Three studies were conducted to investigate if children think that physical growth is determined by inheritance. All three studies employed the “switched‐at‐birth” task. Study 1 investigated if children and adults thought that height and weight were determined by parentage. Study 2 examined preschoolers and elementary school children's and adults' understanding of the role of parentage and nutrition in determining weight. Study 2a examined children's and adults' understanding of the role of parentage and nutrition in determining height. Results indicate that overall, preschoolers and third graders have more of an inheritance bias for height than for weight. However, when nutrition was introduced, apart from third graders, all other grades reasoned that nutrition played a greater role in determining weight. Overall, these results indicate that even young children have a rudimentary but differentiated theory of the role of inheritance in determining height and weight. Highlights Participants were presented with switched‐at‐birth tasks to determine if they thought that parentage and/or nutrition influences height and weight. Young children viewed height as determined more by parentage than weight, suggesting that they have a differentiated and autonomous theory for the determination of height.