Researchers who examined family genealogies from Finland found that the presence of same-sex elder siblings increased the probability that people would disperse to new lands, whereas having opposite-sex siblings had less influence.
Younger brothers' high dispersal was strongly linked to lack of land resources. In contrast, women appeared to compete over availability of mates, but women with elder sisters dispersed to secure a wealthy husband, rather than due to lack of mates per se.
"The results are interesting because having elder sisters for women and elder brothers for men, but not the number of opposite-sex siblings, also meant lower chance to marry and smaller family size, meaning that sibling competition can guide who disperses in our family," said Dr. Aïda Nitsch, lead author of the Journal of Evolutionary Biology study.
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