The Sibling Niche Differentiation Model (Sulloway, 1996) suggests that an individual’s birth order acting as a proxy for within-family environmental factors like age, size and strength relative to ones siblings influences the strategies used to gain resources and minimize sibling conflict. Recent within-family birth order research (for example Paulhus, Trapnell and Chen, 1999; Healey & Ellis, 2007) has found a systematic effect of birth order on personality, with firstborn siblings found to be more conscientious and secondborn siblings more open to experience. However, an examination of birth-order effects by independent raters, has been lacking in the birth order literature. Furthermore no prior examination comparing the type of stimulus material used to elicit participant responses has been conducted. Study 1 (N = 203) sought to replicate previous birth order findings for the two Big-5 traits Conscientiousness and Openness to Experience, while also testing an alternative explanation (hypo-masculinization hypothesis) for observed birth-order differences (Beer & Horn, 2000). Study 2 compared the efficacy of four different types of stimulus material (rankings, ratings, independent ratings and real-world scenarios) in observing birth order effects (combined N = 544), while also testing novel predictions about the saliency and generalisability of birth-order effects on personality outside the context of the family. General support was found for the Sibling Niche Differentiation Model across studies and across stimulus materials, but limited support was found for the nature of within family personality differences between siblings extending to contexts outside the family environment.

The Sibling Niche Differentiation Model (Sulloway, 1996) suggests that an individual’s birth order acting as a proxy for within-family environmental factors like age, size and strength relative to ones siblings influences the strategies used to gain resources and minimize sibling conflict. Recent within-family birth order research (for example Paulhus, Trapnell and Chen, 1999; Healey & Ellis, 2007) has found a systematic effect of birth order on personality, with firstborn siblings found to be more conscientious and secondborn siblings more open to experience. However, an examination of birth-order effects by independent raters, has been lacking in the birth order literature. Furthermore no prior examination comparing the type of stimulus material used to elicit participant responses has been conducted. Study 1 (N = 203) sought to replicate previous birth order findings for the two Big-5 traits Conscientiousness and Openness to Experience, while also testing an alternative explanation (hypo-masculinization hypothesis) for observed birth-order differences (Beer & Horn, 2000). Study 2 compared the efficacy of four different types of stimulus material (rankings, ratings, independent ratings and real-world scenarios) in observing birth order effects (combined N = 544), while also testing novel predictions about the saliency and generalisability of birth-order effects on personality outside the context of the family. General support was found for the Sibling Niche Differentiation Model across studies and across stimulus materials, but limited support was found for the nature of within family personality differences between siblings extending to contexts outside the family environment.

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