One of the longest-debated and most studied issues in psychology is whether and how our personalities are affected by our birth order and the sex of our siblings. A problem with much previous research is that it’s depended on people self-reporting their own personality, or on siblings or parents providing the personality ratings. These ratings are prone to subjectivity and skewed by people’s expectations about how, say, a younger sibling ought to behave.

A new study focused on one particular finding from this literature: the idea that men with older sisters are less competitive. To get round the subjectiveness of previous research, Hiroko Okudaira and her colleagues invited participants (135 high school students in an initial experiment; 232 university students in a follow-up) to complete a maze challenge or maths test under a so-called piece-rate payment system (they accrue modest points for each correct answer) then under a tournament system (a larger reward is available, but only if they beat the other people randomly assigned to their group). Crucially, in a later trial, the participants got to choose which system they would prefer to perform under: piece-rate or tournament.

In the first experiment involving mazes and high-school students, the boys more often chose to enter a tournament system than girls (61 per cent vs. 23.4 per cent, respectively). But focusing only on those boys with an older sister, their rate of entry into the tournament option was much lower than other boys, at just 38 per cent. In the second experiment with uni students and maths problems, men again showed more competitiveness than women, but men with an older sister were 21 per cent less likely to enter the tournament option than other men.

The researchers found that these results held even after controlling for the influence of other potentially complicating traits such as risk-aversion and over-confidence. They did this by giving participants the chance to convert an earlier piece-rate trial into a tournament trial. Making this choice would reveal something about a person’s risk-taking aversion, their confidence and so on, without involving competitiveness (because there was no prospect of actually performing again).

Why should boys and men with older sisters be less competitive? There are at least two complementary explanations: one has to do with “role assimilation”, which describes the way people absorb some of the gender-stereotypical traits of their siblings. The other has to do with birth-order: later borns are often found to be less competitive than first borns (evolutionarily speaking, first borns are under more pressure to meet parental expectations and must then compete to defend their stakes against younger rivals).

An intriguing detail is that while having an older sister reduced the competitiveness of most boys and men, this did not hold true for those who also had an older brother or younger sister, presumably because of counter-acting influences of these siblings and effects of birth-order. Meanwhile, women with an older sister were more competitive, leading them to behave more like an average man than an average woman, in terms of their preference for competition. The researchers speculated this is because having an older sister, with shared interests and needs, increases female competition and conflict in the family.

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Okudaira, H., Kinari, Y., Mizutani, N., Ohtake, F., & Kawaguchi, A. (2015). Older sisters and younger brothers: The impact of siblings on preference for competition Personality and Individual Differences, 82, 81-89 DOI: 10.1016/j.paid.2015.02.037

—further reading—

The taste for competition peaks at age 50

Born to lead

Christian Jarrett (@Psych_Writer) is Editor of BPS Research Digest



