  One of the most contested questions about human behaviour is whether there
are inherent sex or gender differences in the formation and maintenance of
social bonds. On one hand, female and male brains are structurally almost
identical, and while there are sex differences in the endocrine system, these
are small, while much of gendered identity and behaviour is learned. On the
other hand, sex differences in some aspects of social behaviour have deep
evolutionary roots, and are widely present in non-human animals. This
observational study recorded the frequency of same-aged, adult human groups
appearing in public spaces through 2636 hours, recording group formation by
1.2mn people via 170 research assistants in 46 countries across the world. The
results show (a) a significant sex-gender difference in same-sex-same-age
frequency, in that ~50% more female-female than male-male pairs are observed in
public spaces globally, and (b) that despite regional variation, the patterns
holds up in every global region. This is the first study of sex-gender
difference in dyadic social behaviour across the world on this scale, and the
first global study that uses direct rather than internet-based observations.
