Diffusion of cumulative advantage? How wealth and health trajectories co-evolve across the life course among older adults.

Behavioral science is replete with laboratory studies. Standardized environments, controlled experiments, and clever manipulations have produced insights on plausible mechanisms of human behavior, cognition, and development. Additionally, clinical research in rehabilitation—recovery after an injury or medical condition or improvement of functional skills in individuals with disabilities—tends to measure intervention effectiveness using standardized assessments conducted in a controlled lab or clinic setting. However, actual behavior, cognition, development, and recovery take place in the real world. Thus, fully understanding these processes requires studying them outside the lab. For infant researchers, whose methods have always relied heavily on observation of natural behavior due to the limited capacity of our participants to follow experimental instructions or provide verbal reports of their mental activities, it is a natural and necessary shift to focus on actual natural behavior “in the wild.” In exchange for experimental control, researchers studying infants in the real world gain ecological validity, insights into the vital roles of context and environment, and a wealth of theory-generating data on how complex developmental processes actually unfold (). Similarly, the historical shift in rehabilitation research toward focusing on function and participation requires measuring function and participation in real-world settings (). Therefore, the development of methods for measuring real-world behavior is essential for both developmental and rehabilitation science. One meaningful aspect of everyday behavior in early development is : the physical configuration of the body in relation to gravity and environmental supports. A common approach to measuring body position is by classification into recognizable categories, such as standing, sitting, prone, supine, and being held (; , ). Body position can be measured for brief observation periods using manual video coding (; ; ; ), and for longer periods of time using retrospective (; ; ; ) or real-time (; ; ; ) parent report; newer methods using wearable sensors will be described in the section below. Body position is important for development because it constrains how infants interact with the world; these interactions, in turn, provide learning and interaction opportunities that shape developmental outcomes (; ). For example, being held in arms or worn in a sling or carrier decreases crying, increases parent speech and responsiveness, and facilitates attachment (; ; ; ). Sitting—compared to lying prone or supine—improves infants’ ability to explore objects, expands their view of the environment, and enriches social interaction with caregivers (; , ; ; ; ; ; ). Standing further expands visual, social, and object-related interactions (; ; ; ; , , ; ; ; ). Crucially, the more time infants spend in particular positions throughout their everyday lives, the more exposure they have to the corresponding learning opportunities.