Although shared environmental factors affect cognitive development during childhood, non‐shared environmental factors experienced differently by children in the same family appear to be the major source of environmental influence on cognitive development after childhood. However, specific shared and non‐shared aspects of environmental measures relevant to cognitive development have not been differentiated. Data from the Colorado Adoption Project (CAP) for 184 members of 92 sibling pairs, studied separately when each child was 12 and 24 months of age, were used to construct a non‐shared scale and a shared scale from items of the Home Observation for Measurement of the Environment (HOME). Items that were not significantly correlated for siblings were selected as “non‐shared” items; the remaining items were selected as “shared” items. Comparisons of sibling correlations for the shared and non‐shared HOME scales for adoptive and non‐adoptive siblings indicate the shared scale is primarily a measure of shared environment and the nonshared scale primarily assesses non‐shared environment.