Development of size constancy in children: a test of the metacognitive theory.

Two studies investigated children's abilities to estimate the sizes of distant objects. Each included a size estimation task and a size-distance knowledge test, which assessed children's understanding of the effects of distance on objects' image sizes and perceived sizes. In Study 1 (N = 79, age range = 5-10 years), high-knowledge children (those with above-median size-distance knowledge scores) made nearly accurate size estimates from a distance of 61 m and often reported using deliberate strategies to estimate size, whereas low-knowledge children underestimated size at this distance and typically reported no strategy use. In Study 2 (N = 60, age range = 6-11 years), high-knowledge children made nearly accurate size estimates from 61 m when given objective-size instructions and underestimated size when given apparent-size instructions. Low-knowledge children underestimated size in response to both instruction sets. The results suggest that age-related changes in size estimation accuracy result from the development of cognitive abilities necessary for using deliberate strategies to supplement perception.