He revitalized the scientific debate on birth order by putting forward his Family Niche Theory of birth-order effects in his book. His theory was based on evolutionary considerations, he put forward the argument that the adaptation of different roles within the family facilitates cooperation and reduces competition among siblings and enhancing sibship’s fitness, further on characterizing siblings as Darwin’s finches. The birth order is a reflection of disparity in power, age, and size and therefore determine the particular niche to be occupied by siblings within the family. Personality differences between the siblings is a result of these specific adaptation within the family system. The differences in personality being dependent one’s birth order within the family according to Sulloway (1996) can be expressed through Big five personality traits, which consist of the following five dimensions:
Extraversion (E), Neuroticism (N), Agreeableness (A), Conscientiousness (C) and Openness to experience (O).
Sulloway hypothesized for that firstborns to be higher in E, C and N but lower in A and O than the later borns. These dimensions have been briefly explained below,
Extraversion: Being talkative, sociable, enjoying social situations and