Adler pointed out that birth order affects the child in a variety of aspects of development. These notions were more or less a controversial standpoint in the growing research relating to developmental psychology and as the era ventured into contemporary psychology, it was found that birth order did in fact affect the areas of development previously refuted. For example, there is a contemporary theory with respect to personality development that has given birth to the Big Five or the Five Factor Model of personality, which includes Openness, Conscientiousness, Extraversion, Agreeableness, and Neuroticism, five scientifically quantifiable factors that can be used to define human personality at the highest level of organization (Goldberg, 1993). This model contains within it the basic structure that is used to build upon personality traits and has thus become an important tool for organizing and categorizing research based data, which was previously haphazard and unorganized. This model is a purely descriptive, data driven empirical centric model of personality development and aims to draft a structure upon which personality development research may be structured upon. Within that model, Openness is described as a curiosity to explore new things and gain exposure in fields such as art, emotions, new experiences and adventure; Conscientiousness goes to the root of behavior, consisting of insights on self-discipline, aiming to achieve something, acting dutifully, and planning a pattern of life out instead of adhering to mere spontaneousness; Extroversion relates to signs of positive energy, an urgency displaying positive emotions, and being instigated to interact in the company of others so as to be more sociable; Agreeableness has to do with the flexibility of acceptance to incoming ideas, cooperation with others as opposed to hostility and antagonism; Neuroticism relates to the stresses of life affecting the mind that may result from anger, anxiety, depression and vulnerability. These 5 qualities define the personality of the person and the idea that birth order may affect them was explored through researches later on. Based on this notion, it has been argued that birth order does effect the personality development of firstborns in so much as making them less agreeable, more conscientious, more socially dominant and less inclined to accept new ideas as compared with children born later (Sulloway, 2001). According to Sulloway, the birth order does have a marked effect on personality development of children.
Such claims, however, have been met with mixed reviews however. Researchers have both supported Sulloway's claims as well as rejected them on grounds of contrary evidence and/or lack of empirical support. One of the criticisms of his theories is that there is no distinction made between family size and birth order and both are conjoined together in consideration for his purposes. Moreover, the results seem to have stemmed from the reports of people already familiar with the subject's birth order which includes families and acquaintances of families (Harris, 2006). Moreover, it is argued that the personality development is continuous and lasts the entire lifetime of the person with influences derived from the gene pool as well as social environments (Lamb & Sutton-Smith, 1982). What this means is that any personality changes that may have been observed during childhood through interactions with siblings and parents may be heightened, alleviated, changed or replaced by later experiences in life and are thus uncertain.