 The second is the research of Flavell,  which studied children's growing abilities to judge other people's conceptual and perceptual perspectives.
<meaning-changed>  The second is the research of Flavell,  which studied children's growing abilities to judge other people's conceptual and perceptual perspectives.
 The second is the research of John H. Flavell (1968),Flavell, John H., et al. 1968. The development of role-taking and communication skills in children. New York: John Wiley which studied children's growing abilities to judge other people's conceptual and perceptual perspectives.