Effect of number, order, and spacing of siblings on child and adult outcomes: an overview of current research

The impact of the number, order, and spacing of siblings on child and adult outcomes has been the topic of research by scholars in 4 separate fields (human biology, psychology, sociology, and economics), and the barriers to communication between academic disciplines are strong. Also most researchers have had to work with data sets gathered for other purposes. This has resulted in a relative inadequacy of research. Social scientists have 3 theories concerning the relationship between the number, order, and spacing of siblings and child and adult outcomes: that an increase in the number of siblings or a decrease in the spacing between them dilutes the time and material resources that parents can give to each child and that these resource dilutions hinder the outcome for each child; that account must be taken not only of parental resources but also of the resources given to each child by his/her siblings; and that there is no causal relationship between number, order and spacing of siblings and child outcomes and that any apparent relationships are spurious. In light of these theories, the question arises as to how should the sibling variables be measured. The most important aspect of sibling number is that it is a variable over time. Yet, the proper measurement of sibling number has an additional complication. According to all existing theories, the ages of the other siblings are relevant for the outcome for the given child. All of the relevant information is now available only when it is possible to construct a matrix in which the rows present the age of the given child and the columns the age grouping of the siblings for whom a count of sibling number will be made. Many such matrices could be developed, some much more elaborate than others. For illustrative purposes, Table 1 presents the matrix of the number of siblings for a child who is the first-born among 5 children, all of whom are spaced exactly 3 years apart and all of whom are financially dependent only up to exact age 21. Table 2 presents the matrix for the last-born child among 5 children with characteristics identical to those in Table 1. It can be inferred from these tables that the oldest child in the family, as compared to the youngest child, probably will suffer from a diminution of parental resources, most likely financial resources, in adolescence. The youngest will suffer from a reduction of parental resources, probably time resources, in infancy and early childhood. Research concerned with the consequences of the number and spacing of children should be based on data sets for which some version of this matrix can be constructed.