Studies have shown that as family size increases, it becomes harder to determine the personality of each individual sibling. However, in the cases of a large families (+6 siblings, See Figure 4), the personality traits of the youngest three siblings significantly differed from the oldest three siblings. In taking the Eysenck’s Big Three Model dimensions of personality test, youngest siblings scored significantly higher for extraversion compared to the oldest siblings. Youngest siblings also had significantly higher psychoticism scores compared to oldest siblings.  

If personality is a tactic used by siblings in order to secure their status within the family, increased extraversion in laterborn siblings may be a means to gather parental attention. Extraversion is usually marked by sociability, impulsiveness, activity, liveliness and excitability. As family size increases, the level of activity due to interaction among the members also grows. Older siblings, being use to a more subdued household, might try to restrain behavior, where younger siblings, use to the large family size, might actively seek out ways for excitement and attention. As a result, older siblings may score lower in terms of extraversion than younger siblings.  

As family size decreases, more and more siblings adhere to the stereotypical sibling behavior. Scientists speculate that with a smaller family size, there are fewer factors that may impact a child’s personality. Studies have shown, that in smaller families, birth order holds a significant portion in developing a child’s personality.