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.
First-borns' attachment to their parents is directly related to their jealous behaviour. In a study by Volling, four classes of children were identified based on their different responses of jealousy to new infant siblings and parent interactions. Regulated Exploration Children: 60% of children fall into this category. These children closely watch their parents interact with their newborn sibling, approach them positively and sometimes join the interaction. They show fewer behaviour problems in the months following the new birth and do not display problematic behaviours during the parent-infant interaction. These children are considered secure as they act how a child would be expected to act in a familiar home setting with their parents present as secure bases to explore the environment.Approach-Avoidant Children: 30% of children fall into this category. These children observe parent-infant interaction closely and are less likely to approach the infant and the parent. They are anxious to explore the new environment as they tend to seek little comfort from their parents.Anxious-Clingy Children: 6% of children fell into this category. These children have an intense interest in parent-infant interaction and a strong desire to seek proximity and contact with the parent, and sometimes intrude on parent-child interaction.Disruptive Children: 2.7% of children fall into this category. These children are emotionally reactive and aggressive. They have difficulty regulating their negative emotions and may be likely to externalize it as negative behaviour around the newborn.
Implicit theories about relationships are associated with the ways children think of strategies to deal with a new situation. Children can fall into two categories of implicit theorizing. They may be malleable theorists and believe that they can affect change on situations and people. Alternatively, they may be fixed theorists, believing situations and people are not changeable. These implicit beliefs determine both the intensity of their jealous feelings, and how long those jealous feelings last.Malleable Theorists display engaging behaviours, like interacting with the parent or sibling in an attempt to improve the situation. They tend to have more intense and longer-lasting feelings of jealousy because they spend more time ruminating on the situation and constructing ways to make it better.Fixed Theorists display non-engaging behaviours, for example retreating to their room because they believe none of their actions will affect or improve the situation. They tend to have less intense and shorter lasting feelings of jealousy than malleable theorists.