It has been long said that your personality is based on the order in which you were born into your family. When most people hear this news, they are not surprised. However, it is not only which number child you are, but how big the size of your family is that can determine your attitude and characteristics.
It is one thing to be the only middle child in a family of three children, but to be one of many middle children, makes a large difference. Families with more children have less resources. Less parents, less attention and less money to be equally divided between their offspring.
It has always been said that opposites attract. Although this statement may be partially true, when searching for a spouse, people tend to migrate towards someone with similar personality traits as their own. If personality correlates with birth order, than shouldn't spouses correlate on personality? From recent studies, it has been proven that birth order helps determine not only our spouse but our friendships. Firstborns are more likely to associate with firstborns, middle-borns with middle-borns, last-borns with last-borns.
WHO HAS WHAT
So when it comes down to it, which siblings posses which of the five factors to their personality traits? According to the study from Sulloway in 1996 and 1999 the conclusions came about easily. The first born child tends to be higher in Conscientiousness, Neuroticism, and Extraversion, and lower in Agreeableness and Openness. First borns are typically responsible and competitive, unlike their later-born siblings who try to distinguish themselves by being playful, and rebellious. Later-borns tend to rate themselves high in agreeableness and high in openness to experience.
Of course no one likes being compared to their siblings, and maybe this is why we distinguish ourselves from each other. All Humans are different and personalities will vary no matter what, however the evidence correlating birth order and specific traits is reliable and helpful information when trying to understand why we are the way we are.