Early and Mid-Childhood Sibling Relations 357 Dunn, 2002 ). In particular, research on the development of social understanding describes how children construct knowledge about their social worlds, namely, other ' s perspectives, feelings, thoughts, beliefs, and goals, and how this knowledge is linked to the behavior of individuals. Carpendale and Lewis argued that children develop this understanding in the context of close relationships and sometimes show precocious social knowledge in their daily interpersonal interactions (e.g., teasing, play, conversations, and internal state language) before demonstrating it on more formal assessments such as theory -of -mind measures (de Rosnay & Hughes, 2006 ). Some contexts, such as play and confl ict, are conducive to promoting the use of internal state language, although wide individual differences have been noted (Howe, Rinaldi, Jennings, & Petrakos, 2002 ). Hinde (1979) argued that relationships are defi ned by two main types of interactions. First, complementary or hierarchical interactions are characterized by unequal distributions of power and knowledge (i.e., parent -child relationships), which are also evident in teacher -learner, leader -follower, and caretaking exchanges. Second, reciprocal or mutually returned, equal interactions, typical of peer relations, are seen in play and confl ict. Sibling interactions are defi ned by a unique combination of both complementary and reciprocal exchanges. Age, birth order, and knowledge differences between siblings are apparent in their complementary exchanges, yet siblings also engage in reciprocal interactions. Dunn (1983) argued that the long and shared history of reciprocity creates critical opportunities to promote social understanding, because as siblings co -construct shared meanings during mutual and returned exchanges an understanding of the other ' s perspective may be required. In fact, Dunn labeled reciprocal interactions as the " building blocks " of relationships, given the opportunities they afford for understanding the self and others. Nevertheless, the importance of complementary interactions in children ' s development should also be acknowledged, while keeping in mind that many exchanges contain elements of both types of interactions (Howe & Recchia, 2005 ).
Four major characteristics of the sibling relationship have been identifi ed in early and middle childhood (Howe & Recchia, 2008 ). First, sibling relations are defi ned by strong positive, negative, and sometimes ambivalent affect, making it hard to overlook or ignore one ' s sibling. Unlike friendships, which are voluntary and will cease if children do not get along, siblings are siblings for life, and from the early years they must learn to live together and share both social and nonsocial resources (Laursen, Hartup, & Koplas, 1996 ). Increasingly, the psychological literature has regarded confl ict as a normative feature of the sibling relationship. Second, particularly in early childhood, siblings spend a great deal of time together and come to know each other very well, thus potentially establishing an intimate bond. Constructing a common history translates into opportunities for engaging in play (particularly pretense), for emotional and instrumental support, and for disagreements. Third, there are wide individual differences in sibling relationship quality, which are in turn linked to the kinds of interactions observed between children. Moreover, age differences between siblings result in an asymmetry of power, which has implications for the dynamics of both cooperation and confl ict. Finally, nonshared environmental effects on siblings ' development are evident, and various processes lead to differences between siblings in the same family (Dunn & Plomin, 1990 ), including issues related to differential parental treatment.