Friday, January 7, 2011

Birth Order Influences the Formation of Long-Term Relationships

134 years since Francis Galton opened the birth order effects debate by observing that first-born sons and only sons were over-represented among English scientists, controversy has shrouded the issue such that we haven't quite gotten past whether birth order effects exist or not, let alone properly consider what they are or how they work.

Some scholars assert that the lack of conclusive evidence is due to methodological biases that may allow the researcher to find the result that he or she is looking for. So, in that sense, a researcher who seeks to confirm that the birth order effect exists may find it just as well as a researcher who seeks to disconfirm it might.

In Birth Order Effects in the Formation of Long-Term Relationships, Hartshorne, Salem-Hartshorne and Hartshorne seek to probe the birth effect in a manner that is as methodologically neutral as possible by simply determining if there are any correlations between the sharing of birth order and the likelihood of long-term relationship formation. Their results provide new research material that weighs in favour of the presence of birth order effects, though what drives this effect is still speculative.

By drawing on a sample of 900 US undergraduate students, the researchers found that people are more likely to form and be in long-term relationships, both friendly and romantic, if they share the same birth order than would be expected by chance. For instance, if I were a first-born child, the likelihood of me being close friends with another first-born child is higher than the likelihood of me being close friends with another second- or third-born child. This tendency was also found for romantic partners.

A second and similar web-based study was conducted which gathered responses from American participants (1,911) as well as participants from other parts of the world (713). Similar results were garnered. There was no significant difference detected between American and non-American respondents, suggesting that birth order effects on long-term relationships are not culturally variant.

The researchers also controlled for socioeconomic status and size of family, which is a progressive extension from other earlier studies. This eliminates the confounds of number of siblings one has and the socioeconomic class one belongs to, which can potentially influence one's development because it is a commonly known social phenomenon that wealthier and upper class families tend to have less offspring.

The authors surmise that birth order underlies personality traits and having the same birth order results in greater compatibility between personality types, leading to the formation of closer bonds in both friendships and romantic relationships.

This post was chosen as an Editor's Selection for ResearchBlogging.orgJoshua K. Hartshorne, Nancy Salem-Hartshorne, and Timothy S. Hartshorne (2009). Birth Order Effects in the Formation of Long-Term Relationship Journal of Individual Psychology, 65 (2)


  1. Hi there,

    I don't think it's on PLoS One but if you google the article title, you should be able to download a pdf version of the manuscript.

  2. @William Allison: Why would you expect the paper to be on PLoS One? It wasn't published by PLoS One.

  3. @GamesWithWords I think it was because William Allison was directed here via research blogging editor's selection post which mistakenly attributed the article to being published in PLoS One.

  4. @psycho thamalamus. I saw that selection post now. Unfortunately, JoIP has a very poor web presence. But, as you pointed out, you can find the paper on my website.