Adults with no siblings reported significantly lower levels of conscientiousness & honesty-humility & higher levels of neuroticism and openness than adults with siblings, but mean differences failed to reach the small effect size

Only Children in the 21st Century: Personality Differences between Adults With and Without Siblings are Very, Very Small. Samantha Stronge et al. Journal of Research in Personality, September 10 2019, 103868. https://doi.org/10.1016/j.jrp.2019.103868

Highlights
•    Tested differences in HEXACO personality for adults with and without siblings.
•    Gender and age interactions were non-significant for all traits.
•    ‘Only children’ reported lower conscientiousness and honesty-humility.
•    ‘Only children’ reported higher neuroticism and openness to experience.
•    However, all mean differences failed to reach the threshold of a small effect size.

Abstract: Negative beliefs about only children suggest that they are spoiled and unlikable, with these early personality differences persisting across the lifespan. Early research found little support for the idea, yet, negative views towards only children remain prevalent. The current research re-visited the issue using a large national panel study of New Zealand adults (N = 20,592) to assess mean differences in personality between those with and without siblings. Adults with no siblings reported significantly lower levels of conscientiousness and honesty-humility and higher levels of neuroticism and openness than adults with siblings; however, mean differences failed to reach the threshold of even a small effect size (|d’s| = .08 - .11). Beliefs about only children appear to contradict actual group differences.