Wide disagreements within dyads and inconsistencies in individual parents' estimates might increase the chances of infants incurring injuries.