structural:summarization
Confluence model

Robert Zajonc proposed that the intellectual environment within a family is ever-changing due to three factors, and therefore more permissive of first-born children's intellectual advancement:

In 1996, interest in the science behind birth order was re-sparked when Frank Sulloway ’s book Born To Rebel was published. In this book, Sulloway argues that firstborns are more conscientious, more socially dominant, less agreeable, and less open to new ideas compared to later-borns. While being seemingly empirical and academic, as many studies are cited throughout the book, it is still often criticized as a biased and incomplete account of the whole picture of siblings and birth order. Because it is a novel, the research and theories proposed throughout were not criticized and peer-reviewed by other academics before its release. Literature reviews that have examined many studies and attempted to control for confounding variables tend to find minimal effects for birth order on personality. In her review of the scientific literature, Judith Rich Harris suggests that birth order effects may exist within the context of the family of origin, but that they are not enduring aspects of personality.

In practice, systematic birth order research is a challenge because it is difficult to control for all of the variables that are statistically related to birth order. For example, large families are generally lower in socioeconomic status than small families, so third-born children are more likely than first-born children to come from poorer families. Spacing of children, parenting style, and gender are additional variables to consider.