Previous studies by Salmon and Daly have found that birth order along with sex is correlated with how a child acts and develops within the family. This study bridges the observation to non-kin interpersonal relationships ranging from friends to sexual partners. Only children were not specifically discussed in this article but the behavior of an only child can be compared to firstborn and lastborn data as only children are, by definition, the firstborn and lastborn of their families. Only children are similar to lastborns in that they are the only member of a multi-sibling family to receive "investment undiluted by the needs of a younger rival." Similarly, firstborn children are the only in the family to have experienced this same undiluted investment without any rivals, younger or older. I will be using a combination of results from the firstborns and lastborns to make assumptions about only children.
Catherine Salmon conducted this study by administering a questionnaire to two hundred and forty-five undergraduate students at Simon Fraser University. All students were enrolled in an introductory course in psychology and this survey fulfilled an experimental requirement. After dropping all twins and only children, two hundred and twenty-seven respondents remained. Most subjects were middle-income and their age ranged from eighteen to thirty-two with ninety-one percent being single and the remainder married or residing with their partner. Out of the two hundred and twenty-seven respondents, ninety-six were firstborn, seventy-two were middle-born, and fifty-nine were lastborn.
The results from this study agree with previous studies in suggesting that birth order has a significant impact on familial sentiment, with middleborns being the least family-oriented while firstborns and lastborns have the most obligation towards their family. The study also concludes that middleborns place greater value and more positive views of non-kin friendships than firstborns and lastborns. Both female and male middleborns were the least likely to report having cheated on a sexual partner with the females having a lower occurrence than their male counterparts. Overall, this suggests that middle-born children appear to be specialists in non-kin reciprocal relationships.
This study was limited to college students with a majority being of the participants being typical college age, eighteen - twenty-four. The author suggested that it would be beneficial to look at the interpersonal relationship question with a larger sample with more diverse age groups as the definition of a committed relationship is quite different at age 30 than it was at age 20. When multi-sibling non-human animals are observed, environment plays a large role in determine the pursuit of tactics and strategy. This study does not have the scope to cover all factors and variables in a participant's life span, which may have affected their behaviors. Having more evenly distributed numbers of each birth order would also lend to results that are more reliable.
Birth order in a multi-sibling family life is not something I had much experience with, but the results of this study confirmed my own assumptions after becoming aware of the topic. The study also confirmed my assumptions of only children when compared with firstborn and lastborn children. Only children receive their parent's undivided attention similar to firstborn children before the arrival of siblings and similar to lastborn children never having a younger sibling to competing for attention. According to Keller & Zach, (2002) "Firstborns are preferred over laterborns," which coincides with evolutionary generalization of "firstborns" being the prized child of the family. Only children are the firstborn by true definition as well as the lastborn by coincidence. Firstborn children are frequently the leader or protector of the younger siblings, have high expectations from the family to succeed, and tend to hold power within the family with a deep sense of familial obligation. Lastborn children are frequently referred to as "the baby" of the family, tend to be selfish, and tend to hold much regard to familial sentiment. In reference to interpersonal and romantic relationships, both lastborn and only children's selfish nature tend to shine through and they hold less regard and importance to these relationships. Conversely, a firstborn child's confidence, family dedication, and comfort with power and leadership allow them to succeed in interpersonal relationships, yet holding only a moderate importance to the maintenance of these relationships, which is also similar to the behavior of an only child.